Proceedings Volume 9931

Spintronics IX

Proceedings Volume 9931

Spintronics IX

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Volume Details

Date Published: 22 November 2016
Contents: 32 Sessions, 43 Papers, 85 Presentations
Conference: SPIE Nanoscience + Engineering 2016
Volume Number: 9931

Table of Contents


Table of Contents

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  • Front Matter: Volume 9931
  • Spin Photonics and Spin Plasmonics
  • Spin Hall Effects and Related Phenomena
  • Spin Photonics and Spin Lasers
  • Magnetic Tunnel Junctions
  • Spin Transfer Oscillators and Spin Transfer in AF Materials
  • Spin Hall Effect
  • Ferromagnetic Resonance and SO-induced Voltage
  • MRAMs I
  • Ultra-fast Magnetization Dynamics
  • Ultra-fast Magnetization Dynamics, Spin Coherence, and Spin Resonance
  • Spin Ice
  • Quantum Wells and Quantum Dots I
  • Spin-orbit Torque
  • Quantum Wells and Quantum Dots II
  • Tunnel Hall Effect and Chirality
  • International French-US Laboratory on Nanoelectronics
  • Spin-orbit Coupling
  • Voltage Control and New Devices
  • Skyrmions
  • Magneto-acoustics and Magneto-plasmonics
  • Skyrmions and Chiral Textures
  • Magnonics I
  • Chiral Textures and Majorana Fermions
  • Transition Metal Dichalcogenides
  • Majorana States and Topological Insulators
  • Graphene and Organic Materials
  • Magnonics II
  • Superconductivity I
  • Nanomagnetism
  • Superconductivity II
  • Poster Session
Front Matter: Volume 9931
Front Matter: Volume 9931
This PDF file contains the front matter associated with SPIE Proceedings Volume 9931, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.
Spin Photonics and Spin Plasmonics
The effect of fringe fields from patterned magnetic domains on the electroluminescence of organic light-emitting diodes
Nicholas J. Harmon, Markus Wohlgennant, Michael E. Flatté
Large magnetic field effects, either in conduction or luminescence, have been observed in organic light-emitting diodes (OLEDs) for over a decade now. The physical processes are largely understood when exciton formation and recombination lead to the magnetic field effects. Recently, magnetic field effects in some co-evaporated blends have shown that exciplexes deliver even larger responses. In either case, the magnetic field effects arise from some spin-mixing mechanism and spin-selective processes in either the exciton formation or the exciplex recombination. Precise control of light output is not possible when the spin mixing is either due to hyper-fine fields or differences in the Lande g-factor. We theoretically examine the optical output when a patterned magnetic film is deposited near the OLED. The fringe fields from the magnetic layers supply an additionally source of spin mixing that can be easily controlled. In the absence of other spin mixing mechanisms, the luminescence from exciplexes can be modified by 300%. When other spin-mixing mechanisms are present, fringe fields from remanent magnetic states act as a means to either boost or reduce light emission from those mechanisms. Lastly, we examine the influence of spin decoherence on the optical output.
Rare earth doped III-nitride semiconductors for spintronic and optoelectronic applications (Conference Presentation)
Since last four decades the information and communication technologies are relying on the semiconductor materials. Currently a great deal of attention is being focused on adding spin degree-of-freedom into semiconductor to create a new area of solid-state electronics, called spintronics. In spintronics not only the current but also its spin state is controlled. Such materials need to be good semiconductors for easy integration in typical integrated circuits with high sensitivity to the spin orientation, especially room temperature ferromagnetism being an important desirable property. GaN is considered to be the most important semiconductor after silicon. It is widely used for the production of green, blue, UV, and white LEDs in full color displays, traffic lights, automotive lightings, and general room lighting using white LEDs. GaN-based systems also show promise for microwave and high power electronics intended for radar, satellite, wireless base stations and spintronic applications. Rare earth (Yb, Eu, Er, and Tm) doped GaN shows many interesting optoelectronic and magnetoptic properties e. g. sharp emission from UV through visible to IR, radiation hardness, and ferromagnetism. The talk will be focused on fabrication, optoelectronic (photoluminescence, cathodeluminescence, magnetic, and x-ray photoelectron spectroscopy) properties of some rare earth doped GaN and InGaN semiconductor nanostructures grown by plasma assisted molecular beam epitaxy (MBE) and future applications.
Coherent control of plasmonic Spin Hall effect (Conference Presentation)
Shiyi Xiao, Fan Zhong, Hui Liu, et al.
We demonstrate spin-induced manipulation of surface-plasmon polariton (SPP) by exploiting the plasmonic spin Hall effect. By constructing metasurfaces with plasmonic atoms and varying spin-dependent geometric phase, we establish a holographic interface between an incident plane wave and the SPP on an optical chip. It allows us to gain spin-splitting and flexible control of the shapes and phases of the local SPP orbitals. Furthermore, a linearly polarized incident light with rotating polarization angle can be used to play a motion picture of the orbitals. These investigations provide a feasible route to many applications, including spin-enabled imaging, data storage and integrated optics.
Spin Hall Effects and Related Phenomena
Spin Hall effect in 5d Au: W transition metal alloys (Conference Presentation)
Piotr Laczkowski, Juan Carlos Rojas-Sánchez, Williams Savero-Torres, et al.
The spin Hall effect (SHE) [1] allows for a reciprocal conversion between charge and spin currents using the spin orbit coupling which can be at the core of several promising spintronics devices. The spin orbit interaction is used to produce a transverse flow of spin or charge in response to a longitudinal excitation, these are the direct or inverse SHE. The spin Hall angle (SHA), the ratio of longitudinal and transverse electronic conductivities, is the characterising parameter of this conversion. So far, large SHA have been reported in transition metals like Pt, Pd, W, Beta-Ta and in a few alloys with large spin orbit coupling impurities: CuIr, CuBi or CuPb [2]. In this presentation we will report on our study of the SHA in Au based alloys [3] which exhibits a non-monotonic relation with the impurity concentration. In the regime of diluted alloys this behaviour suggests the dominent side-jump contribution to the spin Hall resistivity, thus allowing precise tuning of the SHA as a function of impurities concentration. We will present our analyses results by using the Lateral Spin Valves, with newly introduced spin-absorption model adapted to the case of the strong spin-orbit interactions and by using complementary Ferromagnetic-Resonance/Spin-Pumping technique thus demonstrating very large SHA of the order of 15 % or even larger. [1] J.E. Hirsch, PRL 83, 1834 (1999). [2] Y. Niimi et al., PRL 106, 126601 (2011), PRL 109, 156602 (2012), PRB 89, 054401 (2014). [3] P. Laczkowski et al., APL 104, 142403 (2014) [4] E. Saitoh, et al., APL 88, 182509 (2006).
Magnetoresistance in ferromagnetic multilayer with strong interfacial spin-orbit coupling (Conference Presentation)
Junyeon Kim, Shutaro Karube, Yan-Ting Chen, et al.
Spin-charge conversion induced by spin-orbit coupling (SOC) is attractive topic for alternative magnetization manipulation and involved various novel phenomena. Particularly Bi-based structure draws interest due to its large Rashba-Edelstein effect (REE) at interface between non-magnetic metal and Bi [1]. A recent report showed that spin-to-charge current conversion becomes more efficient when Bi2O3 is employed on behalf of the Bi [2]. Here we report novel type of magnetoresistance (MR) in Co25Fe75/Cu/Bi2O3 multilayer. This novel MR comes from conversion between spin and charge current at Cu/Bi2O3 interface, and distinctive spin transfer torque dependent on magnetization of the ferromagnetic Co25Fe75 layer. A Co25Fe75 (5)/Cu (0-30)/Bi2O3 (20) (unit:nm) multilayer was deposited with electron beam evaporation on shadow masked Si substrate. Hall bar shaped shadow mask was patterned with photo-lithography method. The MR measurement was performed via 4-point probe method with changing magnitude or angle of external field. Note that external field for angle dependent measurement was 6 T to make sure complete saturation of ferromagnetic layer. We found characteristic resistance drop when the magnetization of ferromagnetic layer is parallel to magnetic direction of spin accumulation, which is similar to spin Hall magnetoresistance (SMR) [3,4]. Further discussion will be given. [1] J. C. Rojas Sanchez et al. Nature Comm. 4, 2944 (2013). [2] S. Karube et al. Appl. Phys. Express. 9, 03301 (2016). [3] H. Nakayama et al. Phys. Rev. Lett. 110, 206601 (2013). [4] J. Kim et al. Phys. Rev. Lett. (in press).
Spin Photonics and Spin Lasers
Leaky modes of dielectric cavities
In the absence of external excitation, light trapped within a dielectric medium generally decays by leaking out—and also by getting absorbed within the medium. We analyze the leaky modes of a parallel-plate slab, a solid glass sphere, and a solid glass cylinder, by examining those solutions of Maxwell’s equations (for dispersive as well as non-dispersive media) which admit of a complex-valued oscillation frequency. Under certain circumstances, these leaky modes constitute a complete set into which an arbitrary distribution of the electromagnetic field residing inside a dielectric body can be expanded. We provide completeness proofs, and also present results of numerical calculations that illustrate the relationship between the leaky modes and the resonances of dielectric cavities formed by a simple parallel-plate slab, a glass sphere, and a glass cylinder.
High-frequency operation of spin vertical-cavity surface-emitting lasers: towards 100 GHz
Nils C. Gerhardt, Markus Lindemann, Tobias Pusch, et al.
Spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) have a high potential to overcome limitations of conventional purely charge-based lasers. Probably the most important feature of such spin-lasers lies in their ultrafast spin and polarization dynamics which are decoupled from the intensity dynamics and their limitations. This yields the potential to modulate the polarization state of spin-VCSELs with frequencies far above the barriers known for the intensity modulation dynamics of conventional VCSELs. Such a quality makes them ideal devices for fast optical interconnects. While in conventional devices relaxation oscillations provide insights in the intensity dynamics and modulation bandwidth, in spin-VCSELs oscillations in the circular polarization degree are an ideal measure for investigating the dynamics of the coupled spin-photon system. These polarization oscillations (POs) can be generated using pulsed spin injection and have been proven to be much faster than intensity dynamics in the devices. Their frequency is mainly dependent on the birefringence in the cavities and can be increased by adding mechanical strain. Using an approach for manipulating the birefringence via mechanical strain we demonstrated tunable POs with frequencies up to 44 GHz, recently. Taking our results for strain-induced birefringence splitting of more than 250 GHz into account, the concept has the potential to overcome conventional limitations and to provide polarization modulation in VCSELs with bit rates beyond 100 Gbit=s. In this paper we investigate numerically the in uence of the spin decay rate on the PO amplitude and frequency in order to investigate potential limitations for future ultrafast polarization modulation schemes.
Comparative analysis of energy-efficient long wavebands vertical cavity surface emitting lasers
Sandeep Dahiya, Hemant Kumar Mishra, Suresh Kumar, et al.
Vertical cavity surface emitting laser (VCSEL) is an important laser source for their evident plentiful applications in optical communication. The present investigation reports a comparison of the modeling and optimization of long wavelengths 1310 nm and 1550 nm high speed short cavity VCSEL for continuous wave operation at various temperature (283-3230K) for various diameters. The continuous wave lasing is demonstrated for the device diameter from 2 to 5 μm with threshold current of 1.07-1.33 mA with threshold power consumption of 1.86-2.57 mW for 1310 nm and threshold current of 0.94-1.24 mA and threshold power consumption 1.67-2.1 mW for 1550 nm VCSEL. The results demonstrate that the threshold current, peak emitted power and power consumption increases with the increase in device diameter. The results confirm that VCSELs with 2 μm diameter is most suitable to achieve energy-efficient operation. Although rollover current increases with the diameter, but, due to the advantage of lower threshold current and power consumption, VCSEL having smaller diameter is best suited. The power conversion efficiency for proposed long wavelength VCSELs is approximately 50% which is extremely useful for low power applications. The proposed VCSELs are suitable for very short reach (<2 m) optical interconnects such as chip-to-chip and board-to-board communication in high performance computers.
Spin-photonic devices based on optical integration of Pancharatnam-Berry phase elements
Junxiao Zhou, Yachao Liu, Yougang Ke, et al.
Development of spin-photonic devices requires the integration of abundant functions and the miniaturization of the elements. Pancharatnam-Berry phase elements have fulfilled these requirements and can be attained by using dielectric metasurfaces with subwavelength nanostructures. Here, we review some of our works on Pancharatnam- Berry phase elements and make an introduction of some integrated spin-photonic devices. We propose to integrate Pancharatnam-Berry phase lens into dynamical phase lens, which can be conveniently used to modulate spin states of photons. By integrating a Pancharatnam-Berry phase lens into a conventional plano-concave lens, we can obtain spin-filtering of photons. Moreover, we demonstrate that the generation of complex wavefronts characterized with different spin states can be implemented by the Pancharatnam-Berry phase lens. Further, based on the spin-dependent property of Pancharatnam-Berry phase element, we realize the three-dimensional photonic spin Hall effect with lateral and longitudinal spin-dependent splitting simultaneously. We foresee that this optical integration concept of designing Pancharatnam-Berry phase elements, which circumvents the limitations of bulky optical components in conventional integrated optics, will significantly impact multipurpose optical elements, particularly spin-based photonics devices.
Magnetic Tunnel Junctions
Electric polarization control of magnetoresistance in complex oxide heterojunctions
Adrian G. Swartz, Hisashi Inoue, Harold Y. Hwang
Lorentzian magnetoresistance (L-MR) has been widely observed in three-terminal ferromagnet-nonmagnet (FM-NM) tunnel junctions. One possible explanation for this behavior is ensemble dephasing (Hanle effect) of a spin accumulation, potentially offering a powerful approach for characterizing the spin lifetime of emerging spintronics materials. However, discrepancies between the extracted spin parameters with known materials properties has cast doubt on this interpretation for most implementations. Here, we have developed a method to control band alignments in perovskite oxide heterostructures through the use of epitaxial interface dipoles, providing a highly effective method for manipulating the Schottky barrier height and contact resistance. Using these atomically engineered heterojunctions, we are able to tune key parameters relevant to various spin accumulation models, providing an experimental platform which can test their applicability. We find that the observed L-MR is inconsistent with an interpretation of spin accumulation in either the NM material or in interface states. Rather, we consider a mechanism analogous to Coulomb blockade in quantum dots, where spin-dependent tunneling through an ensemble of interfacial defect states is controlled by local and external magnetic fields.
Oxygen-vacancy driven tunnelling spintronics across MgO
U. Halisdemir, F. Schleicher, D. J. Kim, et al.
The conservation of an electron’s spin and symmetry as it undergoes solid-state tunnelling within magnetic tunnel junctions (MTJs) is thought to be best understood using MgO-based MTJs1. Yet the very large experimental values of tunnelling magnetoresistance (TMR) that justify this perception are often associated with tunnelling barrier heights well below those suggested by the MgO optical band gap. This combination of high TMR and low RA-product, while spawning spin-transfer/spin-orbit torque experiments and considerable industrial interest, cannot be explained by standard theory. Noting the impact of a tunnel barrier’s altered stoichiometry on TMR2, we reconcile this 10+year-old contradiction between theory and experiment by considering the impact of the MgO barrier’s structural defects3–5. We find that the ground and excited states of oxygen vacancies can promote localized states within the band gap with differing electronic character. By setting symmetry- and temperature-dependent tunnelling barrier heights, they alter symmetry-polarized tunnelling and thus TMR. We will examine how annealing, depending on MgO growth conditions, can alter the nature of these localized states. This oxygen vacancy paradigm of inorganic tunnelling spintronics opens interesting perspectives into endowing the MTJ with additional functionalities, such as optically manipulating the MTJ’s spintronic response.
Novel compact model for multi-level spin torque magnetic tunnel junctions
Sanjay Prajapati, Shivam Verma, Anant Aravind Kulkarni, et al.
Spin-transfer torque (STT) and spin-orbit torque (SOT) based magnetic tunnel junction (MTJ) devices are emerging as strong contenders for the next generation memories. Conventional STT magneto-resistive random access memory (MRAM) offers lower power, non-volatility and CMOS process compatibility. However, higher current requirement during the write operation leads to tunnel barrier reliability issues and larger access devices. SOT-MRAM eliminates the reliability issues with strong spin polarized current (100%) and separate read/write current paths; however, the additional two access transistors in SOT-MRAM results into increased cell area. Multilevel cell (MLC) structure paves a way to circumvent the problems related to the conventional STT/SOT based MTJ devices and provides enhanced integration density at reduced cost per bit. Conventional STT/SOT-MRAM requires a unit cell area of ~10-60 F2 and reported simulations have been based on available single-level MTJ compact models. However, till date no compact model exists that can capture the device physics of MLC-MTJ accurately. Hence, a novel compact model is proposed in this paper to capture the accurate device physics and behaviour of the MLC-MTJs. It is designed for MLCs with different MTJ configurations demonstrated so far, such as series and parallel free layer based MLC-MTJs. The proposed model is coded in Verilog-A, which is compatible with SPICE for circuit level simulations. The model is in close agreement with the experimental results exhibiting an average error of less than 15%.
