This PDF file contains the front matter associated with SPIE Proceedings Volume 11805, including the Title Page, Copyright information, and Table of Contents.

In the previous works the spin Hall effect (SHE) of light was usually detected by the quantum weak measurement technique in which the complete information of the field was lost. In this work, we investigate the spatial evolution of the whole field in the SHE of light on reflection. First, we establish a model to describe the polarization state of reflected light and disclose its relationship with the SHE. Then, it is found that the reflected polarization generally becomes a vector field. The SHE due to the polarization gradient is manifested as a spin-dependent splitting. Further, it is found that both the incident angle and the incident polarization can affect the polarization of reflected light noticeably. Finally, the evolution of the energy flow is analyzed to disclose the underlying physical mechanism.

A promising way for the conversion from charge current to spin current is to exploit the spin Hall effect (SHE). Aside from the usage of elemental nonmagnetic materials, element doping or alloying is a promising way to develop a spin Hall material. In order to reveal the optimum composition for achieving the large SHE in the Cu-Ir binary alloys, we exploited the high-throughput combinatorial technique based on spin Peltier imaging. They discovered that the non-equilibrium Cu-Ir alloys beyond the solubility limit are candidates to achieve the large SHE, in which a large spin-Hall angle of ~ 6% was obtained for Cu76Ir24. In addition to the aspect from SHE, we also found that Cu-Ir is a nonmagnetic spacer layer material allowing us to realize moderately strong antiferromagnetic coupling (AFC) between two ferromagnetic layers separately by Cu-Ir. The simultaneous achievement of AFC and SHE makes the Cu-Ir an useful material for antiferromagnetic spintronics.

Spin-orbit torques (SOTs) allow controlling the magnetization of diverse classes of magnetic multilayers and devices. The mechanism utilizes spin-orbit interactions such as spin Hall effect in heavy metals and/or Rashba effect at ferromagnetic/heavy-metal interface with broken inversion symmetry. The SOTs have damping-like (HDL) and field-like (HFL) effective field components. In this talk, we will present the mechanism of spin-transport in ultrathin magnetic multilayer whose thicknesses span across the characteristic spin-dephasing length, and how it results in HDL and HFL nearby the crossing point of this specific length. To this aim, we have quantified SOTs in a series of samples Pt 8|Co x|Al 1.4|Pt 3 with x = 0.55, 0.7, 0.9, 1.2, 1.4 nm. Our experiments demonstrate the presence of very large field-like torque arising from Co|Al interface for Co thickness smaller than spin-dephasing length. The results suggest the contribution of additional mechanisms of spin-current generation.

The link between magnetization and Spin Hall Effect (SHE) has remained mostly unclear for now. In a first part of this contribution, we study oh the presence of the magnetization affect the SHE, by performing in the weak ferromagnet NiCu Spin Pumping-FMR measurements across the ferromagnetic / paramagnetic critical temperature. We show that the high spin Hall effects which can be obtained in 3d ferromagnets seems to be independent of the magnetic phase. In a second part, we show that the spin absorption process in a ferromagnetic material depends on the spin orientation relative to the magnetization. Using a ferromagnet to absorb the pure spin current created within a lateral spin valve, we evidence and quantify a sizable orientation dependence of the spin absorption in Co, CoFe, and NiFe. These experiments allow us to determine the spin-mixing conductance, an elusive but fundamental parameter of the spin-dependent transport.

We develop an approach for toggling magnon processes at nanoscale. We demonstrate an experimental proof-of-concept in magnetic tunnel junction nanodevices, consisting of a free layer and a synthetic antiferromagnet. By triggering the spin-flop transition in the synthetic antiferromagnet and utilizing its nonuniform dipole field, we controllably modify magnon interaction in the free layer. We achieve its tunability by at least one order of magnitude and realize two distinct dissipative states. The results open up an avenue for controlling magnon processes by external stimuli at nanoscale and show prospects for a variety of spin-torque applications, magnetic neural networks, and hybrid quantum information technologies. An immediate consequence of modified magnon interaction is nanomagnet's response to spin-torques. In particular, we show that degenerate resonant three-magnon process inverts an antidamping spin-torque into a torque that enhances dissipation. Supported by NSF-ECCS-1810541.

We theoretically investigate the crystalline anisotropy of topological superconductivity (TS) in phase controlled planar Josephson junctions (JJs) subjected to Rashba and Dresselhaus spin-orbit couplings and in-plane magnetic fields. We show how the interplay between the magnetic field direction and the orientation of the junction with respect to its crystallographic axes can affect the TS. Our results explain previous experiments demonstrating the high sensibility of TS to the in-plane magnetic field direction. The anisotropy can be used to electrically tune between BDI and D symmetry classes in a controlled fashion and thereby optimize the stability and localization of Majorana bound states in planar JJs. Our findings can be used as a guide for achieving the most favorable conditions when engineering TS in planar JJs and can be particularly relevant for setups containing non-collinear junctions, which have been proposed for fusion and braiding operations on multiple Majorana pairs.

In recent years, various novel phenomena such as π-state (π-junction) and long-ranged supercurrents emerged in ferromagnetic Josephson junctions (superconductor/ferromagnet/superconductor junctions) have been studied so actively. An attractive device application of the ferromagnetic π-junction is a flux-bias-free superconducting flux quantum bit (qubit). By inserting the π-junction in the superconducting loop of the flux qubit, an external flux bias corresponding to half flux quantum, which is required for the operation of the conventional flux qubits, becomes unnecessary. This flux-bias-free feature is a great advantage in the realization of large-scale quantum circuits with many qubits. In the talk, we will present recent progress on the development of NbN-based ferromagnetic Josephson junctions which is suitable for the quantum circuits. We also show the experimental results on the several types of the quantum circuits with the NbN-based ferromagnetic π-junction.

