Spintronics XVII

The combined manifestation of high-temperature superconductivity & topological phase transition is of great interest for exhibiting Majorana zero modes at relatively high temperatures. Recently, such possibilities were demonstrated in the context of Fe-based superconductors (Fe-SC), where topological surface states were observed in FeSexTe1-x. Here, we show the possibility of engineering high-temperature topological superconductors with Fe-based superconductors in their bulk and ultra-thin heterostructure phases by exploiting the interplay between electron, lattice, & topology to address some important questions through the first principles DFT+dynamical mean-field theory method.

We present results for Josephson junctions based on three different heterostructures: InAs/Al, BiSb/W, and Cd3As2/Al. Junctions based on each of these heterostructures are predicted to have unique properties, and can in principle be tuned into a topological state. We show how the response of these junctions to microwave radiation can be used to extract detailed information on the microscopic electronic properties and to realize a microwave-tunable superconducting diode effect.

Recently, the Josephson diode effect (JDE) in Josephson junctions (JJs) has attracted a great deal of interests. It is expected that JDE may find important applications such as passive on-chip gyrators and circulators. Such devices would be particularly impactful in quantum information applications. JDE occurs when both inversion symmetry and time reversal symmetry (TRS) are broken. Normally, external magnetic fields or magnetic heterostructures are exploited to break TRS. In this talk, we will present our recent observation of zero-magnetic-field JDE in topological superconducting quantum interference devices (SQUIDs) made of Dirac semimetal Cd3As2. We argue that a phase coupling between the surface and bulk superconducting channels, a unique phenomenon recently identified in the observations of fractional Josephson effect and Leggett modes in Cd3As2, can break TRS and, therefore, give rise to the zero-field JDE. We further show that the efficiency of the JDE can be readily varied by the geometry of JJ arms in the SQUIDs. Our results should have important implications in superconducting electronic circuit applications.

Quantum design of Cooper quartets in a double quantum dot system coupled to ordinary superconducting leads is presented as a novel platform for the study of an elusive many-body state of matter, that is at the basis of the phenomenon of charge-4e superconductivity. A fundamentally novel, maximally correlated ground state, in the form of a superposition of vacuum |0⟩ and four-electron state |4e⟩, emerges as a narrow resonance and it is promoted by an attractive interdot interaction. A novel phenomenology in the dissipationless transport regime is elucidated, that yields typical flux quantization in units of h/4e and manifests in non-local multi-terminal coherence and in two-Cooper pair transport properties mediated by the quartet ground state. The results open the way to the exploration of correlation effects and non-local coherence in hybrid superconducting devices, parity-protected quantum computing schemes and more generally, the work poses the basis for the design and simulation of novel correlated states of matter starting from ordinary ingredients available in a quantum solid state lab.

Motivated by the recent experimental realization of a minimal Kitaev chain in quantum dot systems [1], I will describe our results about the dynamics and fusion of Majorana zero-modes at or near the "sweet spot" t=Δ [2] . I will also talk about our recent findings of exotic "multi-site" MZMs in Y-shape (and X-shape) Kitaev wires [3]. Finally, I will present results for "non-trivial" fusion in canonical chains and in Y-shape Kitaev wires at the sweet spot [4] . 1. Dvir, T., Wang, G., van Loo, N. et al. Realization of a minimal chain in coupled quantum dots. Nature 614, 445450 (2023). 2. Pandey, B., Mohanta, N., Dagotto, E. Out-of-equilibrium Majorana zero modes in interacting Kitaev chains. Phys. Rev. B107, L060304 (2023). 3. Pandey, B., Kaushal, N., Dagotto, E. Majorana zero modes in Y-shape interacting Kitaev wires. npj Quantum Materials 8, 51 (2023) . 4. Pandey, B., Okamoto, S., Dagotto, E. Unexpected results for the non-trivial fusion of Majorana zero modes in interacting quantum-dot arrays. arXiv:2311.15079v2 (2023).

Superconducting quantum circuits are one of the leading quantum computing platforms. To advance superconducting quantum computing to a point of practical importance, it is critical to identify and address material imperfections that lead to decoherence. In this talk, I will show how terahertz Scanning Near-field Optical Microscopy (SNOM) can be used to probe functional devices such as coplanar microwave resonators and inform the processing of new materials for quantum technology.