Spin Transfer Oscillators and Spin Transfer in AF Materials
Superparamagnetic tunnel junctions for bio-inspired computing (Conference Presentation)
Julie Grollier, Jacob Torrejon, Mathieu Riou, et al.
The brain displays many features typical of non-linear dynamical networks, such as synchronization or chaotic behaviour. These observations have inspired a whole class of models that harness the power of complex non-linear dynamical networks for computing. In this framework, neurons are modeled as non-linear oscillators, and synapses as the coupling between oscillators. These abstract models are very good at processing waveforms for pattern recognition or at generating precise time sequences useful for robotic motion. However there are very few hardware implementations of these systems, because large numbers of interacting non-linear oscillators are indeed. In this talk, I will show that coupled spin-torque nano-oscillators are very promising for realizing cognitive computing at the nanometer and nanosecond scale, and will present our first results in this direction.
Electrical synchronization of spin-torque oscillators driven by self-emitted high frequency current (Conference Presentation)
Sumito Tsunegi, Romain Lebrun, Eva Grimaldi, et al.
The rich physics of spin transfer nano-oscillators (STNO) has provoked a huge interest to create a new generation of multi-functional microwave spintronic devices [1]. It has been often emphasized that their nonlinear behavior gives a unique opportunity to tune their radiofrequency (rf) properties but at the cost of large phase noise, not compatible with practical applications. To tackle this issue as well as to open the opportunities to new developments for non-boolean computations [1], one strategy is to use electrical synchronization of STOs through the rf current. Thereby, it is crucial to understand how the synchronization forces transmitted through the electric current. In this talk, we will first present the results of an experimental study showing the self-synchronization of STNO by re-injecting its rf current after a certain delay time [2]. In the second part, we demonstrate that the synchronization of two vortex-STNOs connected in parallel can be tuned either by an artificial delay or by the spin transfer torques [3]. The synchronization of spin-torque oscillators, combined with the drastic improvement of the rf-features (linewidth decreases by a factor of 2 and power increases by a factor of 4) in the synchronized state, marks an important milestone towards a new generation of rf-devices based on STNO. The authors acknowledge the financial support from ANR agency (SPINNOVA: ANR-11-NANO-0016) and EU grant (MOSAIC: ICT-FP7-317950). [1] N. Locatelli, V. Cros, and J. Grollier, Nat Mater 13, 11 (2014). [2] S. Tsunegi et al., arXiv:1509.05583 (2015) [3] R. Lebrun et al., arXiv:1601.01247 (2016)
Theory of spin transport in antiferromagnets (Conference Presentation)
Aurélien Manchon, Hamed Saidaoui, Collins Akosa
Antiferromagnets (AF) have long remained an intriguing and exotic state of matter, their application being restricted to enabling interfacial exchange bias in spin-valves. Their role in the expanding field of applied spintronics has been mostly passive and the in-depth investigation of their basic properties considered as fundamental condensed matter physics. A conceptual breakthrough was achieved ten years ago with the proposal that spin transfer torque could be used to electrically control the direction of the order parameter of AF spin valves, henceforth making these materials potential candidates for low energy spin devices. In spite of substantial theoretical efforts and experimental attempts to observe such a torque, the difficulty to independently detect the direction of the AF order parameter has remained a major obstacle. In this talk, I will first introduce the original concept of spin transfer torque in AF spin-valves, demonstrating that it is strongly limited by the spin decoherence and dramatically vanishes in the presence of disorder, leaving little hope to observe this effect experimentally. Then, I will present the newly proposed concept of spin-orbit torque that utilizes bulk or interfacial the spin-orbit coupling in non-centrosymmetric magnets to directly generate a torque on the AF order parameter. This torque, being local, is much more robust against impurities, as will be demonstrated for the specific case of interfacial Rashba spin-orbit coupling. Finally, I will discuss about spin motive force and torques in antiferromagnetic textures, intriguing effects that remained to be experimentally observed.
Spin transfer in antiferromagnets (Conference Presentation)
Since antiferromagnets (AFMs) have no spontaneous magnetization unlike ferromagnetic materials, it is not easy to manipulate the magnetic moments in AFMs by external magnetic field. However, recent theoretical studies suggest that it is possible to manipulate the magnetization in AFMs by spin-transfer-torque in a similar manner to ferromagnetic materials. In this study, we perform spin-toque ferromagnetic resonance (ST-FMR) measurements on FeNi/NiO/Pt multilayers to experimentally investigate the interaction between the spin current and the magnetic moments of antiferromagnetic NiO. The spin current is injected to the NiO by the spin Hall effect in Pt. The monotonous change in the FMR linewidth of this system with respect to the spin current can be interpreted in a way that the spin current is transferred through the NiO and interacts with the FeNi. This intriguing spin current transport can be explained by the angular momentum transfer mediated by the antiferromagnetic magnons. The results assure that the spin current exerts a torque on the NiO magnetic moments and excites their dynamics. In the talk, recent results will be also discussed.
Spin injection and absorption in antiferromagnets (Conference Presentation)
Lamprini Frangou, Pablo Merodio, Abhijit Ghosh, et al.
The antiferromagnetic order is expected to have a high potential in next-generation spintronic applications. It is resistant to perturbation by magnetic fields, produces no stray fields, displays ultrafast dynamics and may generate large magneto-transport effects. In spintronic materials, spin currents are key to unravelling spin dependent transport phenomena. Here, spin pumping results from the non-equilibrium magnetization dynamics of a ferromagnetic spin injector, which pumps a spin current into an adjacent spin sink. This spin sink absorbs the current to an extent which depends on its spin-dependent properties. The properties of the spin sink can be recorded either through the changes induced in ferromagnetic damping or through direct electrical means, such as by measuring the inverse spin Hall voltage. In this talk, we will deal with the injection of a spin current in thin antiferromagnetic sinks. Measurements of the spin penetration depths and absorption mechanisms were obtained for polycrystalline Ir20Mn80 and Fe50Mn50 films (Appl. Phys. Lett. 104, 032406 (2014)). More interestingly, spins propagate more efficiently in layers where the magnetic order is fluctuating rather than static. The experimental data were compared to some of the recently developed theories and converted into interfacial spin mixing conductance enhancements. These findings help us progress towards the development of more efficient spin sources, while also providing an alternative method to probe magnetic phase transitions (Phys. Rev. Lett. in press (2016)). This type of alternative method is particularly needed to deal with the case of thin materials with no net magnetic moments, such as thin antiferromagnets.
Spin Hall Effect
Spin-pumping and spin-Hall magnetoresistance (SMR) at transition metal interfaces: case of (Co/Pt) (Conference Presentation)
Henri Jaffres, Jean-Marie George, Piotr Laczowski, et al.
Spintronic phenomena are made possible via the diffusion of spin-currents or the generation of spin-accumulation. Spinorbitronics uses the electronic spin-orbit coupling (SOC) and emerges as a new route to create spin-currents in the transverse direction of the charge flow. This is made possible via the intrinsic spin Hall conduction (SHE) of heavy metals or extrinsic spin-Hall effect of metallic alloys. SHE borrows its concept from the anomalous Hall effect (AHE) where the relativistic spin-orbit coupling (SOC) promotes an asymmetric deflection of the spin-current. SHE is now at the base of magnetization commutation and domain wall moving via spin-orbit torque (SOT) and spin-transfer torque operations in the FMR regime. However, the exact anatomy of SOT at spin-orbit active interfaces like Co/Pt is still missing. In the case of Pt, recent studies have put forward the major role played by i) the spin-memory loss (SML) and the electronic transparency at 3d/5d interfaces and ii) the inhomogeneity of the conductivity in the current-in-plane (CIP) geometry to explain the discrepancy in the SHE. Ingredients to consider then are the profiles of both the conductivity and spin-current across the multilayers and spin-transmission. In this talk, we will present robust SMR measurements observed on NiCo/Pt multilayer stacks characterized by a perpendicular magnetic anisotropy (PMA). The SMR occurs for both in-plane magnetization rotation or from nominal out-of-plane to the in-plane direction transverse to the current flow. This clearly departs from standard AMR or pure interfacial anisotropic-AMR symmetries. We analyze in large details our SMR signals for the whole series of samples owing to two main guidelines: i) we consider the exact conductivity profile across the multilayers, in particular near the Co/Pt interface, via the Camley-Barnas approach and ii) we derive the spin current profile generated by SHE along the perpendicular direction responsible for SMR. We consider pure interfacial spin dissipation by SML (decoherence, interfacial enhanced scattering) and give out a general analytical expression for SMR. Our conclusions go towards a robust value of the spin-Hall conductivity and SML like previously published. The CIP spin-Hall angle, of the order of 0.10 is larger than the one found in spin-pumping experiments (CPP geometry) owing to the smaller conductivity at the Co/Pt interface, in agreement with the results of STT-FMR experiments.
Spin Hall effects in metallic multilayers (Conference Presentation)
Georg Woltersdorf, Dahai H. Wei, Martin Obstbaum, et al.
We study the direct as well as the inverse SHE. In the case of the direct SHE a dc charge current is applied in the plane of a ferromagnet/normal metal layer stack and the SHE creates a spin polarization at the surface of the normal metal leading to the injection of a spin current into the ferromagnet. This spin current is absorbed in the ferromagnet and causes a spin transfer torque. Using time and spatially resolved Kerr microscopy we measure the transferred spin momentum and compute the spin Hall angle. In a second set of experiments using identical samples pure spin currents are injected by the spin pumping effect from the ferromagnet into the normal metal. The spin current injected by spin pumping has a large ac component transverse to the static magnetization direction and a very small dc component parallel to the magnetization direction. The inverse SHE converts these spin current into charge current. The corresponding inverse SHE voltages induced by spin pumping at ferromagnetic resonance are measured in permalloy/platinum and permalloy/gold multilayers in various excitation geometries and as a function of frequency in order to separate the contributions of anisotropic magnetoresistance and SHE. In addition, we present experimental evidence for the ac component of inverse SHE voltages generated by spin pumping.
Spin-transfer torque in ferromagnetic bilayers generated by anomalous Hall effect and anisotropic magnetoresistance
Tomohiro Taniguchi, Julie Grollier, M. D. Stiles
We propose an experimental scheme to determine the spin-transfer torque efficiency excited by the spin-orbit interaction in ferromagnetic bilayers from the measurement of the longitudinal magnetoresistace. Solving a diffusive spin-transport theory with appropriate boundary conditions gives an analytical formula of the longitudinal charge current density. The longitudinal charge current has a term that is proportional to the square of the spin-transfer torque efficiency and that also depends on the ratio of the film thickness to the spin diffusion length of the ferromagnet. Extracting this contribution from measurements of the longitudinal resistivity as a function of the thickness can give the spin-transfer torque efficiency.
Spin polarized tunneling study on spin hall metals and topological insulators (Conference Presentation)
Luqiao Liu
Spin orbit interactions give rise to interesting physics phenomena in solid state materials such as the spin Hall effect (SHE) and topological insulator surface states. Those effects have been extensively studied using various electrical detection methods. However, to date most experiments focus only on characterizing electrons near the Fermi surface, while spin-orbit interaction is expected to be energy dependent. Here we developed a tunneling spectroscopy technique to measure spin Hall materials and topological insulators under finite bias voltages. By electrically injecting spin polarized electrons into spin Hall metals or topological insulators using tunnel junctions and measuring the induced transverse voltage, we are able to study SHE in typical 5d transition metals and the spin momentum locking in topological insulators. For spin Hall effect metals, the magnitude of the spin Hall angle has been a highly controversial topic in previous studies. Results obtained from various techniques can differ by more than an order of magnitude. Our results from this transport measurement turned out to be consistent with the values obtained from spin Hall torque measurements, which can help to address the long debating issue. Besides the magnitude, the voltage dependent spectra from our experiment also provide useful information in distinguishing between different potential mechanisms. Finally, because of the impedance matching capability of tunnel junctions, the spin polarized tunneling technique can also be used as a powerful tool to measure resistive materials such as the topological insulators. Orders of magnitude improvement in the effective spin Hall angle was demonstrated through our measurement
Ferromagnetic Resonance and SO-induced Voltage
Dynamic detection of spin accumulation in ferromagnet-semiconductor devices by ferromagnetic resonance (Conference Presentation)
Paul A. Crowell, Changjiang Liu, Sahil Patel, et al.
A distinguishing feature of spin accumulation in ferromagnet-semiconductor devices is its precession in a magnetic field. This is the basis for detection techniques such as the Hanle effect, but these approaches become ineffective as the spin lifetime in the semiconductor decreases. For this reason, no electrical Hanle measurement has been demonstrated in GaAs at room temperature. We show here that by forcing the magnetization in the ferromagnet to precess at resonance instead of relying only on the Larmor precession of the spin accumulation in the semiconductor, an electrically generated spin accumulation can be detected up to 300~K. The injection bias and temperature dependence of the measured spin signal agree with those obtained using traditional methods. We further show that this new approach enables a measurement of short spin lifetimes (< 100~psec), a regime that is not accessible in semiconductors using traditional Hanle techniques. The measurements were carried out on epitaxial Heusler alloy (Co2FeSi or Co2MnSi)/n-GaAs heterostructures. Lateral spin valve devices were fabricated by electron beam and photolithography. We compare measurements carried out by the new FMR-based technique with traditional non-local and three-terminal Hanle measurements. A full model appropriate for the measurements will be introduced, and a broader discussion in the context of spin pumping experimenments will be included in the talk. The new technique provides a simple and powerful means for detecting spin accumulation at high temperatures. Reference: C. Liu, S. J. Patel, T. A. Peterson, C. C. Geppert, K. D. Christie, C. J. Palmstrøm, and P. A. Crowell, “Dynamic detection of electron spin accumulation in ferromagnet-semiconductor devices by ferromagnetic resonance,” Nature Communications 7, 10296 (2016).
Torque-mixing magnetic resonance spectroscopy (Conference Presentation)
Joseph Losby, Fatemeh Fani Sani, Dylan T. Grandmont, et al.
An optomechanical platform for magnetic resonance spectroscopy will be presented. The method relies on frequency mixing of orthogonal RF fields to yield a torque amplitude (arising from the transverse component of a precessing dipole moment, in analogy to magnetic resonance detection by electromagnetic induction) on a miniaturized resonant mechanical torsion sensor. In contrast to induction, the method is fully broadband and allows for simultaneous observation of the equilibrium net magnetic moment alongside the associated magnetization dynamics. To illustrate the method, comprehensive electron spin resonance spectra of a mesoscopic, single-crystal YIG disk at room temperature will be presented, along with situations where torque spectroscopy can offer complimentary information to existing magnetic resonance detection techniques. The authors are very grateful for support from NSERC, CRC, AITF, and NINT. Reference: Science 350, 798 (2015).
Driving magnetization dynamics with interfacial spin-orbit torques (Conference Presentation)
Axel F. Hoffmann, Wei Zhang, Joseph Sklenar, et al.
Bulk spin Hall effects are well know to provide spin orbit torques, which can be used to drive magnetization dynamics [1]. But one of the reoccurring questions is to what extend spin orbit torques may also originate at the interface between materials with strong spin orbit coupling and the ferromagnets. Using spin torque driven ferromagnetic resonance we show for two systems, where interfacial torques dominate, that they can be large enough to be practically useful. First, we show spin transfer torque driven magnetization dynamics based on Rashba-Edelstein effects at the Bi/Ag interface [2]. Second, we will show that combining permalloy with monolayer MoS2 gives rise to sizable spin-orbit torques. Given the monolayer nature of MoS2 it is clear that bilk spin Hall effects are negligible and therefore the spin transfer torques are completely interfacial in nature. Interestingly the spin orbit torques with MoS2 show a distinct dependence on the orientation of the magnetization in the permalloy, and become strongly enhanced, when the magnetization is pointing perpendicular to the interfacial plane. This work was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division. [1] A. Hoffmann, IEEE Trans. Mag. 49, 5172 (2013). [2] W. Zhang et al., J. Appl. Phys. 117, 17C727 (2015). [3] M. B. Jungfleisch et al., arXiv:1508.01410.