With non-Abelian statistics and nonlocal degrees of freedom, Majorana bound states (MBS) are suitable for implementing fault-tolerant topological quantum computing. While the main efforts to realize MBS have focused on one-dimensional systems, it requires delicate parameter tuning and its geometric constraints pose significant challenges for the demonstration of non-Abelian statistics. Building on recent experimental advances in planar Josephson junctions (JJs), we propose how to overcome this obstacle in topological JJs and demonstrate non-Abelian statistics with phase or mini-gate control, detected by charge sensing using quantum point contacts. Our proposals, supported by the experiments, would constitute an important milestone towards topological quantum computing.

In this work, we focus on the emergence of Majorana zero modes in heterostructures composed of superconducting and ferromagnetic materials. We numerically determine self-consistent solutions to the Bogoliubov-de Gennes equations suitable for our system. For the first part, we consider a conventional superconductor sandwiched by two conical ferromagnets. We vary the direction of the conical axes and the strength of Zeeman field for both ferromagnetic regions to study the stability of the topological properties of our system. For the second part, we turn our attention to a trilayer consisting of two superconductors and a conical ferromagnet in the geometry of a conventional Josephson junction. By choosing appropriate parameters, the Josephson junction can also host Majorana zero modes and exhibit topological signatures.

We theoretically investigate the emergence of topological superconductivity in dc-SQUIDs in the presence of Rashba spin-orbit coupling and an in-plane magnetic field. The transmission of the individual planar Josephson junctions (JJs) can be controlled by top gates, switching from a single JJ to a dc-SQUID behavior. The transition to the topological phase in the single JJ configuration is sensible to the direction of the in-plane magnetic field and we show that it is accompanied by minima in the critical current, serving as experimental signatures to identify the phase transition. Furthermore, the topological phase transitions in each of the JJs can be individually tuned by top gates. We show that there are distinctive signatures in the critical current and phase shift of the dc-SQUID for cases when none, either or both junctions are in the topological regime. We also investigate the effects of electrostatic disorder on the topological superconducting state of single JJs.

The remarkable experimental discovery of femtosecond scale laser pulse-driven demagnetization process in ferromagnetic nickel in mid ’90s spurred a flurry of experimental and theoretical research activities in ultrafast magnetization dynamics. Standard theoretical description of magnetization dynamics based on Landau-Lifshitz-Gilbert equation is justified only for slow enough spin phenomena while those occurring at very short time scales are much less understood. The past two decades have observed a remarkable theoretical development predicting the emergence of dynamical inertia in magnetization dynamics at very short time scale, focusing mostly on the dynamics of a single spin, leading to the prediction of a new type of spin motion called nutation. We advance this theoretical progress by considering inertial effect on the dynamics of a system of interacting spins. We demonstrate the occurrence of a new type of collective mode referred to as nutation wave, shown to have massive relativistic dispersion relation with characteristic speed and frequency well exceeding those of the more familiar spin wave. These excellent properties make nutation wave a prospective candidate to be a platform for optically-driven ultrafast spintronic devices.

Invited talk: an abridged version of two very recent Physical Review B papers consisting of Titov et al. In the first one it is shown how the analogy of the inertial magnetization Langevin equation with that pertaining to a current carrying loop can be used to write the corresponding Fokker-Planck equation in phase space. In paper II it is shown how the analogy with a symmetric top allows one to calculate inertial magnetization observables in the zero damping limit.

This presentation recording was recorded for the SPIE Optics + Photonics 2021 symposium

Ultrashort optical pulses applied to ferromagnets excite spin polarized hot electrons. Such an ultrashort-pulse excitation is followed by spin-flip scattering due to spin-orbit coupling, "intrinsic" magnetization dynamics and spin-polarized transport. I will present some of our work concerning the interplay of spin-flip scattering due to spin-orbit coupling and exchange scattering in a model system containing itinerant electrons which are exchange-coupled to localized electronic bands. I will also present results on spin-dependent transport of optically excited hot electrons in ferromagnet-metal heterostructures which have gained interest as THz emitters in recent years.

This presentation recording was recorded for the SPIE Optics + Photonics 2021 symposium

Terahertz (THz) time-domain spectroscopy is an emerging technique to probe and manipulate spins on ultrafast time scales. In this talk, I will highlight recent results of studying spintronic phenomena at terahertz rates, which holds great promise for next-generation THz photonic applications such as broadband THz generation and detection.

THz emission spectroscopy reveals to be a very powerful experimental method to investigate the properties of Rashba or topological insulator surface states. The THz emission can be also used in heavy metallic or in more general Rashba systems. We prove here the ability of the present method. In 3d/5d transient metal bilayers and beyond heavy metal structures, Rashba states and Topological insulators are expected candidates for spintronic-terahertz domains due to their high spin to charge conversion properties. In this scheme, we are interested in the samples based on 2D electron gas, topological insulators and Heusler alloys with strong spin-orbit coupling.

Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale. However, the use of ultra-fast optics in integrated circuits and memories would require a major paradigm shift. An ultrafast electrical control of the magnetization is far preferable for integrated systems. In a recent work, we demonstrate reliable and deterministic control of the out-of-plane magnetization of a 1 nm-thick Co layer with single 6 ps-wide electrical pulses that induce spin orbit torques on the magnetization. These experiments show that spintronic phenomena can be exploited on picosecond time-scales for full magnetic control and should launch a new regime of ultrafast spin torque studies and applications.