Low symmetry conductors, including 2D topological insulators, topological insulator nanowires and other two and one-dimensional nanostructures with spin-orbit interactions or angular momentum to linear momentum coupling, can exhibit resistance that depends linearly on the magnetic field and is proportional to the electric current. We demonstrate that besides the current, characterized by generation function that relaxes on the time scale of momentum relaxation time, there are also effects associated with non-equilibrium alignment of angular momentum or with spin orientation, when the generation functions relax on much longer time scales of spin or angular momentum relaxation. The emerging currents are generated as a result of spin or angular momentum relaxation or precession in effective fields characterizing spectra of these systems. We demonstrate that these mechanisms can contribute to the enhancement of the value of the effect, allowing to determine best materials for sensors and devices

We present memory devices based on valley-spin hall (VSH) effect in monolayer WSe2 and discuss how we utilize their unique properties for logic-memory integration in two applications: (i) energy-efficient non-volatile flip-flops (memory-in-logic) and (ii) compute-enabled crossbar arrays for binary neural networks (logic-in-memory). We discuss the appealing attributes of the VSH-devices enabled by coupling WSe2 with perpendicular magnetic anisotropy (PMA) magnets and show their utility in mitigating the design conflicts of memory devices. We utilize the VSH-devices to design non-volatile flip-flops, which feature energy efficient data backup (by virtue of VSH-based write) and efficient restore operation (due to differential data storage in a single VSH-device). Further, we show how the true and complementary bit-storage along with the integrated back gate of VSH-devices enable the design of compact and low power logic-in-memory primitives (capable of computing dot products of inputs and weights) for binary neural networks.

The ability to generate spin polarization and spin current on femtosecond time-scales is of vital importance for spintronics, quantum information science and novel spin-based low-energy electronics modalities. Current approaches relying on superdiffusive spin-transport in ferromagnetic structures are however limiting due to the need for external magnetic fields. Here we show instead by time-, spin- and angle-resolved photoemission that the combination of C60 and WSe2, two diamagnetic materials, create a 2D heterostructure that upon ultrafast excitation generates transient spin polarization without the need for an external magnetic field or ferromagnets. Optical excitation drives electron-transfer from C60 to WSe2 in a few hundred fs, and the charge-separation is accompanied by the build-up of a layer-dependent electric field across the heterointerface. This electric field induces a time- and layer-dependent Stark shift of the energy levels and valence bands at the interface, which due to spin-valley-layer locking in WSe2 results in a transient spin polarization. This constitutes a new path for ultrafast spin manipulation without the need for external magnetic fields and may pave th

We have developed a nonlocal graphene-on-germanium spin interconnect devoid of ferromagnets where spin-polarized carriers are optically oriented in Ge and are then transferred to graphene at room temperature. Spins are locally injected by means of a confocal microscope with circularly-polarized light at resonance with the direct band gap of Ge, and then detected after the diffusion in graphene by exploiting the inverse spin-Hall effect (ISHE) in a thin Pt bar evaporated on top of graphene. We selectively transfer either spin-polarized electrons or holes with opposite spin polarizations by applying a bias voltage at the junction between graphene and Ge. The spin diffusion length of both spin-polarized carriers reaches 200 μm for graphene at room temperature, demonstrating the possibility to transport spins with negligible losses in graphene-based long-range devices.

Manipulating magnetization through spin current and laser excitation in magnets, along with the study of antiferromagnetic magnons, holds promise for spintronics and quantum applications. In this talk, we present our discovery of ultrastrong magnon–magnon coupling in the gigahertz band within a two-dimensional (2D) van der Waals (vdW) antiferromagnet. We demonstrate the tunability of coupling strength by temperature and magneto-crystalline anisotropy. Additionally, we share findings on spin Hall magnetoresistance (SMR) in a device based on a 2D vdW antiferromagnet, highlighting the potential to exfoliate 2D antiferromagnets into thin layers and manipulate their magnetization via spin currents. Furthermore, we present evidence that laser excitation can induce different types of spin textures at room temperature and zero field in a 2D vdW ferromagnet. Lastly, we discuss the observation of large SMR in a synthetic antiferromagnet and explore its behavior with temperature and magnetic fields. These discoveries lay a solid foundation for developing quantum and spintronics devices based on 2D vdW magnets and synthetic antiferromagnets.

We present an effective tight-binding model capable of accurately reproducing conduction band of 2D van der Waals Magnetic materials, We apply the developed model to chromium trihalides known as promising 2D magnetic materials for spintronics applications. The model accurately captures the spin-polarized conduction band structure emphasizing a distinctive flat band situated at the lower edge of the conduction band quadruplet. An intriguing observation emerges regarding the substantial contribution of long-range hopping beyond nearest neighbours, revealing electron pathways connecting remote chromium sites. Considering a CrI3 monolayer between graphene layers, we demonstrate that charge transfer leads to a 2D electron gas populating the flat band, with a Wigner parameter around 20, highlighting CrI3 as a promising host for strong-correlated phenomena.