Spin pumping and spin-orbit effects in Ge (Conference Presentation)
Simón Oyarzún, Ashis Kumar Nandy, Fabien Rortais, et al.
The field of spintronics is based on the manipulation of the spin degree of freedom. It uses the carrier spin angular momentum as a basic functional unit in addition to the charge. The first requirement of a semiconductor-based spintronic technology is the efficient generation of spin-polarized carriers into the device heterostructure made of Si or Ge (the materials of mainstream microelectronics) at room temperature. In this presentation, we focus on the generation of a sizeable spin population into Ge by spin pumping. Spin pumping corresponds to the generation of a pure spin current in the Ge film by exciting the ferromagnetic resonance of an adjacent ferromagnetic electrode with microwaves. The pure spin current is then detected using spin-orbit based effects. Our aim is to understand the basic mechanisms of spin pumping into Ge as well as the spin-to-charge conversion by inverse spin Hall effect (ISHE, bulk effect) [1-4] and Rashba-Edelstein effect (interface effect) [5]. The influence of interface states is clearly demonstrated. Moreover, using the spin-split Rashba sub-surface states of the Ge(111) surface, we succeeded in demonstrating a giant conversion of a spin current generated by spin pumping into a charge current by the Rashba-Edelstein effect [6,7]. Our experimental findings are supported by ab-initio calculations. 1. Rojas-Sánchez, J.-C. et al. Spin pumping and inverse spin Hall effect in germanium. Phys. Rev. B 88, (2013). 2. Kato, Y. K., Myers, R. C., Gossard, A. C. and Awschalom, D. D. Observation of the spin Hall effect in semiconductors. Science 306, 1910–1913 (2004). 3. Valenzuela, S. O. and Tinkham, M. Direct electronic measurement of the spin Hall effect. Nature 442, 176–179 (2006). 4. Saitoh, E., Ueda, M., Miyajima, H. and Tatara, G. Conversion of spin current into charge current at room temperature: Inverse spin-Hall effect. Appl Phys Lett 88, 2509 (2006). 5. Bychkov, Y. A. and Rashba, E. I. Oscillatory effects and the magnetic susceptibility of carriers in inversion layers. Journal of Physics C: Solid State Physics 17, 6039–6045 (1984). 6. Edelstein, V. M. Spin polarization of conduction electrons induced by electric current in two-dimensional asymmetric electron systems. Solid State Communications 73, 233–235 (1990). 7. Rojas-Sánchez, J.-C. et al. Spin-to-charge conversion using Rashba coupling at the interface between non-magnetic materials. Nat Comms 4, (2013).
Spin-transfer torque switched magnetic tunnel junctions in magnetic random access memory
Jonathan Z. Sun
Spin-transfer torque (or spin-torque, or STT) based magnetic tunnel junction (MTJ) is at the heart of a new generation of magnetism-based solid-state memory, the so-called spin-transfer-torque magnetic random access memory, or STT-MRAM. Over the past decades, STT-based switchable magnetic tunnel junction has seen progress on many fronts, including the discovery of (001) MgO as the most favored tunnel barrier, which together with (bcc) Fe or FeCo alloy are yielding best demonstrated tunnel magneto-resistance (TMR); the development of perpendicularly magnetized ultrathin CoFeB-type of thin films sufficient to support high density memories with junction sizes demonstrated down to 11nm in diameter; and record-low spin-torque switching threshold current, giving best reported switching efficiency over 5 kBT/μA. Here we review the basic device properties focusing on the perpendicularly magnetized MTJs, both in terms of switching efficiency as measured by sub-threshold, quasi-static methods, and of switching speed at super-threshold, forced switching. We focus on device behaviors important for memory applications that are rooted in fundamental device physics, which highlights the trade-off of device parameters for best suitable system integration.
"Extreme events" in STT-MRAM speed retention and reliability (Conference Presentation)
Xiaobin Wang, Jing Zhang, Zihui Wang, et al.
Fast operation speed, high retention and high reliability are the most attractive features of the spin transfer torque magnetic random access memory (STT-MRAM) based upon perpendicular magnetic tunneling junction (pMTJ). For state-of-the-art pMTJ STT-MRAM, its device performance is fundamentally determined by material “extreme events” physics. For example, nanosecond write bit error rate is determined by extremely high probability (>(1-10^(-7))) stochastic magnetization switching events, retention is determined by magnetization configurations with extremely low switching probability, reliability is determined by extremely low probability (<10^(-15)) tunneling junction break-down events. Despite their critical importance, accurately modeling, testing and prediction of "extreme events" have been a long-standing challenging physics and engineering issue due to their low occurrence rates. In this presentation, we will discuss our unique modeling and testing approaches to understand and predict "extreme events" in STT-MRAM write, read, retention and reliability. Specifically, we will present our model that accurately calculates extremely low write BER for various magnetization configurations. We will review our study of thermal magnetization switching through the dynamic optimal reversal path approach, capable of characterizing extreme thermal magnetization switching events under both low frequency (e.g. static retention) and high frequency (e.g. fast read) excitations. We will also discuss a new MTJ breakdown reliability model that quantifies extreme events uniformly at different failure mode regions.
Discussions on switching mechanism for ultimate reduction in energy consumption for STT-MRAM
H. Yoda, N. Shimomura
Critical switching current, ICsw, of STT (Spin Transfer Torque)-MRAM has been reduced by several orders with PMA (Perpendicular Magnetic Anisotropy)-MTJs and the state-of-the-art writing-charge, Qw, becomes the order of 100fC. With the small Qw, MRAM starts to save energy consumption even for mobile applications. The key to the Qw reduction is a development of MTJs having higher writing-efficiency. Especially coherent switching of storage-layer magnetization was found to be the root key to the high efficiency.
Ultra-fast Magnetization Dynamics
Tracking the ultrafast spin-lattice motion in FePt nanoparticles (Conference Presentation)
Hermann A. Dürr
The prospect of manipulating materials’ properties with femtosecond laser pulses, the shortest stimuli known to mankind, has fascinated researchers for decades. Discoveries of femtosecond demagnetization of ferromagnets and all-optical magnetic switching are fueled by the demand for faster information storage and processing. However, understanding when and how optical energy delivered into band electrons is transferred to spin and lattice degrees of freedom remains among the most challenging and important topics in condensed matter physics. Here we demonstrate for FePt nanoparticles how to disentangle these complex energy pathways. We show that femtosecond demagnetization launches a highly anisotropic ultrafast lattice motion characterized by a- and b-axis expansion and c-axis contraction. Picosecond lattice stress from non-equilibrium phonons increases the a,b-lattice spacing while invar-like near-zero c-axis expansion persists for tens of picoseconds. Our work establishes for a metallic system the existence of intimate spin, electron and lattice coupling, a hallmark usually reserved for strongly correlated electron systems.
Ultrafast spintronics roadmap: from femtosecond spin current pulses to terahertz non-uniform spin dynamics via nano-confined spin transfer torques (Conference Presentation)
Alexey Melnikov, Ilya Razdolski, Alexandr Alekhin, et al.
Further development of spintronics requires miniaturization and reduction of characteristic timescales of spin dynamics combining the nanometer spatial and femtosecond temporal ranges. These demands shift the focus of interest towards the fundamental open question of the interaction of femtosecond spin current (SC) pulses with a ferromagnet (FM). The spatio-temporal properties of the spin transfer torque (STT) exerted by ultrashort SC pulses on the FM open the time domain for studying STT fingerprint on spatially non-uniform magnetization dynamics. Using the sensitivity of magneto-induced second harmonic generation to SC, we develop technique for SC monitoring. With 20 fs resolution, we demonstrate the generation of 250 fs-long SC pulses in Fe/Au/Fe/MgO(001) structures. Their temporal profile indicates (i) nearly-ballistic hot electron transport in Au and (ii) that the pulse duration is primarily determined by the thermalization time of laser-excited hot carriers in Fe. Together with strongly spin-dependent Fe/Au interface transmission calculated for these carriers, this suggests the non-thermal spin-dependent Seebeck effect dominating the generation of ultrashort SC pulses. The analysis of SC transmission/reflection at the Au/Fe interface shows that hot electron spins orthogonal to the Fe magnetization rotate gaining huge parallel (anti-parallel) projection in transmitted (reflected) SC. This is accompanied by a STT-induced perturbation of the magnetization localized at the interface, which excites the inhomogeneous high-frequency spin dynamics in the FM. Time-resolved magneto-optical studies reveal the excitation of several standing spin wave modes in the Fe film with their spectrum extending up to 0.6 THz and indicating the STT spatial confinement to 2 nm.
Optomagnetism and ultrafast spintronics via optical vortices
A. F. Schäffer, J. Wätzel, J. Berakdar
Magnetic switching by circular polarized laser pulses is a promising tool for an ultrafast control of magnetism without the need for external magnetic fields. A principle limitation of the spatial resolution is set by the optical diffraction limit which is a clear disadvantage in view of the trend towards nanoscale magnetic structures to achieve high density storage. Here we suggest to exploit the light-matter interaction to achieve atomistic spatial and femtosecond temporal resolutions. The idea is to drive current loops in fullerenes attached to a scanning tip by virtue of femtosecond optical vortices. Using full- edge quantum simulations we calculate the magnetic field associated with the fullerenes current loops and employ this magnetic field for ferromagnetic resonance studies on magnetic adatoms.
Ultra-fast Magnetization Dynamics, Spin Coherence, and Spin Resonance
THz spectroscopy for THz spintronics (Conference Presentation)
Controlling currents using circularly polarized light and spin-orbit coupling could lead to the development of ultrafast spintronic devices driven by laser pulses and operating at the femtosecond timescale. Here we demonstrate that such a helicity dependent photocurrent can be generated in metallic heterostructures consisting of a single ferromagnetic layer and a non-magnetic one. In particular, using terahertz emission spectroscopy we show that the direction of the generated ultrafast photocurrent is controlled by the helicity of light, the magnetization of the ferromagnetic layer and the growth direction of the layers. We argue that the helicity and magnetization dependent photocurrent in metallic multilayers originates from a combination of the spin-orbit interaction and a lack of center of symmetry at the interface.
A single-atom spin-orbit qubit in Si (Conference Presentation)
Dimitrie Culcer, Joseph Salfi, Sven Rogge
High-fidelity two-qubit entanglement operations pose new challenges for spin qubits. Although spin orbit-coupling (SOC) can simplify entanglement via electric fields and microwave photons, it exposes conventional spin qubits to electrical noise. Here we devise a gate-tunable single-acceptor spin-orbit qubit in silicon having a sweet spot where the electric dipole spin resonance (EDSR) is maximized, and the qubit is simultaneously insensitive to dephasing from low-frequency electrical noise. The sweet spot protects the qubit during rapid single-qubit EDSR and two-qubit dipole-dipole mediated operations, and is only obtained by treating SOC non-perturbatively. More than 10000 one-qubit and 1000 two-qubit operations are possible in the predicted relaxation time, as necessary for surface codes. Moreover, circuit quantum electrodynamics with single dopants is feasible in this scheme, including dispersive single-spin readout, cavity-mediated two-qubit entangement, and strong Jaynes-Cummings coupling. Our approach provides a scalable route for controlling electrical and photon-mediated interactions between spins of individual dopants in silicon.
Coherent manipulation of nuclear spins using spin injection from a half-metallic spin source
Tetsuya Uemura, Takafumi Akiho, Yuya Ebina, et al.
We have developed a novel nuclear magnetic resonance (NMR) system that uses spin injection from a highly polarized spin source. Efficient spin injection into GaAs from a half-metallic spin source of Mn-rich Co2MnSi enabled an efficient dynamic nuclear polarization of Ga and As nuclei in GaAs and a sensitive detection of NMR signals. Moreover, coherent control of nuclear spins, or the Rabi oscillation between two quantum levels formed at Ga nuclei, induced by a pulsed NMR has been demonstrated at a relatively low magnetic field of ∼0.1 T. This provides a novel all-electrical solid-state NMR system with the high spatial resolution and high sensitivity needed to implement scalable nuclear-spin based qubits.
Spin Ice
Four states magnetic dots: a design selection by micromagnetic modeling
D. Louis, T. Hauet, S. Petit-Watelot, et al.
In a context where sub-micrometric magnetic dots are foreseen to play an active role in various new breeds of electronics components such as magnetic memories, magnetic logics or bio-sensors, the use of micromagnetic simulations to optimize their shapes and spatial arrangement with respect to a chosen application has become unavoidable. Prior realizing experimentally magnetic dots presenting four stable magnetic states (4SMS), we performed a micromagnetic study to select a design providing not only four equivalent magnetic states in a single dot but also exhibiting mostly uniform magnetic states.
Vertex micromagnetic energy in artificial square ice
Yann Perrin, Benjamin Canals, Nicolas Rougemaille
Artificial arrays of interacting magnetic elements provide an uncharted arena in which the physics of magnetic frustration and magnetic monopoles can be observed in real space and in real time. These systems offer the formidable opportunity to investigate a wide range of collective magnetic phenomena with a lab-on-chip approach and to explore various theoretical predictions from spin models. Here, we study artificial square ice systems numerically and use micromagnetic simulations to understand how the geometrical parameters of the individual magnetic elements affect the energy levels of an isolated square vertex. More specifically, we address the question of whether the celebrated square ice model could be made relevant for artificial square ice systems. Our work reveals that tuning the geometry alone should not allow the experimental realization of the square ice model when using nanomagnets coupled through the magnetostatic interaction. However, low-aspect ratios combined with small gaps separating neighboring magnetic elements of moderated thickness might permit approaching the ideal case where the degeneracy of the ice rule states is recovered.
Exploring frustrated magnetism with artificial spin ice
Ian Gilbert, B. Robert Ilic
Nanomagnet arrays known as artificial spin ice provide insight into the microscopic details of frustrated magnetism because, unlike natural frustrated magnets, the individual moments can be experimentally resolved and the lattice geometry can be easily tuned. Most studies of artificial spin ice focus on two lattice geometries, the square and the kagome lattices, due to their direct correspondence to natural spin ice materials such as Dy2Ti2O7. In this work, we review experiments on these more unusual lattice geometries and introduce a new type of nanomagnet array, artificial spin glass. Artificial spin glass is a two-dimensional array of nanomagnets with random locations and orientations and is designed to elucidate the more complex frustration found in spin glass materials.
Realizing artificial spin ice states for magnetic colloids on optical trap arrays
Colloids interacting with periodic substrates such as those created with optical traps are an ideal system in which to study various types of phase transitions such as commensurate to incommensurate states and melting behaviors, and they can also be used to create new types of ordering that can be mapped to spin systems. Here we numerically demonstrate how magnetic colloids interacting with an array of elongated two-state traps can be used to realize square artificial spin ice. By tuning the magnetic field, it is possible to precisely control the interaction strength between the colloids, making it possible to observe a transition from a disordered state to an ordered state that obeys the two-in/two-out ice rules. We also examine the dynamics of excitations of the ground state, including pairs of monopoles, and show that the monopoles have emergent attractive interactions. The strength of the interaction can be modified by the magnetic field, permitting the monopole velocity to be tuned.
Artificial ice using superconducting vortices (Conference Presentation)
Juan Trastoy Quintela, Maxime Malnou, Christian Ulysse, et al.
We use magnetic flux quanta (superconducting vortices) on artificial energy landscapes (pinning arrays) to create a new type of artificial ice. This vortex ice shows unusual temperature effects that offer new possibilities in the study of ice systems. We have investigated the matching of the flux lattice to pinning arrays that present geometrical frustration. The pinning arrays are fabricated on YBCO films using masked O+ ion irradiation. The details of the magneto-resistance imply that the flux lattice organizes into a vortex ice. The absence of history-dependent effects suggests that the vortex ice is highly ordered. Due to the technique used for the artificial energy landscape fabrication, we have the ability to change the pinning array geometry using temperature as a control knob. In particular we can switch the geometrical frustration on and off, which opens the door to performing a new type of annealing absent in other artificial ice systems. * Work supported by the French ANR “MASTHER”, and the Fundación Barrié (Galicia, Spain)
Quantum Wells and Quantum Dots I
Spin injection devices with high mobility 2DEG channels (Conference Presentation)
Mariusz Ciorga, Martin Oltscher, Thomas Kuczmik, et al.