We present a scheme to achieve coherent polarization rotation without multipolar or rotating external magnetic bias nor complex cascaded emitters, by exploiting artificially engineered strong uniaxial anisotropy in intermetallic heterostructures of rare-earth and transition metals. By replacing the FM layer of the spintronic emitter with a carefully designed FeCo/TbCo2/FeCo heterostructure, we demonstrated Stoner-Wolfarth-like coherent rotation of the THz polarization only by a unipolar variation of the strength of the hard axis field. In a second step we demonstrated the magnetoelectric control of the polarization direction. These results improve greatly the feasibility of fast polarization switchable integrated THz sources impacting practical applications such as ultrabroadband THz spectroscopic ellipsometry without rotating elements, or polarization modulated high speed wireless data communications, but also fundamental physical studies into ultrafast terahertz optospintronics.

This presentation was recorded for the SPIE Optics and Photonics 2021 conference.

While known for a long time, antiferromagnetically ordered systems have previously been considered, as expressed by Louis Néel in his Nobel Prize Lecture, to be “interesting but useless”. However, since antiferromagnets potentially promises faster operation, enhanced stability with respect to interfering magnetic fields and higher integration due to the absence of dipolar coupling, they could potentially become a game changer for new spintronic devices. The zero net moment makes manipulation using conventional magnetic fields challenging. However recently, these materials have received renewed attention due to possible manipulation based on new approaches such as photons or spin-orbit torques. In this talk, we will present an overview of the key features of antiferromagnets to potentially functionalize their unique properties. This includes writing, reading and transporting information using antiferromagnets. This talk is supported as an IEEE Magnetics Society Distinguished Lecturer.

Ruthenium oxide (RuO2) has several intriguing properties such as electrically-conduction like metal, topological electronic band structure called Dirac nodal line, and room temperature collinear antiferromagnet (AFM) [T. Berlijn et al, Phys. Rev. Lett. 118, 077201 (2017)]. We have discovered a novel spin-orbit torque (SOT) generation in epitaxially grown RuO2 thin films originated from recently predicted magnetic spin Hall effect (MSHE) [R. Gonzalez-Hernandez et al, arXiv:2002.07073(2020)]. The detected both damping-like and field-like torques clearly follow the Néel vector directions against the applied current directions in the epitaxial RuO2(101) and RuO2(100) films which have different Néel vectors on the substrate plane. We further discuss the mechanism of the SOT and the related phenomena in the AFM RuO2 films on this conference.

We unambiguously identify the origin of the current-induced magnetic switching of insulating antiferromagnet/heavy metal bilayers. Previously, different reorientations of the Néel order for the same current direction were reported for different device geometries and different switching mechanisms were proposed. Here, we combine concurrent electrical readout and optical imaging of the switching of antiferromagnetic domains with simulations of the current-induced temperature and strain gradients. By comparing the switching in specially engineered NiO/Pt device and pulsing geometries, we can rule out spin-orbit torque based mechanisms and identify a thermomagnetoelastic mechanism to dominate the switching of antiferromagnetic domains, reconciling previous reports.

Antiferromagnetic thin films attract significant interest for future low-power spintronic devices [1]. Multiferroics, such as bismuth ferrite BiFeO3, in which antiferromagnetism and ferroelectricity coexist at room temperature, appears as a unique platform for spintronic [2] and magnonic devices [3]. The nanoscale structure of its ferroelectric domains has been widely investigated with piezoresponse force microscopy (PFM), revealing unique domain structures and domain wall functionalities [4]. However, the nanoscale magnetic textures present in BiFeO3 and their potential for spin-based technology remain concealed. In this report, we present two different antiferromagnetic spin textures in multiferroic BiFeO3 thin films with different epitaxial strains, using a commercial non-invasive scanning Nitrogen-Vacancy (NV) magnetometer based on a single NV defect in diamond, with a calibrated NV flying height of 60 nm and a proven DC field sensitivity of 1 T/Hz. Two BiFeO3 samples were grown on DyScO3 (110) and SmScO3 (110) substrates (later mentioned as BFO/DSO and BFO/SSO, respectively) using pulsed laser deposition. The striped ferroelectric domains in both samples are first observed by the in-plane PFM. The scanning NV magnetometry (SNVM) confirms the existence of the spin cycloid texture, with zig-zag wiggling angles of 90 and 127, and propagation wavelength of DSO=64 nm andSSO=103 nm, respectively. At the local scale, the combination of PFM and SNVM allows to identify the relative orientation of the ferroelectric polarization and cycloid propagation directions on both sides of a domain wall. For the BFO/DSO sample, the 90-degree in-plane rotation of the ferroelectric polarization imprints the 90-degree in-plane rotation of the cycloidal propagation direction along k1=[-1 1 0], corresponding to the type-I cycloid. On the contrary, in the BFO/SSO sample, the propagation vectors are found to be along k1'=[-2 1 1] and k2'= [1 -2 1] directions in the neighboring domains separated by the 71 domain wall. It is worth to mentioned that in the previous report [5], BFO/SSO, prepared in another growth chamber, showed G-type antiferromagnetic textures, compared to the observed type-II cycloid here. Our results here shed the light on future potential for reconfigurable nanoscale spin textures on multiferroic systems by strain engineering.

This presentation was recorded for the SPIE Optics + Photonics 2021 symposium.

Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) have proven to be a highly promising device technology for high-speed optical communication systems. In spin-lasers, the polarization state of the laser emission can be controlled by the carrier spin state exploiting the transfer of angular momentum between carriers and photons. The resonance frequency of the polarization dynamics can be increased by inducing birefringence into the resonator. Here we discuss the role of the photon lifetime and show results on the influence of this parameter on the static polarization behavior.

Strain-induced birefringence in GaAs-based oxide-confined VCSELs (Vertical-Cavity Surface-Emitting Laser) can split the optical modes into orthogonally polarized components. A polarization switching at very high frequencies can occur between these components, which is of great interest for optical communication systems of the future. In this study, we focus our investigation on the frequency characteristics of the polarization switching between the optical modes, which is caused by polarization self-modulation (PSM) in fiber-coupled systems. Moreover, we analyze the PSM that is originating in different optical modes of the VCSEL and compare multi-mode and single-mode VCSELs.