Taking advantages of metasurfaces, and in particular their polarization dependent response, we designed and experimentally validated CMOS compatible vectorial metasurfaces monolithically integrated with standard VCSELs for on-chip spin-decoupling and phase shaping. Our approach enables accessing the optical spin states of VCSELs in an ultra-compact way with previously unattainable phase controllability. By exploiting spin states as a new degree of freedom for laser wavefront engineering, our platform is capable of operating and reading-out the spin-momentum of lasers associated with injected spin carriers, which would potentially play a pivotal role for the development of emerging spin-optoelectronic devices.

We report on spin polarization modulation characteristics of InAlGaAs vertical-cavity surface-emitting lasers (VCSELs) under the optical injection locking. The results measured with the optical spin modulation technique and simulated with the spin-flip model indicate that a weak optical injection locking condition is suitable to exploit a strong and high-frequency resonance response under the spin polarization modulation. We discuss an effect of polarization switching in VCSELs which can be a cause of variation of the spin polarization modulation response and injection locking condition. Our findings suggest a strategy of the optical injection locking for spin-controlled VCSELs.

Direct high-speed modulation of circular polarization (Pc) of coherent light will open the way for new communication technology and offers the possibility to overcome the main bottleneck of the optical telecommunications. Here, by using spin-orbit torque (SOT) with spin Hall effect to control the spin injector magnetization, we report for the first time to achieve electrical control of the circular polarization of light emitted from a quantum dot based light emitting diode (LED) at room temperature [1]. The circular polarization can be modulated between ±25% at 250K and ±16% at 290K after pulsed current switching injector magnetization. A repetition of more than 60 times do not reveal any degradation at the injector/semiconductor interface. Our achievement will directly contribute the implementation of the new optical telecommunication technology with Pc modulation. [1] P. A. Dainone, et al. “Controlling the helicity of light by electrical magnetization switching”, Nature, to be published.

Chiral molecules are one of the Nature-inspired materials for their selective functionalities. Utilizing the chirality of a semiconductor, selective spin transport can be realized, which will be highly desirable for building semiconducting spintronics. Recently, organo-metal halide perovskites in chiral structures have been developed as a promising chiral semiconductor for spintronics. Its unique organic and inorganic structure allows for the structural chirality transfer from the packing of the organics to the inorganic back bones. However, the chiral perovskites are usually in low dimensions (2D, 1D or 0D) which are less conducting and lack visible light sensitivity due to the band gap widening. In this work, we investigate molecular dopants in the chiral perovskite matrix to improve the conductivity. We have incorporated a variety of p-type molecules in the perovskite structure, and found both dark and photo conductivity improved by 1-3 orders of magnitudes.Finally, we integrated our doped chiral perovskite thin film into a photo-diode device for polarized light sensing demonstration.

We report our recent progress in developing inorganic sheets integrating electromagnetic interference (EMI) shielding and infrared (IR) detection functionalities. This work is crucial as the increasing use of radio-frequency (RF) devices saturates the environment with electromagnetic radiation capable of disrupting the operation of electronic components, including sources and detectors of IR radiation that are widely used in commercial and military applications. Therefore, IR devices must be enclosed by a material that blocks RF waves while allowing the transmission of IR radiation. Here, we show that single-crystalline semiconductor membranes are viable candidates for shielding IR devices from EMI as they provide high shielding effectiveness and optical transmittance. In addition, we demonstrate that thin junction photodiodes can be integrated on the back surface of the semiconductor membrane, making the latter a multi-functional material.

Opto-spintronic semiconductors, that can function as spin-photon interfaces, are needed for future optical communication of spin information. Although the spin polarization of electrons is easily lost in conventional non-magnetic semiconductors at room temperature, the proposed 0D-2D hybrid nanostructures using III-V semiconductor quantum dots (QDs) and dilute nitride GaNAs quantum well (QW) enable the generation of nearly fully spin-polarized electrons in the QDs via defect-enabled spin filtering of GaNAs. In this paper, I focus on the electron spin dynamics in these QD-QW coupled nanostructures and the operation characteristics of spin-functional optical devices using these nanostructures.

This work presents a novel method for achieving controlled polarization-selective enhancement of the Photonic Spin Hall Effect (PSHE) by integrating waveguiding effects with surface plasmonic behavior. The conventional plasmonic wave generation, limited to TM waves, is expanded by introducing waveguiding effects, enabling resonances for TE waves. This dual resonance mechanism contributes to the enhancement of PSHE for both horizontally (H) and vertically (V) polarized waves. Utilizing thin metal layers (Ag and Al) of a few nanometers in conjunction with waveguiding glass layers under 500 nm thickness, significant enhancements of PSHE are demonstrated at the sub-millimeter scale. This integrated approach offers a promising avenue for tailoring and controlling PSHE with applications in advanced photonic devices.