Effective electrical spin injection into two-dimensional electron gas (2DEG) is a prerequisite for many new functionalities in spintronic device concepts, with the Datta-Das spin field effect transistor [1] being a primary example. Here we will discuss some of the results of our studies on spin injection devices with high mobility 2DEG confined in an inverted AlGaAs/GaAs heterojunction and a diluted ferromagnetic semiconductor (Ga,Mn)As employed as a source and a detector of spin-polarized carriers. Firstly we will show that nonlocal spin valve signal in such devices can significantly exceed the prediction of the standard model of spin injection based on spin drift-diffusion equations [2], what leads to conclusion that ballistic transport in the 2D region directly below the injector should be taken into account to fully describe the spin injection process [3]. Furthermore, we demonstrate also a large magnetoresistance (MR) signal of ~20% measured in local configuration, i.e., with spin–polarized current flowing between two ferromagnetic contacts. To our knowledge, this is the highest value of MR observed so far in semiconductor channels. The work has been supported by Deutsche Forschungsgemeinschaft (DFG) through SFB689. [1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990) [2] M. Oltscher et al., Phys. Rev. Lett. 113, 236602 (2014) [3] K. Cheng and S. Zhang, Phys. Rev. B 92, 214402 (2015)
Strain in GaAs quantum wells and layered composites detected by optically pumped NMR (Conference Presentation)
Clifford R. Bowers, John T. Tokarski III, Lauren A. McCarthy, et al.
We present a methodology for characterizing lattice strain effects in crystalline semiconductors based on optically pumped NMR (OPNMR). Lattice strain is detected as an electric quadrupole splitting of the NMR transition. Since OPNMR is an optical technique, it selectively probes strain only in the volume within the optical penetration depth of the laser light. The methodology is demonstrated in (1) variably thinned bulk GaAs layered composites and (2) GaAs quantum well thin films. Thermally induced lattice strain was induced by epoxy-bonding to Si support wafers at 373 K followed by cooling to 1.5 K. The variation of the strain with GaAs layer thickness is shown to be consistent with an analytical model for mechanical bowing. In the GaAs/AlxGa1-xAs thin films, the strain measured from the quadrupole splitting of the 71Ga NMR transition was incorporated into electronic energy band structure calculations which yield the photon energy dependence of the optical absorption and conduction electron spin polarization. The nuclear spin polarization is calculated from the electron spin polarization using an appropriate electron-nuclear cross-relaxation model. Comparison of theory to the experimental data provides new insights into how the optically pumped nuclear spin polarization is affected by strain and quantum confinement. [1] M. Sturge, Phys. Rev. 127, 768 (1962) [2] Y. Sun, et. al., Strain Effects in Semiconductors: Theory and Device Applications (Springer, 2010). [3] P.L. Kuhns et al., Phys. Rev. B. 55, 7824-7830 (1997). [4] R.M. Wood et al., Phys. Rev. B. 90, 155317 (2014)
Access to long-term optical memories using photon echoes retrieved from electron spins in semiconductor quantum wells
S. V. Poltavtsev, L. Langer, I. A. Yugova, et al.
We use spontaneous (two-pulse) and stimulated (three-pulse) photon echoes for studying the coherent evolution of optically excited ensemble of trions which are localized in semiconductor CdTe/CdMgTe quantum well. Application of transverse magnetic field leads to the Larmor precession of the resident electron spins, which shuffles optically induced polarization between optically accessible and inaccessible states. This results in several spectacular phenomena. First, magnetic field induces oscillations of spontaneous photon echo amplitude. Second, in three-pulse excitation scheme, the photon echo decay is extended by several orders of magnitude. In this study, short-lived optical excitation which is created by the first pulse is coherently transferred into a long-lived electron spin state using the second optical pulse. This coherent spin state of electron ensemble persists much longer than any optical excitation in the system, preserving information on initial optical field, which can be retrieved as a photon echo by means of third optical pulse.
Magneto-optical studies of CdSe/CdMnS/CdS core/multi-shell colloidal nanoplatelets
Athos Petrou, Thomas A. Scrace, Joseph R. Murphy, et al.
We studied the photoluminescence (PL)) from CdSe/CdMnS/CdS core/multi-shell colloidal nanoplatelets, a versatile platform to study the interplay of optical properties and nanomagnetism. The photoluminescence (PL) exhibits σ+ polarization in the applied magnetic field. Our measurement detects the presence of even a single magnetic monolayer shell. The PLL consists of a higher and a lower energy component; the latter exhibits a circular polarization peak. The time-resolved PL (trPL) shows a red shift as function of time delay. At early (later) times the trPL spectra coincide with the high (low) energy PL component. A model is proposed to interpret these results.
Spin-orbit Torque
Strain engineered magnetic tunnel junctions and spin-orbit torque switching (Conference Presentation)
Yang Wu, Kulothungasagaran Narayanapillai, Mehrdad Elyasi, et al.
The efficient generation of pure spin currents and manipulation of the magnetization dynamics of magnetic structures is of central importance in the field of spintronics. The spin-orbit effect is one of the promising ways to generate spin currents, in which a charge current can be converted to a transverse spin current due to the spin-orbit interaction. We investigate the spin dynamics in the presence of strong spin-orbit coupling materials such as LaAlO3/SrTiO3 oxide heterostructures. Angle dependent magnetoresistance measurements are employed to detect and understand the current-induced spin-orbit torques, and an effective field of 2.35 T is observed for a dc-current of 200 uA. In order to understand the interaction between light and spin currents, we use a femtosecond laser to excite an ultrafast transient spin current and subsequent terahertz (THz) emission in nonmagnet (NM)/ferromagnet (FM)/oxide heterostructures. The THz emission strongly relies on spin-orbit interaction, and is tailored by the magnitude and sign of the effective spin Hall angle of the NM. Our results can be utilized for ultrafast spintronic devices and tunable THz sources.
Quantum Wells and Quantum Dots II
Exciton spin coherence in InGaAs/GaAs quantum dots revisited by heterodyne pump-probe experiment (Withdrawal Notice)
B. Eble, B. Siarry, F. Bernardot, et al.
Publisher’s Note, 1 December 2016: This paper, originally published on 11/9/2016, was withdrawn at request of the authors.
Tunnel Hall Effect and Chirality
Spin injection, spin detection, and resonant spin transmission through single and double ferromagnetic/nonmagnetic Esaki barriers
We study the contact resistance of strongly doped ferromagnetic/non-magnetic semiconductors structure (p+ - F/n+ -N), working as spin injectors and spin extractors. Because of the strong effect that the barrier shape has on the tunneling probabilities, we evaluate, as accurately as possible, the quantum-mechanical spin-dependent transmission across the Esaki barrier built up at the p-n interface. To simplify the discussion and calculation of transmission coefficients through the Esaki barrier, we concentrate here on the structure p+ - F/n+ -N, without a stop layer I. We evaluate the spin injection and spin extraction transmission coefficients T↑↑ and T↓↓, and the spin transmission polarization as functions of bias potential, exchange interaction energy and Fermi energy level for specific realizations of the structure p+-Ga1-xMnxAs/n+-GaAs.
Tunnelling anomalous and planar Hall effects (Conference Presentation)
Alex Matos-Abiague, Benedikt Scharf, Jong E. Han, et al.
We theoretically show how the interplay between spin-orbit coupling (SOC) and magnetism can result in a finite tunneling Hall conductance, transverse to the applied bias. For two-dimensional tunnel junctions with a ferromagnetic lead and magnetization perpendicular to the current flow, the detected anomalous Hall voltage can be used to extract information not only about the spin polarization but also about the strength of the interfacial SOC. In contrast, a tunneling current across a ferromagnetic barrier on the surface of a three-dimensional topological insulator (TI) can induce a planar Hall response even when the magnetization is oriented along the current flow[1]. The tunneling nature of the states contributing to the planar Hall conductance can be switched from the ordinary to the Klein regimes by the electrostatic control of the barrier strength. This allows for an enhancement of the transverse response and a giant Hall angle, with the tunneling planar Hall conductance exceeding the longitudinal component. Despite the simplicity of a single ferromagnetic region, the TI/ferromagnet system exhibits a variety of functionalities. In addition to a spin-valve operation for magnetic sensing and storing information, positive, negative, and negative differential conductances can be tuned by properly adjusting the barrier potential and/or varying the magnetization direction. Such different resistive behaviors in the same system are attractive for potential applications in reconfigurable spintronic devices. [1] B. Scharf, A. Matos-Abiague, J. E. Han, E. M. Hankiewicz, and I. Zutic, arXiv:1601.01009 (2016).
Spin oscillations and Zitterbewegung of free carriers in semiconductors (Conference Presentation)
Sergey Tarasenko
We discuss the results of recent theoretical and experimental study of coupled spin-charge dynamics and noise of free carriers in three- and two-dimensional semiconductor structures. (i) Due to the Brownian motion of electrons and spin-orbit interaction, the temporal and spatial correlations of spin fluctuations emerging in the electron gas are coupled and the fluctuations probed at spatially separated spots of the sample are correlated. The spin correlations at large delay times are determined by the long-lived waves of spin density and drastically increase in the regime of a persistent spin helix. The measurement of spatial spin fluctuations provides direct access to the parameters of spin-orbit coupling and spin transport in conditions close to thermal equilibrium. (ii) The spin precession of electrons in a magnetic field gives rise to a trembling orbital motion of the carriers, a phenomenon similar to Zitterbewegung that free relativistic particles can experience. The trembling motion emerges in the absence of an ac driving force and caused by a quantum interference between the spin split states. The phenomenon can be studied by measuring the macroscopic ac electric current of the coherent trembling motion of spin-polarized electrons or, alternatively, by detecting the electric noise at the frequency of the Larmor precession at thermal equilibrium.
Spin-orbit assisted chiral-tunneling at semiconductor tunnel junctions: study with advanced 30-band k x p methods
Huong T. Dang, E. Erina, Hoai T. L. Nguyen, et al.
In this paper, we report on theoretical investigations and advanced k • p calculations of carrier forward scattering asymmetry (or transmission asymmetry in tunnel junction) vs. their incidence through magnetic tunnel junctions (MTJ) made of semiconductors involving spin-orbit interactions (SOI). This study represents an extension to our previous contribution1 dealing with the role, on the electronic forward and backward transmission-reflection asymmetry, of the Dresselhaus interaction in the conduction band (CB) of MTJs with antiparallel magnetized electrodes. The role of the atomic-SOI in the p-type valence band (VB) of semiconductors is investigated in a second step. We first developed a perturbative scattering method based on Green’s function formalism and applied to both the orbitally non-degenerated CB and degenerated VB to explain the calculated asymmetry in terms of orbital-moment tunneling branching and chirality arguments. This particular asymmetry features are perfectly reproduced by advanced k • p tunneling approaches (30-band) in rather close agreement with the Green’s function methods at the first perturbation order in the SOI strength parameter. This forward scattering asymmetry leads to skew-tunneling effects involving the branching of evanescent states within the barrier. Recent experiments involving non-linear resistance variations vs. the transverse magnetization direction or current direction in the in-plane current geometry may be invoked by the phenomenon we discuss.
International French-US Laboratory on Nanoelectronics
Domain wall dynamics under electric field in CoFeB-MgO structures with perpendicular magnetic anisotropy (Conference Presentation)
One crucial breakthrough in spin electronics has recently been achieved regarding the possibility to move magnetic domain walls (DWs) in magnetic tracks using the sole action of an electrical current instead of a conventional magnetic field. Here, we will present our recent results of DW dynamics obtained in Ta-CoFeB-MgO nanodevices with perpendicular magnetic anisotropy (PMA), which are widely used in STT-RAM applications, and discuss the critical problems to be addressed for implementation into a memory device. Using NV center microscopy to map DW pinning along a magnetic wire, we will first show1 that Ta/CoFeB(1nm)/MgO structures exhibit a very low density of pinning defects with respect to others materials with PMA. Then, we will focus on the possibility to use Electric Field Effect to control domain wall motion with low power dissipation. We will demonstrate gate voltage modulation of DW dynamics using different approaches based on dielectrics, piezoelectrics and ionic liquid layers.
Simple transition metal oxides (Conference Presentation)
Ivan K. Schuller, Ali C. Basaran, Jose de la Venta, et al.
Hybrid materials allow the engineering of new material properties by creative uses of proximity effects. When two dissimilar materials are in close physical proximity the properties of each one may be radically modified or occasionally a completely new material emerges. In the area of magnetism, controlling the magnetic properties of ferromagnetic thin films without magnetic fields is an on- going challenge with multiple technological implications for low- energy consumption memory and logic devices. Interesting possibilities include ferromagnets in proximity to dissimilar materials such as antiferromagnets or oxides that undergo metal-insulator transitions. The proximity of ferromagnets to antiferromagnets has given rise to the extensively studied Exchange Bias[1]. Our recent investigations in this field have addressed crucial issues regarding the importance of the antiferromagnetic [2-3] and ferromagnetic [4] bulk for the Exchange Bias and the unusual short time dynamics [5]. In a series of recent studies, we have investigated the magnetic properties of different hybrids of ferromagnets (Ni, Co and Fe) and oxides, which undergo metal-insulator and structural phase transitions. Both the static as well as dynamical properties of the ferromagnets are drastically affected. Static properties such as the coercivity, anisotropy and magnetization [6-8] and dynamical properties such as the microwave response are clearly modified by the proximity effect and give raise to interesting perhaps useful properties. Work supported by US-AFOSR and US-DOE
Coherent x-ray imaging of spins on nanoscale (Conference Presentation)
Understanding electronic structure at nanometer resolution is crucial to understanding physics such as phase separation and emergent behavior in correlated electron materials. Nondestructive probes which have the ability to see beyond surfaces on nanometer length and sub-picosecond time scales can greatly enhance our understanding of these systems and will impact development of future technologies, such as magnetic storage. Polarized x-rays are an appealing choice of probe due to their penetrating power, elemental and magnetic specificity, and high spatial resolution. The resolution of traditional x-ray microscopy is limited by the nanometer precision required to fabricate x-ray optics. In this thesis, a novel approach to lensless imaging of an extended magnetic nanostructure is presented. We demonstrate this approach by imaging ferrimagnetic "maze" domains in a Gd/Fe multilayer with perpendicular anisotropy. A series of dichroic coherent diffraction patterns, ptychographically recorded, are numerically inverted using non-convex and non-linear optimization theory, and we follow the magnetic domain configuration evolution through part of its magnetization hysteresis loop by applying an external magnetic field. Unlike holographic methods, it does not require a reference wave or precision optics, and so is a far simpler experiment. In addition, it enables the imaging of samples with arbitrarily large spatial dimensions, at a spatial resolution limited solely by the coherent x-ray flux and wavelength. It can readily be extended to other non-magnetic systems that exhibit circular or linear dichroism. This approach is scalable to imaging with diffraction-limited resolution, a prospect rapidly becoming a reality in view of the new generation of phenomenally brilliant x-ray sources.
Domain wall motion in ferromagnetically and antiferromagnetically coupled nanowires (Conference Presentation)
Majd Kuteifan, Sidi Fu, Stephane Mangin, et al.
demonstrate that the DW velocity can be significantly increased in antiferomagnetically coupled nanowires. The DW velocity increase is related to the exchange fields and reduction or elimination of the magnetostatic effects, which lead to reduction or elimination of the Walker breakdown. In addition, the reduction of the magnetostatic effects results in the reduction of the effects due to the pinning sites and disorder present in most nanomagnetic systems. The reduction of the pinning site and disorder effects further leads to a steadier DW motion. The study includes an analytical model for explaining how and why the Walker breakdown is overcome as well as numerical study supporting the analytical model and providing insights into the effects of the material and structural disorder. The numerical study is based on micromagnetic simulations solving the Landau-Lifshitz-Gilbert equation with continuous spin transfer torque components. The parameter space considered in the models and simulations includes the material properties, various types of disorder, and the exchange coupling in coupled systems. In addition, we discuss various aspects associated with modeling the DW motion in thin nanowires with disorder, including simulation speed, numerical stability, and the simulation model creation.
Spin-orbit Coupling
New relativistic Hamiltonian: the angular magnetoelectric coupling
Charles Paillard, Ritwik Mondal, Marco Berritta, et al.
Spin-Orbit Coupling (SOC) is a ubiquitous phenomenon in the spintronics area, as it plays a major role in allowing for enhancing many well-known phenomena, such as the Dzyaloshinskii-Moriya interaction, magnetocrystalline anisotropy, the Rashba effect, etc. However, the usual expression of the SOC interaction