To date, the emergence of topological vector fields has almost exclusively been associated with a global inversion symmetry breaking that causes a vector spin exchange, known as Dzyaloshinskii-Moriya interaction. Engineering a locally varying vector spin exchange has been proposed as an alternative to stabilize anisotropic topological states. I will present experimental evidence of 3D chiral spin textures stabilized in amorphous iron germanium thick films with local inversion symmetry breaking and DMI [1]. Lorentz microscopy with exit wave reconstruction revealed both isotropic Bloch skyrmions and anisotropic solitons, which are accompanied by a reduced orbital-to-spin moment ratio, underling the importance of disordered electron orbitals and random DMI. Persistent switching of anisotropic skyrmions corroborate variations in magnetic anisotropy and exchange, and confirm a degenerate spin chirality and particle-like properties.

We consider a new mechanism for the spin current swapping effect. The effect manifests in the appearance of the transverse spin current qyx of the spin projection along x in the direction of y, in response to the longitudinal spin current of spin projection along y flowing in x direction with swapped indices qxy, hence the name spin swapping. We show that in the presence of a chiral spin texture primary spin current produces the transverse spin current. This spin-related transport phenomenon is similar to that known for electron scattering on a charged impurity with the account of correlation between the spin rotation and the scattering angle due to spin-orbit interaction. The discussed spin swapping mechanism originates from the electron spin correlations in real space due to an exchange interaction with chiral spin textures such as magnetic skyrmions. The spin swapping effect exists already in the first Born approximation.

We elaborate that Rashba spin-orbit coupling causes an out-of-plane polarized helical edge spin current at the boundaries of 2D metals. In the presence of a magnetization pointing perpendicular to the edge, an edge charge current is also produced, which can be either chiral or nonchiral depending on whether the magnetization lies in-plane or out-of-plane. The spin polarization near the edge develops a transverse component orthogonal to the magnetization, which tends to cause a noncollinear magnetic order between the two edges. If the magnetization only occupies a region near one edge, or in an irregular shaped quantum dot, this transverse component renders a gate voltage-induced magnetoelectric torque without the need of a bias voltage. We also argue that these phenomena are generic effects of a variety of spin-orbit couplings irrespective of the detail of the band structure, as also demonstrated for the Dresselhaus spin-orbit coupling and graphene nanoribbons.

We investigate the applicability of spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) for optical transmitters in analog radio-over-fiber systems by using spin-flip rate equations. In addition to the current modulation for generating orthogonal frequency division multiplexing (OFDM) signals, we use the spin polarization modulation to excite a high-frequency polarization oscillation corresponding to a millimeter-wave carrier frequency. The polarization oscillation is converted to intensity modulation by using a polarizer, and the millimeter-wave carrier and the OFDM signals can be combined. Our results indicate that OFDM signals with millimeter-wave carrier can be generated by using the spin- VCSEL, and bandwidth of the current modulation in the spin-VCSEL limits data rate of the OFDM signals.

Spin-controlled lasers are highly interesting photonic devices and have been shown to provide ultra-fast polarization dynamics in excess of 200 GHz. Another class of modern semiconductor lasers are high-beta emitters which benefit from enhanced light-matter interaction due to strong mode confinement in low-mode-volume microcavities. We combine the advantages of both laser types to demonstrate spin-lasing in high-beta microlasers for the first time. For this purpose, we realize bimodal high-beta quantum dot micropillar lasers for which the mode splitting and the polarization-oszillation frequency can be engineered via the pillar cross-section. The microlasers show very pronounced spin-lasing effects with polarization oscillation frequencies up to 16 GHz.

We consider a transverse electrical and spin current response to a longitudinal electric field in a metallic or semiconductor system characterized by various types of chirality. Using the similar theoretical approach we study a skew scattering on magnetic skyrmions leading to topological Hall effect, tunneling anomalous Hall effect (TAHE) of electrons and holes across an interface between magnetic semiconductors and electron scattering on a magnetic center in a semiconductor. We demonstrate how the chiral symmetry of the system manifests itself in the Hall response and its dependence on the electron spin polarization.

Parafermions or Fibonacci anyons leading to universal quantum computing, require strongly interacting systems. A leading contender is the fractional quantum Hall effect, where helical channels can arise from counter- propagating chiral modes. These modes have been considered weakly interacting. However, experiments on transport in helical channels in the fractional quantum Hall effect at a 2/3 filling shows current passing through helical channels on the boundary between polarized and unpolarized quantum Hall liquids nine-fold smaller than expected. This current can increase three-fold when nuclei near the boundary are spin polarized. We develop a microscopic theory of strongly interacting helical states and show that emerging helical Luttinger liquid manifests itself as unequally populated charge, spin and neutral modes in polarized and unpolarized fractional quantum Hall liquids. We show that at strong coupling counter-propagating modes of opposite spin polarization emerge at the sample edges, providing a viable path for generating proximity topological superconductivity and parafermions. Current, calculated in strongly interacting picture is in agreement with the experimental data.

The lowest achievable blocking temperature limits magnetic ordering in highly frustrated thermally active artificial kagome spin ice. By exploiting the interfacial Dzyaloshinskii-Moriya interaction, we can lower the blocking temperature of individual nanomagnets without strongly affecting their magnetic moments, thus leaving the critical transition temperatures unchanged. Using this approach, we demonstrate that a seven-ring kagome structure consisting of 30 nanomagnets can be thermally annealed into its ground state. Furthermore, the spin-ice correlations extracted from extended kagome lattices are found to exhibit the quantitative signatures of long-range charge-order, thereby giving experimental evidence for the theoretically predicted continuous transition to a charge-ordered state.