This presentation will provide an overview of a research effort aimed at a deep understanding of spin injection and transport in metallic non-local spin valves (NLSVs). These NLSVs enable experimental quantification of essential spin transport parameters such as spin polarization, spin diffusion length, and spin lifetime, also forming the basis for a proposed next-generation hard-disk-drive reader technology known as spin accumulation sensors. The focus will be recent progress towards spin accumulation sensors, spanning integration of ideal ferromagnetic and nonmagnetic metals, exploration of the sub-10-nm thickness regime, understanding of the feasible range of resistance-area products in devices with signal-boosting interfacial barriers, unexpected enhancement of the nonmagnetic spin diffusion length, and an emerging understanding of signal limitations in the optimal resistance-area product range. Work supported by the US National Science Foundation and the Advanced Storage Research Consortium.

The ability to generate pure-spin currents using the non-local spin valve (NLSV) has established it as an opportune device for testing spin transport on nanoscopic dimensions, with potential technological applications. The geometry has been used extensively to probe relaxation in a host of materials including non-magnetic metals. Nevertheless, interpretation of the non local ‘spin signal’ obtained in such a device relies on a precise ability to separate out pure spin transport and background effects. In this talk I will present insights obtained from all-metallic NLSVs. Recent results, systematically investigating the spin signal generated in such devices has allowed us to isolate several effects which limit the generation and detection of spin signals in NLSVs. I will present findings on the interface, impurity scattering and thermoelectric contributions in such devices, including the background signals they generate. I will also discuss the concept of thermal nanoscale conversion: using a heated scanning probe tip to locally modify the properties of magnetic thin films or devices. I will show how this offers new ways to design spintronic and non-local devices.

Spin superfluidity (SSF) is a non-diffusive transport mechanism analogous to superconductivity. However, the device geometries required for realizing SSF in conventional candidate materials are incompatible with current fabrication methods. In this work, we investigate the efficiency of SSF in the underexplored lateral device geometry. Starting from the Landau–Lifshitz–Gilbert equation we show that SSF efficiency is maximized whenever the area of the injector is much larger than that of the detector and transport channel. This unique tunability enables efficient, scale-independent spin transport. Furthermore, we connect the nonlocal resistivity to the unrecognized reciprocal of spin Hall magnetoresistance (SMR): spin pumping–induced SMR.

Magnetic skyrmions exhibit favorable properties for neuromorphic computing. However, an experimental demonstration of the weighted sum operation is still missing. Here, we leverage the particle-like characteristics of magnetic skyrmions to perform the weighted sum operation in a compact, biologically-inspired manner. We demonstrate the precise control of the number of skyrmions, determined by electrical pulse inputs multiplied by the track synaptic weights. Magneto-ionic effects enable non-volatile and reversible tuning of magnetic properties, allowing gate voltage manipulation of synaptic weights. Detection of the skyrmion number is accomplished through non-perturbative anomalous Hall voltage measurements. We experimentally validate the weighted sum operation using two electrical inputs in a crossbar array configuration with two tracks. This ensures efficient execution of the fundamental weighted sum operation, a cornerstone for neuromorphic computing. Our experimental demonstration is scalable to accommodate multiple inputs and outputs using a crossbar array design, potentially approaching the energy efficiency observed in biological systems.

Nonlinear random projection machines are efficient neural networks capable of classifying real-life data with lower computational demands compared to standard artificial neural networks. They are well-suited for hardware implementation using nonlinear devices, enabling the creation of low-power hardware neural networks. We implement such a network using vortex-based spin-torque oscillators (STVOs), magnetic tunnel junctions (MTJs) that transform input signals nonlinearly at low power. We identify three physical parameters affecting the STVO dynamics and the network's performance during data classification. We demonstrate their impact on a simplified nonlinear separation task and optimize them using ultrafast data-driven simulations for image recognition on the MNIST dataset. This approach holds potential for further hyperparameter optimization in STVO-based hardware random projection machines, and for the efficient development of custom neural architectures tailored for neuromorphic data classification.

Manipulation of interfacial magnetism utilizing voltage pulses can lead to energy-efficient scalable nanomagnetic devices. Through voltage-controlled magnetic anisotropy (VCMA), we had previously shown the potential to achieve non-volatile magnetoresistive random-access memory (MRAM) technology that is 100 times more energy-efficient than commercially available spin-transfer torque MRAM [1]. Building on prior work on VCMA-based skyrmion-mediated reversal of ferromagnetic states and its scaling to 20 nm [2], we will present new experimental demonstrations of manipulation of skyrmions in magnetoionic heterostructures with an electric field. This talk will also focus on energy-efficient magnetoionic control of skyrmions in (Co/Ni)N-based heterostructures for memory application. Furthermore, our talk also demonstrates implementing physical reservoir computing, a neuromorphic process typically used for classifying and predicting temporal data, with the energy-efficient magnetoionic process. References: [1] Bhattacharya et al. ACS applied materials & interfaces, 10(20), 17455-17462 (2018). [2] Rajib et al. Scientific reports, 11(1), 20914 (2021).