ħ/4m2c2 [E×p] • σ (1)

where p is the momentum operator, E the electric field, σ the vector of Pauli matrices, breaks the gauge invariance required by the electronic Hamiltonian. On the other hand, very recently, a new phenomenological interaction, coupling the angular momentum of light and magnetic moments, has been proposed based on symmetry arguments:

ξ/2 [r × (E × B)] M, (2)

with M the magnetization, r the position, and ξ the interaction strength constant. This interaction has been demonstrated to contribute and/or give rise, in a straightforward way, to various magnetoelectric phenomena,such as the anomalous Hall effect (AHE), the anisotropic magnetoresistance (AMR), the planar Hall effect and Rashba-like effects, or the spin-current model in multiferroics. This last model is known to be the origin of the cycloidal spin arrangement in bismuth ferrite for instance. However, the coupling of the angular momentum of light with magnetic moments lacked a fundamental theoretical basis.

Starting from the Dirac equation, we derive a relativistic interaction Hamiltonian which linearly couples the angular momentum density of the electromagnetic (EM) field and the electrons spin. We name this coupling the Angular MagnetoElectric (AME) coupling. We show that in the limit of uniform magnetic field, the AME coupling yields an interaction exactly of the form of Eq. (2), thereby giving a firm theoretical basis to earlier works. The AME coupling can be expressed as:

ξ [E × A] • σ (3)