We provide a comprehensive progress update on spin wave logic circuits. First, we will present experimental data on magnetic bit readout using spin waves. The data are collected for Y3Fe2(FeO4)3 waveguide matrix with cobalt magnets placed on top of the waveguides. The magnetization direction of the magnets is recognized by the level of the inductive voltage produced by the spin waves. This approach allows us to retrieve information from a number of bits in parallel. Second, we will present experimental data on magnetic database search using spin wave superposition. The data are collected for the multi-port YIG devices. The applying of wave superposition makes it possible to speed up the search procedure compared to conventional magnetic memory. Finally, we will present experimental data on prime factorization using spin wave multi-port interferometers. The shortcomings and physical limits of spin wave logic devices will be also discussed.

Co/Pd thin film multilayers show large Perpendicular Magnetic Anisotropy (PMA) which is useful in MRAM devices for perpendicular magnetic recording. Co/Pd systems have been studied extensively through the use of ultrafast optical pump-probe methods in order to measure the Time Resolved Photo-excited Precession of Magnetization (TRPEPM). Most studies have been conducted at high laser fluence (> 1 mJ/cm2), where heating near the curie temperature occurs. In this study, we present low fluence measurements between 0.42 to 3.14 μJ/cm2 in Co/Pd systems with differing Co thickness between 0.4 to 0.74 nm to probe the role of interface anisotropy in low-power excitation.

We employ a reinforcement learning strategy for finding switching schemes for deterministic switching of a spin-orbit torque magnetoresistive random access memory cell. The free layer of the memory cell is perpendicularly magnetized, and the spin-orbit torques are generated by currents through two orthogonal heavy metal wires. A rewarding scheme for the reinforcement learning approach is defined such that the objective of the algorithm is to find a pulse sequence that leads to fast deterministic field-free switching of the memory cell. The reliability of the found switching scheme is tested by performing micromagnetic simulations. The results show that a neural network model trained on fixed material parameters is able to reverse the memory cell magnetization for a wide range of material parameters and can be used to derive a writing pulse sequence for fast and deterministic spin-orbit torque switching of a perpendicular free layer.

The relativistic charge carriers in monolayer graphene can be manipulated in manners akin to conventional optics (electron-optics): angle-dependent Klein tunneling collimates an electron beam (analogous to a laser), while a Veselago refraction process focuses it (analogous to an optical lens). Both processes have been previously investigated, but the collimation and focusing efficiency have been reported to be relatively low even in state-of-the-art ballistic pn-junction devices. In this talk, we will present a novel device architecture of a graphene microcavity defined by carefully-engineered local strain and electrostatic fields. We create a controlled electron-optic interference process at zero magnetic field as a consequence of consecutive Veselago refractions in the microcavity, which we utilize to localize uncollimated electrons and further improve collimation efficiency.

Conventional magnetic field sensors are fabricated on flat substrates and are rigid. Extending 2D structures into 3D space relying on the flexible electronics approaches allows to enrich conventional or to launch novel functionalities of spintronic-based devices. Here, we will review fundamentals of 3D curved magnetic thin films and primarily focus on their application potential for eMobility, virtual and augmented reality appliances. The technology platform relies on high-performance magnetoresistive and Hall effect sensors fabricated on ultrathin polymeric foils and paves the way towards skin-compliant devices enabling touchless interactivity with our surroundings. Flexile magnetosensitive elements impact emerging research and technology fields of smart skins, soft robotics and human-machine interfaces. In this talk, recent fundamental and technological advancements on flexible magnetoelectronics will be reviewed.

Spin-orbit torque offers an efficient route to manipulate the magnetic state of magnetic materials, which is of great importance for energy-efficient applications of various spintronic devices like memory, logic, oscillator, and neuromorphic computing. Here, we propose a strategy for the realization of a spin torque gate magnetic field sensor with an extremely simple structure by utilizing the longitudinal field dependence of the spin-orbit torque driven magnetization switching. This sensor does not require any magnetic bias to achieve a linear response to the external field, which is the main cause of high cost of all types of magnetoresistance sensors. In addition, zero offset can be achieved in the spin torque gate sensor without complicated offset compensation circuit. By employing the WTe2/Ti/CoFeB structure with both large spin-orbit torque and well-defined PMA, we demonstrate that the sensor can work linearly in the range of ±3-10 Oe with nearly zero dc offset.

Diamond has emerged as a unique material for a variety of applications, both because it is very robust and because it has defects with interesting properties. One of these defects, the nitrogen-vacancy center (NV center), has a single spin associated with it that shows quantum behavior up to room temperature. Our group is harnessing the properties of single NV centers for high resolution magnetic sensing applications. In this talk, I will introduce the basic concepts and emerging applications of diamond-based quantum sensors. I will discuss the challenges in the fabrication of diamond probes and their integration into scanning probe microscopy (SPM) systems. I will then present some illustrative examples of applications in nanoscale magnetism, including the imaging of antiferromagnetic domains and domain walls, the flow of current in graphene devices, and magnetic resonance imaging of nuclear spins with atomic spatial resolution.

Domain walls (DWs) in magnetic nanowires have been of intense interest due to proposals to use them to represent data in logic and memory devices. However, these have been challenging to realise because DWs behaviour is highly stochastic, making conventional digital devices unreliable. Here, we show how embracing DW stochasticity as a functional feature can facilitate novel computational devices. We first present results showing how integrating tuneable stochastic DW pinning into DW logic networks allows “stochastic computing”, where numbers are represented by random bit streams and individual logic gates perform complex mathematical operations. We then go on to demonstrate how DW stochasticity can be used to facilitate neuromorphic devices: (a) a neural network where the probabilities of DW propagation through nanowires perform the roles of synaptic weights and (b) a reservoir computing platform based on the emergent dynamics of DWs within an extended nanowire ensemble.