This talk will consider the future of metal halide perovskite (MHP) photovoltaic (PV) technologies as photovoltaic deployment reaches the terawatt scale. The requirements for significantly increasing PV deployment beyond current rates and what the implications are for technologies attempting to meet this challenge will be addressed. In particular how issues of CO2 impacts and sustainability inform near and longer-term research development and deployment goals for MHP enabled PV will be discussed. To facilitate this, an overview of current state of the art results for MHP based single junction, and multi-junctions in all-perovskite or hybrid configurations with other PV technologies will be presented. This will also include examination of performance of MHP-PVs along both efficiency and reliability axes for not only cells but also modules placed in context of the success of technologies that are currently widely deployed.

The recent advent of robust, refractory (having a high melting point and chemical stability at temperatures above 2000°C) photonic materials such as plasmonic ceramics, specifically, transition metal nitrides (TMNs), MXenes and transparent conducting oxides (TCOs) is currently driving the development of durable, compact, chip-compatible devices for sustainable energy, harsh-environment sensing, defense and intelligence, information technology, aerospace, chemical and oil & gas industries. These materials offer high-temperature and chemical stability, great tailorability of their optical properties, strong plasmonic behavior, optical nonlinearities, and high photothermal conversion efficiencies. This lecture will discuss advanced machine-learning-assisted photonic designs, materials optimization, and fabrication approaches for the development of efficient thermophotovoltaic (TPV) systems, lightsail spacecrafts, and high-T sensors utilizing TMN metasurfaces. We also explore the potential of TMNs (titanium nitride, zirconium nitride) and TCOs for switchable photonics, high-harmonic-based XUV generation, refractory metasurfaces for energy conversion, high-power applications, photodynamic therapy and photochemistry/photocatalysis. The development of environmentally-friendly, large-scale fabrication techniques will be discussed, and the emphasis will be put on novel machine-learning-driven design frameworks that leverage the emerging quantum solvers for meta-device optimization and bridge the areas of materials engineering, photonic design, and quantum technologies.

Artificial intelligence (AI) techniques have boosted the capability of optical imaging, sensing, and communication. Concurrently, photonics facilitate the tangible realization of deep neural networks, offering potential benefits in terms of latency, throughput, and energy efficiency. In this talk, I will discuss our efforts in AI photonics with two examples. The first involves employing a convolutional neural network for achieving single-shot full-field measurement of optical signals. The second example pertains to implementing a deep neural network with a multiple-scattering system featuring structural nonlinearity, thereby enabling nonlinear computations using linear optics.

Amorphous alloys of cobalt with a small concentration <10% of nonmagnetic rare earths (Sc, Y, La, Lu) allow us to infer the magnetic behavior of pure amorphous Co, which should exhibit a Curie temperature approaching 2000 K and a local random anisotropy energy of about 10 K/atom, which is exchange averaged in the strong ferromagnetic exchange to give soft ferromagnetic films with negligible coercivity. The amorphous films can therefore carry strongly spin-polarized and orbital currents, that could open new opportunities in orbitronic electron transport. Corresponding amorphous iron alloys are quite different, with a broad distribution of mainly ferromagnetic with some antiferromagnetic nearest-neighbor coupling, that leads to spin freezing at a temperature of order 100 K. Analysis of the spin flop transition near compensation in amorphous R-Co alloys with non-S-state rare earths, where the random anisotropy experienced by the rare earth is not exchange averaged in the Co-R exchange field, leads to the conclusion that strong random pinning of the R moments to the local anisotropy directions cannot be described by the a simple random axial anisotropy model.

Metallic ferrimagnets, where two antiferromagnetically coupled subsystems are not fully compensated, are of potential interest for applications ranging from skyrmion memories, to all-optical switching, to ionic gating that changes the magnetization compensation temperature. Rare earth-transition metal Metallic ferrimagnets show a high degree of tunability, but also show unusual and in some cases unexplained magnetic features, including very strong sensitivity to compositional and other disorder, "silent" magnetic components that are measurable via SQUID magnetometry, but do not contribute to electrical transport, and changes in magnetic anisotropy. In this talk I will present our group's exploration of these effects in Co-Gd alloys and bilayers, and our work to incorporate these materials in useful spintronic devices.