with A being the vector potential. Interestingly, the AME coupling was shown to be complementary to the traditional SOC, and together they restore the gauge invariance of the Hamiltonian. As an illustration of the AME coupling, we straightforwardly derived a relativistic correction to the so-called Inverse Faraday Effect (IFE), which is the emergence of an effective magnetic field under illumination by a circularly polarized light.
Birefringent coherent diffraction imaging
Dmitry Karpov, Tomy dos Santos Rolo, Hannah Rich, et al.
Directional dependence of the index of refraction contains a wealth of information about anisotropic optical properties in semiconducting and insulating materials. Here we present a novel high-resolution lens-less technique that uses birefringence as a contrast mechanism to map the index of refraction and dielectric permittivity in optically anisotropic materials. We applied this approach successfully to a liquid crystal polymer film using polarized light from helium neon laser. This approach is scalable to imaging with diffraction-limited resolution, a prospect rapidly becoming a reality in view of emergent brilliant X-ray sources. Applications of this novel imaging technique are in disruptive technologies, including novel electronic devices, in which both charge and spin carry information as in multiferroic materials and photonic materials such as light modulators and optical storage.
Chiral asymmetry driven by unidirectional magnetic anisotropy in Spin-Orbitronic systems
Paolo Perna, Fernando Ajejas, Davide Maccariello, et al.
Spin-Orbit (SO) effects of a ferromagnetic (FM) layer can be artificially modified by interfacial exchange coupling with an anti-ferro magnet (AFM). Non-symmetric magnetization reversals as well as asymmetric transport behaviors are distinctive signatures of the symmetry-breaking induced by such interfacial coupling. We present a complete picture of the symmetry of the SO effects by studying the magneto-transport properties of single FM film and FM/AFM systems (exchanged-biased bilayer and spin-valve structures) with specific in-plane magnetic anisotropy. Single FM films with a well-defined (two-fold) uniaxial magnetic anisotropy display symmetric magnetization reversals and magneto-resistance responses for any value and direction of the applied magnetic field. On the contrary, in the exchange-biased structures, the exchange interaction at the interface between the FM and AFM layers is responsible of chiral asymmetries in magnetization reversal pathways as well as in the magneto-resistance behaviors. Such asymmetries are directly related to the additional unidirectional (one-fold) magnetic anisotropy imposed by the AFM. In particular, chiral reversals and MR responses are found around the magnetization hard-axis direction. This has been shown in FM/AFM bilayer and spin-valve (where the MR outputs are related to different transport phenomena, i.e. anisotropic magneto-resistance and giant magneto-resistance respectively), hence indicating that the chiral asymmetries are intrinsic of systems with unidirectional anisotropy.
Voltage Control and New Devices
Giant voltage controlled magnetic anisotropy in heavy metal/ferromagnet/insulator junctions (Conference Presentation)
The realization of the MeRAM is based on the voltage control of the interfacial magnetocrystalline anisotropy (MCA) of heavy-metal/ferromagnet/insulator (HM/FM/I) nanojunctions, where the non-magnetic HM contact electrode (Ta, Pd, Pt, Au) has strong spin-orbit coupling (SOC). Employing ab initio electronic structure calculations we have investigated the effect of electric-field (E-field) and epitaxial strain on the MCA of Ta/FeCo/MgO heterostructure. We predict that uniaxial strain leads to a wide range of interesting voltage behavior of the MCA ranging from linear behavior with positive or negative magnetoelectronic coefficient, to non-monotonic ⋁-shape or inverse-⋀-shape E-field dependence with asymmetric magnetoelectronic coefficients. The calculations reveal that under a 4% compressive strain on MgO reaches the giant value of ~ 1126 fJ/(V.m). The underlying mechanism is the synergistic effects of strain and E-field on the orbital characters, the energy level shifts of the SOC d-states, and the dielectric constant of MgO. These results demonstrate for the first time the feasibility of highly sensitive E-field-controlled MCA through strain engineering, which in turn open a viable pathway towards tailoring magnetoelectric properties for spintronic applications. * This research was supported by NSF Grant No. ERC-TANMS-1160504
Ultralow energy electric field control of magnetism: can we get to 1 AttoJoule/bit? (Conference Presentation)
Ramamoorthy Ramesh
Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics. Our work so far has clearly demonstrated the possibility of reversible, electric field switching and control of the state and direction of magnetization. I will present our results to date.
Magnetic phase transitions in multiferroics (Conference Presentation)
Piero Torelli
An ambitious objective of the modern magnetism is the control of the magnetization of materials via the application of an electric field [1]. This objective is obviously driven by the important fallout on the electronic industry that this possibility would open. The magnetization control with electric field is now possible in those materials that display a spontaneous coupling between ferroelectric (FE) and ferromagnetic (FM) orders that are called multiferroics. Unfortunately the low magnetoelectric coupling coefficient like in BiFeO3 [1] and sometime the low Tc have for the moment limited the use of these materials in real devices. Two main routes are currently explored to overcome this limitations: the doping of mutiferroics with substitutional magnetic impurities and the realization of heterostructures in which different properties like ferroelectricity and ferromagnetism are artificially coupled to obtain high magnetoelectric coupling coefficient. The advantages and drawbacks of these two different solution will be presented together with two model examples: the Bi2FeCrO6 double peroskite [3] and the Fe/BaTiO3 interface [4]. In the first example we present the magnetic properties of Bi2FeCrO6: a double peroskite obtained by replacing half of the (111) crystal planes of Fe, in the BiFeO3 structure, with Cr planes. This new structure is predicted to display an artificial ferrimagnetic structure in spite of the normal antiferromagnetic configuration that will lead to an increase of the magnetoelectric coupling [5]. In the second case we present the chemical and magnetic analysis of the Fe/BaTiO3 interface, probably the most representative FE/FM interface, in which we have observed the formation of iron oxide at the interface that surprisingly shows a magnetic phase transition driven by the FE state of the BaTiO3. [1] F. Matsukura et al. Nature Nanotech. 10, 209 (2015) [2] C. Ederer and C. J. Fennie, J. Phys. Condens. Matter 20, 434219 (2008) [3] G. Vinai et al. APL Mater. 3, 116107 (2015) [4] G. Radaelli et al. Nat. Commun. 5, 3404 (2014) [5] P. Baettig et al., Phys. Rev. B 72, 214105 (2005)
Layer coupling and read disturbances in a buffered magnetic logic environment
Thomas Windbacher, Alexander Makarov, Viktor Sverdlov, et al.
There are two major obstacles impeding computing systems from further advancements: The power dissipation due to leakage and the energy spent for the information transfer between memory and processor(s). The first issue is commonly handled by shutting down unused circuit parts, however, when the dormant circuits are turned on again, their previous state must be recovered. This is commonly realized by retrieving the required information from the memory, which exacerbates the limited bandwidth between memory and processor(s). In order to circumvent these limitations, we have proposed a non-volatile buffered magnetic logic grid with instant-on capability. Non-volatile magnetic flip flops and spin-transfer torque majority gates are combined to a compact regular structure, which enables a small layout foot print as well as it guarantees the reduction of the information transfer due to a shared buffer. In the proposed structure the information is passed from one magnetic layer to another by first running a current through the magnetic layer to be read, which subsequently generates a magnetization orientation encoded spin-transfer torque, when the polarized electron spins enter the next layer. Since the current passing through the junction also exerts a spin-transfer torque on the read layer, its magnetization orientation could be destabilized which might cause a read disturbance. However, during our simulations it was also found out that the stray fields of neighboring layers have a non-negligible influence on the proposed copy operation. In this work we investigate these potential read disturbances in detail for a 2-bit shift register for varying stray field strength by changing the thickness of the interconnection layer. We found that for closer proximity the acting stray fields not only stabilize but also speed up the copy procedure, while for increasing interconnection layer thickness oscillating domain walls are formed and the copy operation becomes unreliable.
CMAT non-volatile spintronic computing: complementary MTJ logic
Joseph S. Friedman
Magnetic tunnel junctions (MTJs) have thoroughly demonstrated their utility as a non-volatile memory storage element, inspiring their application to a memory-in-logic computer that would overcome the von Neumann bottleneck. However, MTJ logic gates must be able to cause other MTJs to switch, thus ensuring the cascading capability fundamental to efficient computing. Complementary MTJ logic (CMAT) provides a simple circuit structure through which MTJs can be cascaded directly to perform logic operations. In this novel logic family, charge pulses resulting from MTJ switching create magnetic fields that switch other MTJs, providing impetus for further development of MTJs for computing applications.
Topological spin-orbitronics (Conference Presentation)
Nicolas Reyren, Davide Maccariello, Juan-Carlos Rojas-Sánchez, et al.
Spintronics evolves along new paths involving non-magnetic materials having large spin-obit coupling, typically 5d metals, allowing for example large spin-to-charge current conversion (spin Hall and Rashba-Edelstein effects). These heavy metals have other effects: in proximity of magnetic thin films they can burst out the Dzyaloshinskii-Moriya interaction leading to the stabilization of chiral magnetic structures. Another source of recent interest relies on “non-trivial topologies”, either of the band structure of the topological insulators, or of the spin textures in magnetic thin films. We will discuss our recent progress to control the topological textures known as skyrmions in multilayers made of heavy metals and magnetic layers. Aiming at using skyrmions as magnetic bits in “racetrack memory” structures, one of the present challenges is to efficiently move skyrmions with dimensions of a few tens of nanometers. The topology of these magnetic structures imposes peculiar dynamics, interesting both in fundamental and applied perspectives. Simulations indicate that spin-orbit torques, through the absorption of the spin current generated by a nearby layer, should be the most efficient method. The conducting surfaces of topological insulators at which the carriers’ spin and momentum are locked, can display better spin-to-charge conversion than what is found using heavy metals. However, the control of the interfaces is crucial to conserve the Dirac cone and the associated spin-momentum locking. We demonstrate by ARPES and spin pumping experiments how the properties of the α-Sn thin film topological insulator are preserved and can be used for spintronics, maybe to move skyrmions!
Skyrmions à la carte (Conference Presentation)
Bertrand Dupe, Gustav Bihlmayer, Marie Böttcher, et al.
Due to their unique topological and dynamic properties skyrmions in magnetic materials offer attractive perspectives for future spintronic applications [1]. Recently, it has been discovered that magnetic skyrmions of Néel-type symmetry cannot only occur in ultra-thin transition metal films at surfaces [2,3] but also in asymmetric multilayers due to strong Dzyaloshinskii-Moriya (DMI) interactions [4]. We carry out first-principles calculations in order to study the stabilization mechanism of skyrmions in multilayers. Here, we predict the emergence of skyrmions in a new class of multilayers based on [4d/Fe2/5d]n, i.e. structures composed of Fe biatomic layers sandwiched between 4d- and 5d-transition-metal layers [5]. In these composite structures, the exchange and the Dzyaloshinskii-Moriya interactions which control skyrmion formation can be tuned separately by the two interfaces. This allows engineering skyrmions as shown by density functional theory, Monte Carlo and spin dynamics simulations. [1] A. Fert, et al., Nature Nano. 8, 152 (2013). [2] N. Romming, et al., Science 341, 636 (2013). [3] B. Dupé, et al., Nature Comm. 5, 4030 (2014). [4] C. Moreau-Luchaire, et al., Nature Nano. (2016) doi: 10.1038/nnano.2015. [5] B. Dupé, et al., submitted (arXiv :1503.08098).
Magneto-acoustics and Magneto-plasmonics
Transient grating generation of ultrafast surface magneto-elastic waves
Chia-Lin Chang, Julius Janušonis, Vladimir M. Vlasov, et al.
We present a simple experimental approach to generating and detecting surface-propagating magneto-elastic waves. Using the ultrafast optical transient grating geometry, we drive in-plane propagating surface acoustic waves which couple to, and resonantly drive, magnetization precession in thin magnetic films. The optical approach provides for the real-time detection of both elastic wave transients as well as the tightly coupled magnetization precession in independent detection channels and thus reveals the tight coupling between the two when an appropriate magnetic field is applied. We discuss the experimental geometry and resulting linear magneto-elastic responses. We briefly touch upon nonlinear magnetoelastic properties, which is the focus of our current work.
The road towards nonlinear magneto-plasmonics
Wei Zheng, Xiao Liu, Günter Lüpke, et al.
Nonlinear magneto-plasmonics (NMP) describes systems where nonlinear optics, magnetics and plasmonics are all involved. NMP can be referred to as interdisciplinary studies at the intersection of Nonlinear Plasmonics (NP), Magneto- Plasmonics (MP), and nanoscience. In NMP systems, nanostructures are the bases, Surface Plasmons (SPs) work as catalyst due to strong field enhancement effects, and the nonlinear magneto-optical Kerr effect (nonlinear MOKE) plays an important role as a characterization method. Many new effects were discovered recently, which include enhanced magnetization-induced harmonic generation, controlled and enhanced magnetic contrast, magneto-chiral effect, correlation between giant magnetroresistance (GMR) and nonlinear MOKE, etc. We review the structures, experiments, findings, and the applications of NMP.
Skyrmions and Chiral Textures
Chiral damping in magnetic domain-walls (Conference Presentation)
Emilie Jue, C.K. Safeer, Marc Drouard, et al.
The Dzyaloshinskii-Moriya interaction is responsible for chiral magnetic textures (skyrmions, spin spiral structures, …) in systems with structural inversion asymmetry and high spin-orbit coupling. It has been shown that the domain wall (DW) dynamics in such materials can be explained by chiral DWs with (partly or fully) Néel structure, whose stability derives from an interfacial DMI [1]. In this work, we show that DMI is not the only effect inducing chiral dynamics and demonstrate the existence of a chiral damping [2]. This result is supported by the study of the asymmetry induced by an in-plane magnetic field on field induced domain wall motion in perpendicularly magnetized asymmetric Pt/Co/Pt trilayers. Whereas the asymmetry of the DW motion is consistent with the spatial symmetries expected with the DMI, we show that this asymmetry cannot be attributed to an effective field but originates from a purely dissipative mechanism. The observation of chiral damping, not only enriches the spectrum of physical phenomena engendered by the SIA, but since it can coexist with DMI it is essential for conceiving DW and skyrmion devices. [1] A. Thiaville, et al., EPL 100, 57002 (2012) [2] E. Jué, et al., Nat. Mater., in press (doi: 10.1038/nmat4518)