Skyrmion manipulation with VCMA can lead to small footprint nanomagnetic memory [1-2]. This talk will focus on experimental demonstration of VCMA induced nonvolatile creation and annihilation of skyrmions in an antiferromagnet/ferromagnet/oxide heterostructure film. This could provide a pathway for using intermediate skyrmion states to enable robust magnetization reversal with VCMA. We will discuss its scaling to lateral dimensions below 50 nm, oscillations > 50 GHz and application to neuromorphic computing.

We propose a four-terminal domain wall-magnetic tunnel junction (DW-MTJ) neuron that enables the first-ever purely spintronic multilayer perceptron with unsupervised learning. The leaky integrate-and-fire neuron has a ferromagnetic DW track coupled to a binary MTJ by an electrically insulated layer. Current through the DW track performs integration by moving the DW. Leaking occurs by moving the DW in the opposite direction of integration due to either dipolar magnetic field, anisotropy gradient, or shape variation. When the DW passes underneath the MTJ, it fires by switching between the resistive and conductive states. In a crossbar perceptron, the DW track of each neuron is connected to the analog three-terminal DW-MTJ synapses and the MTJ terminals cascade multiple layers. Finally, an unsupervised learning algorithm results from the feedback between the neuron MTJ and the analog synapses, providing best results of 98.11% accuracy on the Wisconsin breast cancer clustering task.

In this work , we describe the design, realisation and characterization of the magnetic version of the Galton Board, an archetypal statistical device originally designed to exemplify normal distributions. Although simple in its macroscopic form, achieving an equivalent nanoscale system poses many challenges related to the generation of sufficiently similar nanometric particles and the strong influence that nanoscale defects can have in the stochasticity of random processes. We demonstrate how the quasi-particle nature and the chaotic dynamics of magnetic domain-walls can be harnessed to create nanoscale stochastic devices [1]. Furthermore, we show how the direction of an externally applied magnetic field can be employed to controllably tune the probability distribution at the output of the devices, and how the removal of elements inside the array can be used to modify such distribution.

We present a quantitative theory of the suppression of the optical linewidth due to charge fluctuation noise in a p–n diode. We connect the local electric field with the voltage across the diode, allowing for the identification of the defect depth from the experimental threshold voltage. Furthermore, we show that an accurate description of the decoherence of such spin centers requires a complete spin–1 formalism that yields a bi-exponential decoherence process, and predict how reduced charge fluctuation noise suppresses the spin center's decoherence rate. The material is based on work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award Number DE-SC0019250. undefined

We report the demonstration of adiabatic rapid passage on single solid-state quantum emitters based on semiconductor quantum dots. By extending our earlier experiments employing femtosecond pulse shaping for rapid and arbitrary qubit rotations to the strong-driving regime, we demonstrate full suppression of decoherence tied to LA phonon coupling. Our results will support the development of single photon sources and distributed quantum networks using semiconductor quantum dots.

Spins behave quantum mechanically in solids. If there are strong interactions between spins, the effective repulsion force constraints the behavior of spins and intriguing quantum phenomena such as a magnon crystal, Bose-Einstein condensation (BEC), and spin liquid, appear when a magnetic field whose Zeeman energy is comparable to the interaction strength is applied at low temperatures. In the present study, such magnetic field induced quantum phase transitions have been investigated using ultrahigh magnetic fields exceeding 100 T. Specifically, the magnon crystals in SrCu2(BO3)2, the magnon BEC in TlCuCl3, and the spin liquid in a-RuCl3 were observed. In addition to the pure spin systems, systems that possess a strong spin-lattice coupling exhibit more variety of phenomena including structural phase transitions in the ultrahigh fields: The spin state BEC in LaCoO3 and the insulator-metal transition in V1-xWxO2 have been recently studied in magnetic fields of up to 600 T.

Advances in chemical synthesis enable the design of nanocrystals with targeted architecture, functionalized by transition metal doping. As a consequence of pronounced exchange interactions between charge carriers and dopants, this class of materials combines optical, electronic and magnetic activity even up to room temperature. Ensemble doping leads to collective spin phenomena like optically and electrically triggered magnetization as well as spin fluctuations, probed down to the level of single quantum dots. We found strong anisotropy effects paving the path towards directed magnetic polaron formation. Incorporation of single magnetic impurities yield unique discoveries like huge zero-field exchange splittings, which allows probing the spin state of an individual atom, or digital magnetic doping in magic size nanocluster.

Spin-orbit torque (SOT) magnetization switching is an efficient method to control magnetization. In SOT switching, controlling a field-like torque strength is indispensable to reduce the critical current density; however, this is difficult because the field-like torque is intrinsic to the material system used. Here, we show that it can be suppressed in a spin-orbit ferromagnet single layer of (Ga,Mn)As by a current-induced Oersted field due to its strong Dresselhaus spin-orbit coupling and non-uniform current distribution. We obtained an extremely low switching current density of 4.6×10^4 A/cm^2, three orders of magnitude smaller than that observed in typical metal bilayers.

In this work we argue that the electron skew scattering on paramagnetic impurities in non-magnetic semiconductors possesses a remarkable fingerprint, allowing us to differentiate it directly from other microscopic mechanisms of the emergent Hall response. We demonstrate theoretically that the exchange interaction between the impurity magnetic moment and itinerant carriers leads to the emergence of an electric Hall current persisting even at zero electron spin polarization. We describe two microscopic mechanisms behind this effect, and propose an essentially all-electric scheme based on a spin-injection ferromagnetic-semiconductor device, which allows one to reveal the effect of paramagnetic impurities on the Hall phenomena via the detection of the spin polarization-independent terms in the Hall voltage.

About a decade ago, the discovery of monolayers of transition metal dichalcogenides opened a new frontier in the study of optically excited states in semiconductors. These materials exhibit a plethora of robust excitonic states, such as the optically accessible bright excitons, momentum- and spin-forbidden dark excitons, and hot excitons. In today’s talk, I will discuss studies in my lab of photoexcited two-dimensional semiconductors using time-resolved photoemission spectroscopy.