Recent findings unveil the presence of uncompensated magnetic moments and a topological magnetic structure in noncollinear magnets, suggesting promising applications in spintronics. However, a comprehensive exploration of the magnetic excitations (magnons) and the coupling with phonons in noncollinear magnets, which carries profound physical implications, is still lacking. Here, we present a notable observation of a giant hysteretic Spin Seebeck Effect (SSE) anomaly, marked by a sign reversal at large magnetic fields, within a (001) TbIG film. The SSE enhancement at high fields reaches an impressive 4200% at around 105 K, exhibiting a nonmonotonic temperature dependence. Remarkably, the high-field hysteresis of SSE is linked to a magnetic transition involving a double-umbrella spin texture in TbIG. Theoretical calculations support nearly parallel dispersion curves of magnons and phonons around this transition, resulting in a high density of field-tuned magnon polarons and an exceptionally large SSE. This investigation provides valuable insights into the evolution of magnon dispersions in double-umbrella TbIG and holds the potential to enhance the efficiency of SSE devices.

For magnetic oxides, competition between various types of exchange interactions has often led to striking physical properties that are highly tunable by external fields. Such tunability is desirable for spintronic applications. In this talk, I will show how one can use electric field to control magnetic domain structures and interfacial ferroelectricity in oxides thin films and heterostructures, giving rise to the ability to control spin-dependent transport using electric field. The electric field control of magnetic domain structures in oxides is achieved based on the understanding of the physical origin of domain formation in oxides, which is well beyond conventional Landau-Lifshitz theory. We have successfully fabricated various oxides-based spintronic devices, which all exhibit promising functionality with low energy consumption. To finish, I will discuss the future of oxides spintronics from my own perspective.

Solid state magneto-ionic (MI) effects, which can be achieved through controlled ionic migration at atomic scale interfaces in magnetic nanostructures, have shown promise for energy-efficient nanoelectronics. Our recent efforts reach outside of the often-explored oxygen-based MI systems, and have focused on alternative ionic species, including nitrogen. Two nitrogen-based systems, Ta/CoFe/MnN/Ta and the all-nitride Mn4N/MnNx, show MI manipulation of magnetic properties including saturation magnetization and exchange bias. Such MI systems are valuable platforms to gain quantitative understanding at buried interfaces and they demonstrate contrasts with oxygen-based MI effects in terms of operating principles, switching speed, and reversibility.

Probabilistic computers, with inherent randomness built into their bits, promise orders of magnitude improvement in efficiency over deterministic computers for solving many complex problems related to natural phenomena and modern society which are inherently probabilistic. In this work we present novel choices of magnetic materials and material parameters to design probabilistic bits (p-bits), the basic building block of a probabilistic computer, with enhanced speed and/or robustness to errors. Through stochastic Landau-Lifshitz-Gilbert (LLG) equation simulations, we study the dynamics of magnetic p-bits as a function of experimentally variable material parameters such as magnetic order (ferromagnetic, ferrimagnetic, and octupole order in noncollinear antiferromagnets), energy barrier, Gilbert damping, saturation magnetization, and volume. We highlight previously unexplored regimes in this parameter space which could lead to orders of magnitude improvement in speed and/or robustness to experimental fluctuations over the state-of-the-art magnetic p-bits. Our work provides direction and guidance to experimentalists in their efforts to build a spintronic probabilistic computer.

Spintronic terahertz emitters (STEs) have great potential for use in THz time-domain spectroscopy and its exploitation in scientific and industrial applications. While they typically emit broadband single-cycle THz pulses, there is interest in narrowband THz sources, with applications across a broad range of research areas. Here we show that any spintronic terahertz (THz) emitter can be driven to generate narrowband THz pulses, with continuous tuning of the emission frequency and bandwidth by employing temporal shaping of the drive laser pulses to manipulate the ultrafast demagnetization dynamics. Using a regenerative amplifier laser system with 50 fs transform-limited pulses chirped to 6 ps together with a chirped-pulse beating scheme, we demonstrate narrowband THz generation over a frequency range from 0.4 to 2.3 THz, in addition to bandwidths down to 40 GHz using 12 ps chirped pulses. These results highlight how STEs can provide opportunities for narrowband applications over the entire THz spectral range.

Recent developments in nanomagnetism and spintronics have enabled the use of ultrafast spin physics for terahertz (THz) emission. In this presentation, we explore the potential of spintronic THz emitters for more efficient THz radiation and adjustable bandwidth by addressing different crystallographic phases of the magnetic and non-magnetic layers, alloyed layers and antiferromagnetic interlayers.