Electric field driven switching of individual magnetic skyrmions (Conference Presentation)
An interesting class of interface-driven non-collinear spin structures, i.e., chiral domain walls, cycloidal spin spirals and Néel-type skyrmions, have been observed in ultrathin transition metal films grown on heavy-element substrates making use of spin-polarized scanning tunneling microscopy (SP-STM) [1]. Due to a lack of structural inversion symmetry at interfaces, they exhibit a unique rotational sense as a consequence of interfacial Dzyaloshinskii-Moriya (DM) interactions. In this talk, I will present our results based on the investigations of such chiral spin textures under the influence of strain relief and the effect of local electric fields. While a nanoskyrmion lattice was revealed for Fe monolayers (ML) grown on Ir(111), a cycloidal spin spiral ground state has been resolved on Fe double-layers (DL) by employing SP-STM with vectorial magnetic field. As a result of a large lattice mismatch between the epitaxially grown Fe-DL film and the underlying Ir(111) substrate, local uniaxial strain relief occurs, leading to dislocation line patterns. Interestingly, the wavevector of spin spirals is strictly guided along the dislocation lines, while the spin spiral's wavefront exhibits a zigzag deformation [2]. By further increasing the Fe coverage to triple-layers (TL), the zigzag spin spiral remains the magnetic ground state, but with an enhanced periodicity as compared to that of Fe-DL. A magnetic phase transition from the spin spiral to a skyrmionic state, and finally to a saturated ferromagnetic state occurs for Fe-TL by applying an external magnetic field. STM-induced writing and deleting of individual skyrmions is demonstrated with a pronounced bias-polarity dependence, suggesting the decisive role of the local electric field between STM tip and Fe film for the switching mechanism [3]. [1] K. von Bergmann, A. Kubetzka, O. Pietzsch, and R. Wiesendanger, J. Phys.: Condens. Matter 26, 394002 (2014) [2] P.-J. Hsu, A. Finco, L. Schmidt, A. Kubetzka, K. von Bergmann, and R. Wiesendanger, Phys. Rev. Lett. 116, 017201 (2016) [3] P.-J. Hsu, A. Kubetzka, A. Finco, N. Romming, K. von Bergmann, and R. Wiesendanger, arXiv:16001.02935 (2016)
Magnonics I
Exchange magnon spintronics in the magnetic insulator yttrium iron garnet (Conference Presentation)
In magnetic insulators, transport of charge is prohibited due to the large bandgap. Spin can still be transported however by spin waves (magnons), the excitations of magnetic systems. The field that studies the properties of spin waves in magnetic insulators is known as magnon spintronics [1]. In the past years, research in the field has been focused on dipolar magnons, which are low-energy spin waves. We have shown [2] that magnons with energy comparable to the thermal energy (exchange magnons) can also transport spin over long distances, characterized by a spin diffusion length λ ≈ 9.5 μm. We have developed a non-local measurement scheme in which exchange magnons are excited and detected making use of the spin Hall- and inverse spin Hall-effect, respectively. This enables the conversion from electronic charge, to electron spin current, to magnonic spin current and vice-versa, using DC electronic signals. This provides a direct interface with conventional electronics and opens up new magnonic device functionalities. Additionally, it allows us to gain insight in the transport of magnons by studying the non-local signal as a function of various parameters, such as an external magnetic field [3] or sample temperature. Finally, studying the long-distance transport of thermal magnons can increase our understanding of the spin Seebeck effect in both the longitudinal and the non-local geometry. [1] A.V. Chumak et al., Nat. Phys. 11, 453-461 (2015) [2] L.J. Cornelissen et al., Nat. Phys. 11, 1022-1026 (2015) [3] L.J. Cornelissen and B.J. van Wees, Phys. Rev. B 93, 020403(R) (2016)
Topological magnon insulators: Chern numbers and surface magnons
Alexander Mook, Jürgen Henk, Ingrid Mertig
Within the framework of a quantum-mechanical Heisenberg model, we address the topology of the magnon band structures of ferromagnetic pyrochlores, in particular Lu2V2O7. The bridge from Chern numbers of bulk magnons to essential properties of topological surface magnons is constructed by the bulk-boundary correspondence. The topological properties of the system which originate from the Dzyaloshinskii-Moriya interaction and show up as nonzero Berry curvatures suggest a method to clearly distinguish topological surface magnons from other, trivial magnons. The method is illustrated with regard to experiments.
Chiral Textures and Majorana Fermions
Shape-dependence of the barrier for skyrmions on a two-lane racetrack
Single magnetic skyrmions are localized whirls in the magnetization with an integer winding number. They have been observed on nano-meter scales up to room temperature in multilayer structures. Due to their small size, topological winding number, and their ability to be manipulated by extremely tiny forces, they are often called interesting candidates for future memory devices. The two-lane racetrack has to exhibit two lanes that are separated by an energy barrier. The information is then encoded in the position of a skyrmion which is located in one of these close-by lanes. The artificial barrier between the lanes can be created by an additional nanostrip on top of the track. Here we study the dependence of the potential barrier on the shape of the additional nanostrip, calculating the potentials for a rectangular, triangular, and parabolic cross section, as well as interpolations between the first two. We find that a narrow barrier is always repulsive and that the height of the potential strongly depends on the shape of the nanostrip, whereas the shape of the potential is more universal. We finally show that the shape-dependence is redundant for possible applications.
Majorana bound states in magnetic skyrmions (Conference Presentation)
Peter Stano
Magnetic skyrmions are highly mobile nanoscale topological spin textures. We show, both analytically and numerically, that a magnetic skyrmion of an even azimuthal winding number placed in proximity to an s-wave superconductor hosts a zero-energy Majorana bound state in its core, when the exchange coupling between the itinerant electrons and the skyrmion is strong. This Majorana bound state is stabilized by the presence of a spin-orbit interaction. We propose the use of a superconducting tri-junction to realize non-Abelian statistics of such Majorana bound states.
Gate control of spin polarization in a quantum Hall regime toward reconfigurable network of helical channels (Conference Presentation)
Leonid Rokhinson, Aleksandr Kazakov, George Simion, et al.
Several experiments in nanowires detected signatures of Majorana fermions, building block for topologicaly protected quantum computer. Now the focus of research is shifting toward systems where non-Abelian statistics of excitations can be demonstrated. To achieve this goal we are developing a new dilute magnetic semiconductor-based platform where non-Abelian excitations can be created and manipulated in a two-dimensional plane, with support for Majorana and higher order non-Abelian excitations. Here we report development of heterostructures where spin polarization of a two-dimensional electron gas in a quantum Hall regime can be controlled locally by electrostatic gating. This is demonstrated via voltage induced shift of quantum Hall ferromagnetic transition in the CdTe quantum wells with engineered placement of paramagnetic Mn impurities. The structures can be used to form helical domain walls in integer quantum Hall regime which, coupled to an s-wave superconductor, are expected to support Majorana zero modes. These heterostructures can be used as a testbed to study gate-reconfigurable domain walls networks.
Transition Metal Dichalcogenides
Spin and valley polarization in MoS2, MoSe2, and WSe2 monolayers (Conference Presentation)
Cedric Robert, Gang Wang, Fabian Cadiz, et al.
The spectacular progress in controlling the electronic properties of graphene has triggered research in alternative atomically thin two-dimensional crystals. Monolayers (ML) of transition-metal dichalcogenides such as MoS2 have emerged as very promising nanostructures for optical and spintronics applications. Inversion symmetry breaking together with the large spin-orbit interaction leads to a coupling of carrier spin and k-space valley physics, i.e., the circular polarization (σ+ or σ−) of the absorbed or emitted photon can be directly associated with selective carrier excitation in one of the two nonequivalent K valleys (K+ or K−, respectively). We have investigated the spin and valley properties for both neutral and charged excitons in transition metal dichalcogenide monolayer MoS2, MoSe2 and WSe2 with cw and time-resolved polarized photoluminescence spectroscopy [1,2]. The key role played by exciton exchange interaction will be presented [3]. We also demonstrate that the optical alignment of excitons (“exciton valley coherence”) can be achieved following one or two photon excitation [1,4]. Finally recent results on magneto-photoluminescence spectroscopy on MoSe2 and WSe2 in Faraday configuration up to 9 T will be presented; the results will be discussed in the framework of a k.p theory [5]. [1] G. Wang et al, PRL 114, 97403 (2015) [2] G. Wang et al, Nature Com. 6, 10110 (2015) [3] J. P. Echeverry, ArXiv 1601.07351 (2016) [4] G. Wang et al, PRL 115, 117401 (2015) [5] G. Wang et al, 2D Mat. 2, 34002 (2015)
Ultrafast valley dynamics in atomically thin transition metal dichalcogenides (Conference Presentation)
Monolayer transition metal dichalcogenides such as MoS2, WS2, MoSe2, WSe2, and MoTe2 received a lot of attention recently due to their atomic thickness in combination with an optical band gap in the visible or infrared. These properties render them a promising material class for new opto-electronic devices. Strong spin-orbit coupling together with the absence of inversion symmetry leads to an emission of polarized photoluminescence after excitation with circularly polarized light. Therefore, the K and K’ valley of the semiconductor can be selectively addressed by left and right handed circularly polarized light, which is interesting for valleytronic applications. To understand the mechanisms governing the creation and destruction of valley polarization, time-resolved experiments are necessary. While stationary photoluminescence experiments show nearly perfect valley polarization, indicating very slow intervalley scattering processes, first valley-selective pump-probe experiments yielded a strong signal immediately after optical excitation in both the pumped and unpumped valley, suggesting a small valley polarization. To understand this behavior, we performed a joint experiment-theory study on the time-resolved valley dynamics in atomically thin WS2. We find strong intervalley Coulomb coupling governing the dynamics in the atomically thin semiconductor. Our results are also applicable to the other transition metal dichalcogenides MoS2, MoSe2, and WSe2, where strong intervalley Coulomb coupling is expected.
Many-body and stark effects in transition metal dichalcogenides monolayers (Conference Presentation)
Benedikt Scharf, Tobias Frank, Martin Gmitra, et al.
Inversion symmetry breaking combined with strong spin-orbit coupling in transition metal dichalcogenides such as MoS2 offers important opportunities for spintronics. We investigate excitons in MoS2 monolayers in an applied in-plane electric field. Tight-binding and Bethe-Salpeter equation calculations predict a large quadratic Stark shift. The scaling of the Stark shifts with the exciton binding energy and the dielectric environment provides a path to engineering the MoS2 electro-optical response. Our results suggest that the excitonic Stark effect can be observed experimentally in a MoS2 monolayer and we explain its implications for spintronic devices.
Optical control of exciton states and enhanced valley Zeeman splitting in WS2 (Conference Presentation)
George Kioseoglou, Marc Currie, Aubrey T. Hanbicki, et al.
Monolayer transition metal dichalcogenides, MX2 (M = Mo, W and X = S, Se), are direct-gap semiconductors with many interesting properties capable of producing an all-surface material applicable to sensing, single-atom storage and other quantum-based technologies. Here we report on the optical control of single layers of MX2 such that the photoluminescence (PL) is solely from the trion state. After trion isolation, changes in the Raman spectra were observed: there is a decrease in the intensity of the out of plane mode and an enhancement of the 2LA mode. The effect is reversible, and our results suggest that the changes of the strength of a particular excitonic state are due to surface interactions with ambient environment. In addition, spatial non-uniformity is probed by studying variations of strain and the PL emission as a function of position on our sample. The boundaries of mechanically exfoliated MX2 as well as boundaries intentionally created via fs laser ablation were investigated. The edges exhibit significant Raman shifts as well as remarkably enhanced PL emission compared to their respective central area. Finally, we probe the degree of circular polarization of the emitted PL as a function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. This work was supported by the FP7-REGPOT-2012-2013-1, under grant agreement 316165.
Majorana States and Topological Insulators
Electrical detection of spin-momentum locking in Bi2Se3 (Conference Presentation)
Topological insulators (TIs) exhibit topologically protected metallic surface states populated by massless Dirac fermions with spin-momentum locking – the carrier spin lies in-plane, locked at right angle to the carrier momentum. An unpolarized charge current should thus create a net spin polarization. Here we show direct electrical detection of this bias current induced spin polarization as a voltage measured on a ferromagnetic (FM) metal tunnel barrier surface contact [1]. The voltage measured at this contact is proportional to the projection of the TI spin polarization onto this axis, and similar data are obtained for two different FM contact structures, Fe/Al2O3 and Co/MgO/graphene. From measurements of the carrier type and sign of the spin voltage for n-Bi2Se3 and p-Sb2Te3, we show that transport measurements can be used to determine the chirality of the spin texture [2]. The chirality inverts as one crosses the Dirac point, so that the carrier spin-momentum locking follows a left-hand rule (clockwise chirality) when the Fermi level is above the Dirac point, and right-hand rule below (counter-clockwise chirality). These results demonstrate simple and direct electrical access to the TI Dirac surface state spin system, provide clear evidence for the spin-momentum locking and bias current-induced spin polarization, and enable utilization of these remarkable properties for future technological applications. [1] C. H. Li, O. M. J. van ‘t Erve, J. T. Robinson, Y. Liu, L. Li , and B. T. Jonker, Nature Nanotech. 9, 218 (2014). [2] C. H. Li, O. M. J. van ‘t Erve, Y. Y. Li, L. Li and B. T. Jonker, under review.
Topological phases in InAs1−xSbx: from novel topological semimetal to Majorana wire (Conference Presentation)
Georg W. Winkler, QuanSheng Wu, Matthias Troyer, et al.
Superconductor proximitized one-dimensional semiconductor nanowires with strong spin-orbit interaction (SOI) are at this time the most promising candidates for the realization of topological quantum information processing. 1-6 In current experiments the SOI originates predominantly from extrinsic fields, induced by finite size effects and applied gate voltages. The dependence of the topological transition in these devices on microscopic details makes scaling to a large number of devices difficult unless a material with dominant intrinsic bulk SOI is used. Here we show that wires made of certain ordered alloys InAs1-xSbx have spin splittings up to 20 times larger than those reached in pristine InSb wires.7, 8