This paper considers homomorphism of the Lie group SU(2) to the Lie group SO(3) of all rotations of 3- dimensional Euclidean space from Observers Mathematics point of view. In our work, we proved that in Observers Mathematics the probability of spin-j transformation is a homomorphism (representation) of SU(2 ) to the set of matrix transformations of a linear space of polynomial functions is less than 1, and got corresponding results for elementary fermions and bosons. As a continuation of these results we proved here the following theorems: Theorem 1. In Observers Mathematics the probability of two-to-one transformation of SU(2) to SO(3) is Lie groups homomorphism (representation) is less than 1. Theorem 2. The probability of two-to-one transformation and spin-j transformation (j = 1) are equivalent in Observers Mathematics is less than 1.

Magnetic tunnel junctions (MTJs) show great promise for implementation in high-performance STT-MRAM and novel computing regimes such as magnetic logic and neuromorphic computing. However, a handful of material setbacks stand in the way of the adoption of leading MgO MTJs over other emerging technologies, such as Resistive-RAM junctions, in next-generation architectures. Here, we explore the properties of iron / scandium nitride (ScN) magnetoresistive junctions using density functional theory (DFT) and find ScN a promising barrier material given its novel electron symmetry filtering properties, high TMR, and low RA-product. Magnetoresistance ratios exceeding 1900% are enabled by Δ2’ symmetry filtering through the barrier, in addition to the traditional Δ1 symmetries observed in MgO MTJs. The electronic properties of the diffusive Fe/ScN interface are resolved, with predicted half-metallicity that could amplify MR in realistic low-power ScN devices.

Long-range spin transport at room temperature is one of the indispensable technologies for realizing spintronics devices. In this study, we have investigated electron spin relaxation time of (110)-oriented GaAs superlattice having tunnel-coupled quantum wells for both lateral and vertical spin transport. It was revealed that the spin relaxation time at room temperature was 0.7 ns, about 7 times longer than that of bulk GaAs which has been used for conventional spin transport layer of spin-controlled lasers. This finding provides a novel method of controlling the spin relaxation time at room temperature and indicates that the superlattice structures are promising for spin transport layers in semiconductor-based spintronics devices.

The term “artificial spin ice” (ASI) refers to a class of magnetic metamaterials where magnetic domains can be mapped onto a spin-lattice model. Here, we present broadband ferromagnetic resonance and Brillouin light scattering measurements of ASI and correlate the experimental findings with micromagnetic simulations. We focus on the angular-dependent spin dynamics of different types of ASI made of one single material and bicomponent ASIs composed of two sub-lattices made of dissimilar materials. Our results show that the interaction and the resonant dynamics in ASI can be tuned, not only by the field, geometry and orientation of the lattice, but also by the proper choice of the materials.

Complex dynamics far from equilibrium in two dimensions can be explored in arrays of interacting nano-scale magnetic islands. We discuss how the fine details of how magnetic islands interact can lead to non-intuitive and often striking behaviour that is observable on macroscopic length and time scales. Results from nano-scale squid and Lorentz transmission electron microscopy imaging are analysed using models for thermal activation and Monte Carlo simulations. We find evidence for the emergence of effective anisotropies and surprising consequences for avalanche dynamics can be attributed to correlated, microscopic spin configurations that can arise within individual magnetic elements.

This presentation recording was recorded for the SPIE Optics + Photonics 2021 symposium

We theoretically study the high harmonic generation (HHG) in Kitaev spin liquids, comparing the HHG with those of ferromagnets and semiconductors. We find several new features can be observed in the HHG of the featureless Kitaev spin liquids. Our results would build a bridge between photo science and quantum spin liquids.

Artificial spin ices are arrays of correlated nano-scale magnetic islands that prove an excellent playground in which to study critical phenomena. In this contribution, we discuss how both geometry and the coupling of islands to external fields influence magnetic order. Using Lorentz transmission electron microscopy, we study a transition between antiferromagnetic and ferromagnetic order across a continuum of spin ice geometries. We show how emergent anisotropies can arise in field-driven processes and how relaxation timescales can be adjusted locally within arrays through a coupling to a site-specific bias field. Our work demonstrates artificial spin ice as an excellent testbed in which to probe non-equilibrium phenomena in low-dimensional systems.

Two-dimensional (2d) nano-electronics, plasmonics, spintronics and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Here I will describe how the Relativistic Mott Insulating state of RuCl3, a 2D antiferromagnet, provides a new opportunity to introduce modulation doping into 2D materials. Specifically, we demonstrate and optimize this charge transfer with extensive Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations. Also, we find the doping is exceptionally local, can occur through hBN, works with various exfoliated, CVD, and MBE materials. Time permitting, I will discuss new opportunities this opens for nanoplasmonic, optoelectronics, and correlated phases.

Magnetic micro and nanostructures resembling macrospins have, to date, been lithographically patterned and fixed to a predefined 2D or, most recently, 2.5D layout. I will present an alternate route facilitating interfacial self-assembly and jamming of superparamagnetic nanoparticles at curved liquid-liquid interfaces to create macrospin systems. The mechanical jamming of superparamagnetic nanoparticles is key to freeze both structural and magnetic short-range order and transform the paramagnetic ensemble into a ferromagnetic systems with remanent magnetization and rigid microscopic shape. We use hydrodynamics experiments to probe how the magnetization of ferromagnetic liquid droplets and their response to external stimuli can be tuned by chemical, structural and magnetic means. Numerical modeling using molecular dynamics and mircomagnetic simulations, usher a path toward nanopatterning structured liquids.