Broadband ultrashort terahertz pulses are a powerful tool to measure the dynamics of low energy excitations in materials, especially as they undergo electronic transitions between different orders. In the first part of my talk, I will discuss dynamic conductivity measurements of in iron based superconductors as prototypical quantum phase transition between a superconducting and metallic grounds state. Our results demonstrate universal scaling relationship as a function of photon energy and thermal energy that elucidates the nature of its ground state. In the second material, I will talk about our investigations of the electronic structure of the one of the Ruddlesden-Popper series of iridates, SrIrO3, which is characterized by a small strain dependent gap near the U point in the Brillouin zone. We find that the gap energy has two distinct regions with a transition below and above T_0 = 110 K. In the final material system, I will discuss the development and characterization of bismuth-doped gallium arsenide for electronic materials applications, where ErAs nano islands can be used to control the absorption in the important 1550 nm telecommunications band.

Topological spin textures such as skyrmions and merons, have been proposed as the building blocks of a series of computational schemes and also for the development of memory elements. Here, we show that upon exfoliation layered ferromagnetic compounds like Fe5-xGeTe2 and Fe3GaTe2, which display topological spin textures, also show a pronounced topological Hall response due to their presence [1,2]. Remarkably, upon applying a magnetic field nearly along a planar direction, we observe a very large and hysteretic topological Hall at zero field. This implies that the magnetic field is writing topological spin textures which are detectable via a Hall voltage, paving the way for the possible development of topological spin texture based memory and computational schemes at room temperature. [1] Brian W. Casas et al., Adv. Mater. 35, 2212087 (2023). [2] Alex Moon et al., ACS Nano 18, 4216–4228 (2024)

The emergent two-dimensional (2D) layered magnets provide ideal platforms to enable the atomically thin magneto-optical and magnetoelectric devices. Though many have envisioned that 2D magnets should allow efficient control of magnetism by a variety of external stimuli, true breakthroughs are still lacking, with limited proof-of-concept demonstrations reported thus far. There appear to be fundamental obstacles for efficient control, e.g., through electrical and optical means. In this talk I will analyze the challenges and present our theoretical and experimental progress on efficient electrical and optical control of 2D magnets. Specifically, the results show that the voltage of a few volts can effectively change the magnetic anisotropy of 2D magnets and the laser shinning of tens of uW/um^2 can effectively affect the domain behaviors of 2D magnets. These efficient controls of 2D magnets potentially open up new avenues towards low-power spintronics and photonics.

The presented work discusses the challenges and advances in synthesizing thin films of topological Kondo insulator, samarium hexaboride (SmB6), and ferromagnetic manganese diselenide (MnxSey) by chemical vapor deposition (CVD) using powder precursors for applications in spintronics. Issues related to scalability, crystallinity, and grain size will be highlighted, which have implications for device performance. Samarium hexaboride thin films were synthesized by CVD using samarium chloride, SmCl3, and boron/boron oxide powders at 1075°C on p-type silicon with nickel nanoparticles acting as a catalyst for growth. achieving rough cube-like structures with an average grain size of ~1.3 μm. CVD growth of MnxSey on a sapphire substrate is obtained using Se and manganese chloride, MnCl2, powders under argon, and hydrogen. Integrating MnxSey with materials exhibiting strong spin-orbit coupling can lead to the emergence of topological spin textures or the realization of efficient spin manipulation mechanisms, crucial for developing next-generation spin-based electronics.

Fe3Ga4 is an exciting compound since the helical spin structure (HSS) results in a competition between two ground states - one ferromagnetic (FM) and one antiferromagnetic (AFM). Through this ground state competition, there are multiple magnetic transitions of interest, where one is a metamagnetic transition FM to AFM at 70 K, then an AFM to FM transition at 370 K. The HSS in Fe3Ga4 allows for the metallicity to be retained in the AFM state, which makes this compound a candidate for room temperature AFM spintronic applications and the helimagnetism can support topological skyrmion particles similar to MnSi and FeGe. Axis-dependent magnetoresistance and magnetic measurements will be presented that provide information on the exchange interactions of the spins with respect to field and temperature in single crystal Fe3Ga4. Through these results, important insights have been developed that further the understanding of the unique magnetic structure in Fe3Ga4.

In the field of in-memory computing for artificial intelligence (AI), the traditional reliance on analog memory for storing synaptic weights presents significant challenges, particularly when using Magnetic Tunnel Junctions (MTJs), which are limited by their inherently binary nature. However, the advent of Binarized Neural Networks (BNNs) has opened a new avenue for leveraging binary storage mechanisms, offering a promising solution for energy-constrained and miniaturized AI systems. This presentation unveils a fully integrated implementation of a BNN using 32k binary memristors. We detail the design and fabrication of our system and its operational excellence even under variable power conditions in an energy harvesting scenario, demonstrating its potential as a resilient and energy-frugal AI platform. While our current work employs hafnium oxide memristors, the design principles and architecture we propose are readily adaptable to MTJs, suggesting a seamless transition pathway to spintronic implementations in the future. We also compare our approach to other research spintronic research endeavors, to prove a comprehensive view of the field's direction.