We consider a stable ordered CuPt-structure at x = 0.5 with alternating (111)-layers of As and Sb. Experimental evidence for the CuPt-ordering of InA0.5Sb0.5 exists.9-13 Furthermore, we find an inverted band ordering realizing a novel type of topological semimetall14-17 with triple degeneracy points.18 We identify the novel semimetal as having both properties of the established topological Dirac 19-21 and Weyl15, 22 semimetals, and thus the triple point can be seen as an interpolation of Dirac and Weyl points. Analog to the Dirac and Weyl semimetals we find surface Fermi arcs19, 21, 23-27 and anomalous transport in the presence of magnetic fields. 28-30

The band inversion can be avoided either by application of experimentally achievable strain or incomplete ordering making the CuPt-ordered InA0.5Sb0.5 a semiconductor with a large intrinsic linear in k bulk spin splitting in the conduction bands. Furthermore, we find large Landé g-factors for thin films of CuPt-ordered InA0.5Sb0.5, which is crucial for the realization of various Majorana scenarios.1, 2, 31, 32 In conclusion, the electronic properties of CuPt-ordered InA0.5Sb0.5 outperform all presently available Majorana platforms.
Magneto-optical effects in topological insulators
In this contribution we study the magneto-optical Faraday effect of topological insulator (TI) films in the presence of an external magnetic field. In the first part, we give a short review of the essential results [Refs. 10-12] in the low-frequency regime. In strong magnetic fields, the low-frequency Faraday effect for TI thin films is found to be quantized at integer multiples of the fine structure constant. In the second part, we present results from our study on the influence of cyclotron and cavity resonance effects in thick TI films. For thick films, we find that the same quantization of the Faraday rotation re-emerges when cavity resonance occurs. At higher frequencies, the interplay of both cyclotron resonance and cavity resonance effects leads to interesting features in the resulting Faraday rotation.
Topological classification with Z2Pack (Conference Presentation)
Dominik Gresch, Alexey A. Soluyanov, Gabriel Autés, et al.
We present a general technique for capturing various non-trivial topologies in the band structure of materials, which often arise from spin-orbit coupling. The technique is aimed at insulators and semimetals. Of insulators, Chern, Z2, and crystalline topological insulators can be identified. Of semimetals, the technique captures non-trivial topologies associated with the presence of Weyl and Dirac points in the spectrum. A public software package -- Z2Pack -- based on this technique will be presented. Z2Pack is an easy-to-use, well documented Python package that computes topological invariants and illustrates non-trivial features of Berry curvature. It works as a post-processing tool with all major first-principles codes, as well as with tight-binding models. As such, it can be used to investigate materials with strong spin-orbit coupling.
Milestones toward Majorana-based quantum computing (Conference Presentation)
Ryan V. Mishmash, David Aasen, Michael Hell, et al.
We introduce a scheme for preparation, manipulation, and readout of Majorana zero modes in semiconducting wires with mesoscopic superconducting islands. Our approach synthesizes recent advances in materials growth with tools commonly used in quantum-dot experiments, including gate-control of tunnel barriers and Coulomb effects, charge sensing, and charge pumping. We outline a sequence of milestones interpolating between zero-mode detection and quantum computing that includes (1) detection of fusion rules for non-Abelian anyons using either proximal charge sensors or pumped current; (2) validation of a prototype topological qubit; and (3) demonstration of non-Abelian statistics by braiding in a branched geometry. The first two milestones require only a single wire with two islands, and additionally enable sensitive measurements of the system’s excitation gap, quasiparticle poisoning rates, residual Majorana zero-mode splittings, and topological-qubit coherence times. These pre-braiding experiments can be adapted to other manipulation and readout schemes as well.
Graphene and Organic Materials
Magneto-strain driven quantum engine on a graphene flake
E. Muñoz, F. J. Peña
A novel proof of principle prototype for a quantum heat engine is proposed, based on the quasi-static tuning of an external magnetic field, in combination with controlled mechanical strain applied to a single graphene flake. The "working fluid" of this engine is composed by a statistical ensemble of Dirac quasiparticles in Landau levels. The cyclic operation of the engine, whose intermediate states are described through a density matrix, is discussed in detail, and its thermodynamic efficiency is calculated in the quasi-static limit.
Perpendicular magnetic anisotropy, unconventional magnetization texture and extraordinary gradual spin reorientation transition of cobalt films in contact with graphene (Conference Presentation)
Nicolas Rougemaille, Anh Duc Vu, Gong Chen, et al.
Owing to its peculiar electronic band structure, high carrier mobility and long spin diffusion length, graphene is a promising two-dimensional material for microelectronics and spintronics. Graphene also shows interesting magnetic properties when in contact with a ferromagnetic metal (FM). For instance, graphene carries a net magnetic moment when deposited on Fe/Ni(111), and a significant spin splitting can be induced in graphene due to proximity with a heavy element. While these results illustrate potential advantages of integrating graphene within a magnetic stack, the influence of graphene on the magnetic properties of a FM is still largely unexplored. In particular, non-magnetic overlayers generally affect the magnetic anisotropy energy (MAE) of thin layers, where interfaces play an important role. We can then wonder how an interface with graphene would influence the MAE of a thin FM film. Using spin-polarized low-energy electron microscopy, we study how a graphene overlayer affects the magnetic properties of atomically flat, nm-thick Co films grown on Ir(111). In this contribution, we report several astonishing magnetic properties of graphene-covered Co films: 1) Perpendicular magnetic anisotropy is favored over an unusually large thickness range, 2) Vectorial magnetic imaging reveals an extraordinarily gradual thickness-dependent spin reorientation transition (SRT), 3) During the SRT, cobalt films are characterized by an unconventional spin texture, 4) Spectroscopy measurements indicate that incident spin-polarized electrons do not suffer substantial spin-dependent collisions a few electron-Volts above the vacuum level. These properties strikingly differ from those of pristine cobalt films and could open new prospects in surface magnetism and spintronics.
Simple and advanced ferromagnet/molecule spinterfaces
M. Gruber, F. Ibrahim, F. Djedhloul, et al.
Spin-polarized charge transfer between a ferromagnet and a molecule can promote molecular ferromagnetism 1, 2 and hybridized interfacial states3, 4. Observations of high spin-polarization of Fermi level states at room temperature5 designate such interfaces as a very promising candidate toward achieving a highly spin-polarized, nanoscale current source at room temperature, when compared to other solutions such as half-metallic systems and solid-state tunnelling over the past decades. We will discuss three aspects of this research. 1) Does the ferromagnet/molecule interface, also called an organic spinterface, exhibit this high spin-polarization as a generic feature? Spin-polarized photoemission experiments reveal that a high spin-polarization of electronics states at the Fermi level also exist at the simple interface between ferromagnetic cobalt and amorphous carbon6. Furthermore, this effect is general to an array of ferromagnetic and molecular candidates7. 2) Integrating molecules with intrinsic properties (e.g. spin crossover molecules) into a spinterface toward enhanced functionality requires lowering the charge transfer onto the molecule8 while magnetizing it1,2. We propose to achieve this by utilizing interlayer exchange coupling within a more advanced organic spinterface architecture. We present results at room temperature across the fcc Co(001)/Cu/manganese phthalocyanine (MnPc) system9. 3) Finally, we discuss how the Co/MnPc spinterface’s ferromagnetism stabilizes antiferromagnetic ordering at room temperature onto subsequent molecules away from the spinterface, which in turn can exchange bias the Co layer at low temperature10. Consequences include tunnelling anisotropic magnetoresistance across a CoPc tunnel barrier11. This augurs new possibilities to transmit spin information across organic semiconductors using spin flip excitations12.
Electron transport and noise spectroscopy in organic magnetic tunnel junctions with PTCDA and Alq3 barriers
Isidoro Martinez, Juan Pedro Cascales, Jhen-Yong Hong, et al.
The possible influence of internal barrier dynamics on spin, charge transport and their fluctuations in organic spintronics remains poorly understood. Here we present investigation of the electron transport and low frequency noise at temperatures down to 0.3K in magnetic tunnel junctions with an organic PTCDA barriers with thickness up to 5 nm in the tunneling regime and with 200 nm thick Alq3 barrier in the hopping regime. We observed high tunneling magneto-resistance at low temperatures (15-40%) and spin dependent super-poissonian shot noise in organic magnetic tunnel junctions (OMTJs) with PTCDA. The Fano factor exceeds 1.5-2 values which could be caused by interfacial states controlled by spin dependent bunching in the tunneling events through the molecules.1 The bias dependence of the low frequency noise in OMTJs with PTCDA barriers which includes both 1/f and random telegraph noise activated at specific biases will also be discussed. On the other hand, the organic junctions with ferromagnetic electrodes and thick Alq3 barriers present sub-poissonian shot noise which depends on the temperature, indicative of variable range hopping.
Magnonics II
Torque, spin and energy currents in chiral magnets (Conference Presentation)
Alexey Kovalev
We predict that a temperature gradient can induce a magnon-mediated intrinsic torque and a transverse spin current in ferromagnets with non-trivial magnon Berry curvature. With the help of a microscopic linear response theory of nonequilibrium magnon-mediated torques and spin currents we identify the interband and intraband components that manifest in ferromagnets with Dzyaloshinskii–Moriya interactions and magnetic textures. In addition to the torque and spin current, we also identify the mechanical torque effect in accordance with the conservation of angular momentum. To illustrate and assess the importance of such effects, we apply our theory to the magnon-mediated spin Nernst and torque responses in a kagome lattice ferromagnet.
Determination of the Dzyaloshinskii Moriya interaction for various hetero-structure (Conference Presentation)
Chun-Yeol You
We investigated non-reciprocal spin wave (SW) dispersion relations by using Brillouin Light Scattering (BLS) in inversion symmetry breaking heterostructures. The non-reciprocal SW dispersion relation is a fingerprint of the existence of asymmetric exchange coupling, Dzyaloshinskii-Moriya Interaction (DMI). The quantification of DMI is important, because it is an essential ingredient of the domain wall motion and skyrmion based logic devices. We obtained DMI energy densities of various NM1/ferromagnetic(Co, CoFeB)/NM2 structures, where NM1,2 are non-magnetic heavy metals (Pt, Ir, Ta) or insulators (MgO, AlOx). We revealed that DMI is proportional to the inverse of ferromagnetic layer thickness, which strong evidence of interface nature of DMI in the heterostructure. In order to exclude the possible source of non-reciprocal SW dispersions, we carefully examined DMI with three independent measurement methods, field strength dependence, SW wave vector magnitude dependence, and SW propagating angle dependences, and found all three measurements gave the same results. It implies our measurement results are more reliable compared with other methods. We also found that sign and strength are sensitive function of materials, interface qualities, layer orders, and annealing conditions. Since the strong spin-orbit coupling (SOC) between ferromagnetic and the heavy metal layers is a source DMI, we also investigated SOC related quantities such as perpendicular magnetic anisotropy, magneto-optical Kerr effect, and spin pumpings. And we found that the relation between SOC and DMI is not the same with other SOC-related physical quantities.
Magnetic domain walls as reconfigurable spin-wave nano-channels (Conference Presentation)
In the research field of magnonics, it is envisaged that spin waves will be used as information carriers, promoting operation based on their wave properties. However, the field still faces major challenges. To become fully competitive, novel schemes for energy-efficient control of spin-wave propagation in two dimensions have to be realized on much smaller length scales than used before. In this presentation, these challenges are addressed with the experimental realization of a novel approach to guide spin waves in reconfigurable, nano-sized magnonic waveguides. For this purpose, two inherent characteristics of magnetism are used: the non-volatility of magnetic remanence states and the nanometre dimensions of domain walls formed within these magnetic configurations. The experimental observation and micromagnetic simulations of spin-wave propagation inside nano-sized domain walls and a first step towards a reconfigurable domain-wall-based magnonic nanocircuitry will be presented.
Off-equilibrium thermodynamics of classical spin systems (Conference Presentation)
Simone Borlenghi Garoia
I will present the results of a systematic micromagnetic study of the spin dynamics in magnetic multilayers in the presence of a temperature gradient. In particular, I will describe the flow of magnetisation and energy currents between the layers by means of a general oscillator model, the discrete nonlinear Schrödinger equation (DNLS). Two new effects are predicted: 1) the rectification of spin-wave (SW) and energy currents: controlling the synchronisation between the spin-oscillators allows to propagate those currents only in one direction. 2) The spin-josepson effect: SW currents is proportional to the sine for the phase differences between spins, in strong analogy will the well known effect of superconductivity. The DNLS model is very general and those properties are expected to emerge in a large class of systems. In this respect, i will elucidate the strong connection with lattice gauge theories, suggesting possible experiments in system with antisymmetric exchange coupling. References: S. Borlenghi et. al., Phys. Rev. Lett. 112, 040703 (2014); Phys. Rev. E 91, 040102(R) (2015)
Superconductivity I
Accessing and probing of the photo-induced hidden state in 1T-TaS2 with time- and angle-resolved photoemission spectroscopy
Isabella Avigo, Igor Vaskivskyi, Manuel Ligges, et al.
A previous time-resolved optical study reported on a metastable hidden electronic state in 1T-TaS2, which is only accessible upon photoexcitation and created under non-equilibrium conditions [1]. The properties of such a state are distinct from those of any other state in the equilibrium phase diagram and it is possible to revert to the thermodynamic initial state either by illuminating with picosecond laser pulses or by applying other thermal erase procedures. In this work we show photoinduced switching to a metastable hidden state on the same material, and probe it by means of both static and time-resolved photoemission spectroscopy, thus having direct access to the electronic structure of the system. From our experimental findings and comparison with other studies, we conclude that we obtain partial switching, leading to a hidden state with persisting insulating nature but significant modifications in the electronic structure and CDW ordering.
Magnetoanisotropic Andreev reflection in ferromagnet/superconductor junctions (Conference Presentation)
Petra Hoegl, Alex Matos-Abiague, Igor Zutic, et al.
Andreev reflection spectroscopy of ferromagnet/superconductor (F/S) junctions is a sensitive probe of the junction interface as well as the spin polarization. We theoretically investigate spin-polarized transport in F/S junctions in the presence of Rashba and Dresselhaus interfacial spin-orbit fields and show that Andreev reflection can be controlled by changing the magnetization orientation. This suggests a similar control of the superconducting proximity effect and Majorana states. We predict a giant in- and out-of-plane magnetoanisotropy of the junction conductance. If the ferromagnet is highly spin polarized - in the half-metal limit - the magnetoanisotropic Andreev reflection depends universally on the spin-orbit fields only. Our results show that Andreev reflection spectroscopy can be used for sensitive probing of interfacial spin-orbit fields in F/S junction. This work has been supported by DFG SFB 689, the International Doctorate Program Topological Insulators of the Elite Network of Bavaria, DOE-BES Grant DE-SC0004890, and ONR N000141310754. P. H\"{o}gl, A. Matos-Abiague, Igor \v{Z}uti\'c, J. Fabian, Phys. Rev. Lett. 115, 116601 (2015)
Epitaxial superconductor-semiconductor two-dimensional systems: platforms for quantum circuits (Conference Presentation)
Javad Shabani
Theory suggests that the interface between a one-dimensional semiconductor (Sm) with strong spin-orbit coupling and a superconductor (S) hosts Majorana modes with nontrivial topological properties. A key challenge in fabrication of such hybrid devices is forming highly transparent contacts between the active electrons in the semiconductor and the superconducting metal. Recently, it has been shown that a near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on InAs nanowires. In this work, we present the first two-dimensional epitaxial superconductor-semiconductor material system that can serve as a platform for topological superconductivity. We show that our material system, Al-InAs, satisfies all the requirements necessary to reach into the topological superconducting regime by individual characterization of the semiconductor two dimensional electron system, superconductivity of Al and performance of S-Sm-S junctions. This exciting development might lead to a number of useful applications ranging from spintronics to quantum computing.
Edge current in a small chiral superconductor
We discuss a theoretical description of the edge current in a chiral superconductor. On the basis of the quasiclassical Green function formalism, we derive a useful expression of the chiral edge current which enable us to understand how Cooper pairs contribute to the electric current. We will show that the chiral edge current is carried by the combinations of two Cooper pairs belonging to different pairing symmetries. One Cooper pair belongs to the usual even-frequency pairing symmetry class. However, the other belongs to the odd-frequency symmetry class.
Probing the spinor nature of electronic states in nanosize non-collinear magnets (Conference Presentation)
Jeison A. Fischer, Dirk Sander, Stuart Parkin
Non-collinear magnets (NCM) exhibit a spatial variation of the magnetization direction, where helical and skyrmionic spin orders in materials have lately attracted considerable interest. This interest is spurred by both, exploring the physical origin of nanoscale NCM and applications in spintronics. Our study advances the understanding of nanoscale NCM by revealing the effect of nanoscale lateral confinement on the physical properties of NCM. We combine spin-polarized scanning tunneling microscopy/spectroscopy (sp-STM/S) and first-principles calculations to study prototypical helical NCM of some nm extension in proximity to both ferromagnetic Co and vacuum regions. We report a non-uniform distortion of the spin helix in an Fe bilayer on Cu(111)[1], where the spin orientation deviates from that of an ideal helical structure. The proximity to either Co or vacuum leads to distortions of the spin orientation within nm range of the respective interface. The distortions give rise to a specific energy dependent phenomenon of non-collinearity between the local magnetization in the sample and the electronic magnetization probed above its surface. This phenomenon is a direct consequence of the spinor nature of the electronic states in NCM. The symmetry breaking due to lateral confinement makes the spinor nature of electronic states observable in sp-STM/S experiments. [1] Phark, S. H.; Fischer, J. A.; Corbetta, M.; Sander, D.; Nakamura, K. and Kirschner, J. Reduced-dimensionality-induced helimagnetism in iron nanoislands Nat. Commun. 5 (2014) 5183.
Growth and oxidation of vanadium ultra-thin buffer layers on Fe(001)
A. Picone, D. Giannotti, M. Finazzi, et al.
The growth and oxidation of vanadium ultra-thin films deposited on Fe(001) have been investigated by combining scanning tunneling microscopy and Auger electron spectroscopy. In the early stages of growth, vanadium develops a structure pseudomorphic to the Fe(001) substrate, nucleating one-layer-thick islands. At higher coverages, the growth proceeds nearly layer-by-layer, up to a thickness of about 5 atomic layers. Upon oxygen exposure, the vanadium film gets oxidized, while no signatures of the formation of iron oxides are detected in Auger spectra. As revealed by scanning tunneling microscopy images, the oxidation increases the surface roughness, suggesting the formation of an amorphous vanadium oxide layer.
Superconductivity II
Andreev bound states in topological superconductors (Conference Presentation)
Yukio Tanaka, Bo Lu, Keiji Yada, et al.
Andreev bound states in topological superconductors Yukio Tanaka1, Lu Bo1,, K. Yada1, A. Yamakage1, M. Sato2 1Department of Applied Physics, Nagoya University 2Yukawa Institute, Kyoto University e-mail: It is known that Andreev bound state is an important ingredient to identify unconventional superconductors [1]. Up to now, there have been several types of Andreev bound states stemming from their topological origins [2-3]. It can be classified into i)dispersionless flat band type realized in cuprate, ii)linear dispersion type realized in chiral superconductor like Sr2RuO4, iii)helical dispersion type realized in non-centrosymmetric superconductor and iv)cone type in the surface state on B-phase of superfluid 3He [3]. It has been noted that certain surfaces of Weyl semimetals have bound states forming open Fermi arcs, which are never seen in typical metallic states. We show that the Fermi arcs enable them to support an even more exotic surface state with crossed flat bands in the superconducting state. We clarify the topological origin of the crossed dispersionless flat bands and the relevant symmetry that stabilizes the cross point. Our symmetry analysis are applicable to known candidate materials of time-reversal breaking Weyl semimetals[4]. [1]S. Kashiwaya and Y. Tanaka, Rep. Prog. Phys. 63 1641 (2000). [2]Y. Tanaka, M. Sato, and N. Nagaosa, J. Phys. Soc. Jpn. 81 011013 (2012). [3] M. Sato, et al., Phys. Rev. Lett. 103 (2009) 020401. [4] B. Lu, K. Yada, M. Sato, and Y. Tanaka, Phys. Rev. Lett. 114 09
Point contact Andreev reflection and the measurement of spin polarization: high fields and novel materials (Conference Presentation)
Plamen Stamenov, Kiril Borisov
Point Contact Andreev Reflection (PCAR) is one of the few available methods for the determination of the Fermi level spin polarisation in metals and degenerate semiconductors. It has traditionally been applied at fixed (liquid He) temperatures, using pure niobium as the superconductor, and at essentially zero applied magnetic fields, all of which limit the amount of information that it can provide – i.e. do not allow for the extraction of the sign of the spin polarisation and make the assignment of the transport regime to ballistic or diffusive almost impossible. Here a series of experiments is described, aimed at the expansion of this parameter space to higher magnetic fields and to higher temperatures. These require redesigned experimental setups and the use of higher performance superconductors. Demonstrations are described of the determination of the sign of the spin polarisation, at fields of more than 5 Tesla using a low-Z superconductor, as well as operations beyond 9.2 K. Doubts about the practical reliability of the PCAR technique are dispersed using systematic series of samples – the heavy rare-earths and comparisons with alternatives, such as spin-polarised field emission, photo-emission and Tedrow-Meservey tunnelling. The specific material examples presented include 3d-metals, order-disorder transition alloys and zero-moment half-metals – Fe, FeAl and MnRuGa, alternative low-Z and high-Z superconductors – MgB2 and NbTi, and magnetic topological insulators, such as Cr- and V-doped (Bi1-xSbx)2Te3.
Wireless majorana fermions: from magnetic tunability to braiding (Conference Presentation)
Geoffrey L. Fatin, Alex Matos-Abiague, Benedikt Scharf, et al.
In condensed-matter systems Majorana bound states (MBSs) are emergent quasiparticles with non-Abelian statistics and particle-antiparticle symmetry. While realizing the non-Abelian braiding statistics under exchange would provide both an ultimate proof for MBS existence and the key element for fault-tolerant topological quantum computing, even theoretical schemes imply a significant complexity to implement such braiding. Frequently examined 1D superconductor/semiconductor wires provide a prototypical example of how to produce MBSs, however braiding statistics are ill-defined in 1D and complex wire networks must be used. By placing an array of magnetic tunnel junctions (MTJs) above a 2D electron gas formed in a semiconductor quantum well grown on the surface of an s-wave superconductor, we have predicted the existence of highly tunable zero-energy MBSs and have proposed a novel scheme by which MBSs could be exchanged [1]. This scheme may then be used to demonstrate the states’ non-Abelian statistics through braiding. The underlying magnetic textures produced by MTJ array provides a pseudo-helical texture which allows for highly-controllable topological phase transitions. By defining a local condition for topological nontriviality which takes into account the local rotation of magnetic texture, effective wire geometries support MBS formation and permit their controlled movement in 2D by altering the shape and orientation of such wires. This scheme then overcomes the requirement for a network of physical wires in order to exchange MBSs, allowing easier manipulation of such states. [1] G. L. Fatin, A. Matos-Abiague, B. Scharf, and I. Zutic, arXiv:1510.08182, preprint.
Poster Session
Dependence of quantum kinetic effects in the spin dynamics of diluted magnetic semiconductors on the excitation conditions
M. Cygorek, F. Ungar, P. I. Tamborenea, et al.
Non-Markovian quantum kinetic features, that cannot be captured by rate equations, have been predicted theoretically in the spin dynamics in diluted magnetic semiconductors excited with a circularly polarized laser. In order to identify situations which are most promising for detecting the genuine quantum kinetic effects in future experiments, we study numerically the strength of these effects for a number of different excitation conditions. In particular, we show that laser pulse durations of the order of the spin-transfer rate or longer are well suited for studying the non-Markovian effects. Furthermore, in the presence of an external magnetic field, the quantum kinetic theory predicts a significantly different stationary value for the carrier spin polarization than Markovian rate equations, which can be attributed to the build-up of strong carrier-impurity correlations.
Geometric phase gradient and spin Hall effect of light
The spin Hall effect (SHE) of light originates from the spin-orbit interaction, which can be explained in terms of two geometric phases: the Rytov-Vladimirskii-Berry phase and the Pancharatnam-Berry phase. Here we present a unified theoretical description of the SHE based on the two types of geometric phase gradients, and observe experimentally the SHE in structured dielectric metasurfaces induced by the PB phase. Unlike the weak real-space spin-Hall shift induced by the SRB phase occurring at interfacial reflection/refraction, the observed SHE occurs in momentum space is large enough to be measured directly.
Frequency tuning of polarization oscillations in spin-lasers
Polarization oscillations can be observed as resonant oscillations of the coupled spin-photon system in spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs). They are a reasonable measure of the polarization dynamics and provide insights to the polarization modulation bandwidth of these devices. These oscillations can be generated using pulsed spin injection and have proven to be much faster than the relaxation oscillations for the intensity dynamics under the same conditions. The oscillation frequency mainly depends on the cavity birefringence, which can be tuned by applying mechanical strain to the VCSEL structure. This provides a direct tool to considerably increase the polarization oscillation frequency and thus the modulation bandwidth. Following this approach we were able to experimentally tune the frequency over a range of 34 GHz. We demonstrated polarization oscillations in spin-VCSELs with frequencies up to 44 GHz recently, only limited by the used mechanical strain setup.1 By measuring the polarization oscillation frequency and the birefringence-governed mode splitting as a function of the applied strain simultaneously, we investigated the correlation between birefringence and polarization oscillations. Here we use an optimized and simplified mount, which potentially allows for larger strain values. The experimental findings are compared to numerical calculations based on the spin-flip model. Taking our previously reported record value of more than 250 GHz for the birefringence splitting in VCSEL cavities into account,2 this technique may pave the road toward high-speed polarization modulation in VCSELs for bit rates above 100 Gb/s.
Anisotropic longitudinal optical conductivity of a spin-orbit coupled system: effect of cubic Dresselhaus coupling
We investigate the effect of the oft-neglected cubic terms of the Dresselhaus spin-orbit coupling on the longitudinal current response of a two-dimensional electron gas with both Rashba and linear Dresselhaus interactions. Changes caused by these nonlinear-in-momentum terms on the absorption spectrum becomes more notable under SU(2) symmetry conditions, when the Rashba and linear Dresselhaus coupling strengths are tuned to be equal. The longitudinal optical response no longer vanishes then and shows a strong dependence on the direction of the externally applied electric field, giving a signature of the relative size of the several spin-orbits contributions. This anisotropic response arises from the non isotropic splitting of the spin states induced by the interplay of Rashba and Dresselhaus couplings. However, the presence of the cubic terms introduces characteristic spectral features and can modify the overall shape of the spectra for some values of the relative sizes of the spin-orbit parameters. In addition to the control through the driven frequency or electrical gating, this directional aspect of the current response suggests new ways of manipulation and supports the use of interband optics as a sensitive probe of spin-orbit mechanisms in semiconductor spintronics.