Hyperbolic nanoparticles provide a versatile platform to widely tune light-matter interactions. Active nanophotonics can be realized by controlling the optical properties of materials with external magnetic fields. Here, we explore the influence of optical anisotropy on the magneto-optical response of hyperbolic nanoparticles across the visible and near infrared spectral range. By using a perturbative approach, we establish a model where the magneto-optical activity of the system is described in terms of the coupling of fundamental electric and magnetic dipole modes, which are induced by the hyperbolic dispersion, with a static magnetic field. Finally, an analytical model is established in the framework of Mie theory to describe the magneto-optical response and identify the contribution of electric and magnetic modes to the total spectrum.

Topological insulators (TI) have gained much interest in the field of spintronics for the generation of pure spin currents. Indeed, three-dimensional TIs are predicted to host exotic properties like topologically protected surface states (TSS), which show Dirac-like band dispersion and spin-momentum locking [1]. One of the main strategies is to take advantages of the spin polarization of the TSSs to manipulate the magnetization of an adjacent ferromagnetic thin film (FM) using the spin-orbit torque (SOT) mechanism [2]. In the past few years, the community attempted to replace the traditional heavy metals by a TI in order to enhance the SOT efficiency with limited success. It now appears that the interface sharpness and the high chemical affinity between Bi-based TIs and classical 3d FMs is a major hurdle to reach the predicted breakthrough in magnetization switching power-efficiency [3]. The emergence of ferromagnetism in two dimensions in 2017, which started a new field in condensed matter physics, could bring a solution to this issue.The van der Waals (vdW) nature of the interaction between the TI and the 2D-FM should limit chemical reactions, interface intermixing and hybridization of state between the two layers.

The spin mixing conductance is an important figure of merit for spin transport across an interface. This is a particularly important number for Spin Orbit Torque Magnetic Random Access Devices, where spin generated in one layer is used to provide the spin torque needed to flip the magnetization in an adjacent layer. Here the spins are generated in either an topological insulator (TI) or an heavy metal (HM). The overall efficiency of such a device depends on both the charge to spin conversion in the spin generation layer and the spin mixing conductance of the interface.

We report on ferromagnetic resonance (FMR) detected spin pumping in Fe60Al40 /Pd and Py/Fe60Al40 bilayers, and laterally patterned Fe60Al40 (FeAl) nanostructures. The magnetic properties of FeAl alloy are tailorable from paramagnetic to ferromagnetic state by variation of the structure through an ion beam irradiation, which makes this material promising for the fabrication of magnetic landscapes and magnonic crystals. Exploiting this tunability, we show that FeAl can be used as spin source and spin sink in spin pumping experiments. Using the material with the identical chemical composition as para- and ferromagnet, we suggest a new pathway for creating lateral spin pumping geometries which are produced with ion beam irradiation of B2 alloys.

Spin-orbit torque is one of the key phenomena in modern spintronics, which enables electric control of magnetic moments. The mechanism of the spin-orbit torque is often interpreted in terms of the spin current generated by an electric field in the presence of spin-orbit coupling. However, the electron can carry angular momentum not only by spin but also by its orbital degrees of freedom. Quite often, orbital angular momentum current can be more efficiently generated by an electric current than the spin current since it does not require spin-orbit coupling. With this consideration, we recently proposed a mechanism of torque generation based on electronic orbital angular momentum current, which is now called orbital torque. In this talk, I will explain the background and idea of the orbital torque mechanism and provide guidelines for experimental observation.

We study the stationary state of Hall-bar devices composed of a load circuit connected to the lateral edges of a Hall-bar. We follow the approach developed in a previous work (Creff et al. J. Appl. Phys 2020) in which the stationary state of a ideal Hall bar is defined by the minimum power dissipation principle. The presence of both the lateral circuit and the magnetic field induces the injection of a current: the so-called Hall current. Analytical expressions for the longitudinal and the transverse currents are derived. The same analysis is performed on the spin-Hall effect, in the framework of the two spin-channel model.

Since the 1980s, the generation and detection of spin currents has relied on ferromagnets. Their switching today relies on spin-orbit torque from heavy metals. Nevertheless, spin injection in semiconductors has rather low efficiency. Ferroelectric Rashba semiconductors (FERSC) [1,2] may constitute a new paradigm for semiconductor spintronics, thanks to the combination of semiconductivity, large spin-orbit interaction, and the non-volatility provided by ferroelectricity. Here we report the room-temperature ferroelectric switching of spin-to-charge conversion in epitaxial GeTe films. We first show that ferroelectricity in GeTe can be reversed by electrical gating despite its high carrier density. Then, we reveal a spin-to-charge conversion as effective as in Pt, but whose sign switches with the ferroelectric polarization. These results open a route towards devices combining spin-logic and memory integrated into a silicon-compatible material.

Spin wave logic circuits using quantum oscillations of spins (magnons) as carriers of information have been proposed for next generation computing with reduced energy demands and the benefit of easy parallelization. Current realizations of magnonic devices have micrometer sized patterns. Here we demonstrate the feasibility of biogenic nanoparticle chains as the first step to truly nanoscale magnonics at room temperature. Our measurements on magnetosome chains (ca 12 magnetite crystals with 35 nm particle size each), combined with micromagnetic simulations, show that the topology of the magnon bands, namely anisotropy, band deformation, and band gaps are determined by local arrangement and orientation of particles, which in turn depends on the genotype of the bacteria. Our biomagnonic approach offers the exciting prospect of genetically engineering magnonic quantum states in nanoconfined geometries. By connecting mutants of magnetotactic bacteria with different arrangements of magnetite

Gate-tunable spin-orbit interaction (SOI) and its related phenomena have been a new aspects of spintronics and spin-orbitronics. In this presentation, recent progress of gate modulation of the inverse spin Hall effect, its reciprocal effect and spin lifetime anisotropy in solids by a gate electric field will be introduced and discussed.