Neural networks are increasing in scale and sophistication, catalyzing the need for efficient hardware. An inevitability when transferring neural networks to hardware is that non-idealities impact performance. Hardware-aware training, where non-idealities are accounted for during training is one way to recover performance, but at the cost of generality. In this work, we demonstrate a binary neural network consisting of an array of 20,000 magnetic tunnel junctions (MTJ) integrated on complementary metal-oxide-semiconductor (CMOS) chips. With 36 dies, we show that even a few defects can degrade the performance of neural networks. We demonstrate hardware-aware training and show that performance recovers close to ideal networks. We then introduce a robust method – statistics-aware training – that compensates for defects regardless of their specific configuration. When evaluated on the MNIST dataset, statistics-aware solutions differ from software-baselines by only 2 %. We quantify the sensitivity of networks trained with statistics-aware and conventional methods and demonstrate that the statistics-aware solution shows less sensitivity to defects when sampling the network loss function.

Superparamagnetic tunnel junctions (SMTJs) and spin-torque nano-oscillators (STNOs) show promise for use in energy-efficient unconventional computing schemes based on stochastic information encodings, operating from nanosecond to microsecond time scales. We demonstrate electrical coupling of SMTJs for emulating neuro-synaptic connections and leverage the phase dynamics of STNOs for innovative approaches to unbiased random number generation, with the potential to mimic fast stochastic binary neurons, paving the way for low-energy, hardware-based stochastic neural networks.

Quantum sensors based on isolated spin defects in diamond, most notably the nitrogen-vacancy (NV) center, have emerged as a new tool for investigating nanoscale magnetism with nanotesla sensitivity and sub-50-nm spatial resolution. Quantum sensors bring some unique features to the table, including non-invasiveness, sensitivity to multiple physical parameters (such as magnetic fields, electric fields or currents), and an absolute quantitative calibration based on natural constants. In this talk, I will introduce the basic concepts and emerging application of the quantum scanning microscope (QSM). After a brief overview of the technology, I will focus on some illustrative examples of application in nanoscale magnetism, including the imaging of domains, domain walls and spin textures in antiferromagnets, the imaging of electric fields from ferroelectrics and multiferroics, and the imaging of current flow in two-dimensional conductors. The talk will conclude with an outlook on future developments and applications of diamond quantum sensors.

Here we propose the use of magnon modes of a magnetic material for entangling spin centers (e.g,, NV-centers) over long distances [1,2]. We first predict the strong coupling between NV-magnon with corresponding cooperativity exceeding unity for different geometries, including ferromagnetic bars, waveguides, and cylindrical structures [1,2]. Secondly, we assess different entangling protocols and their suitability for generating entangled states under realistic experimental conditions. Furthermore, we experimentally determine the magnon-induced NV-NV interaction by combining longitudinal relaxometry measurements with both the fluctuation-dissipation theorem and Kramer-Kronig relations [3]. [1] D. R. Candido, G. D. Fuchs, E. Johnston-Halperin, and M. E. Flatte, Materials for Quantum Technology 1, 011001 (2021). [2] M. Fukami, D. R. Candido, D. D. Awschalom, and M. E. Flatte, PRX Quantum 2, 040314 (2021). [3] M. Fukami, J. C. Marcks, D. R. Candido, L. R. Weiss, B. Soloway, S. E. Sullivan, N. Delegan, F. J. Heremans, M. E. Flatté and D. D. Awschalom, Proc. Natl. Acad. Sci. U.S.A. 121, e2313754120 (2024).

Magnetometers are not only useful for Earth-based applications, but also very important for planetary science and heliophysics investigations. This is the reason they are flown on almost all missions in space. Heritage fluxgate and optically-pumped atomic gas magnetometers are typically large in size, weight, and require watts of power to operate, preventing their infusion onto smaller platforms like CubeSats, drones, landers, and rovers. Here, we report on the development of a 4H silicon carbide (SiC) magnetometer, promising to be a low complexity, lightweight, low power, and inexpensive alternative to these heritage technologies. It measures magnetic field induced changes in spin dependent recombination (SDR) current within a pn junction, both in vector and scalar modes, thereby giving the instrument the ability to self-calibrate in the remoteness of space.

Reservoir computing is a neuromorphic architecture that may offer viable solutions to the growing energy costs of machine learning. In software-based machine learning, computing performance can be readily reconfigured to suit different computational tasks by tuning hyperparameters. This critical functionality is missing in ‘physical’ reservoir computing schemes that exploit nonlinear and history-dependent responses of physical systems for data processing. Here we overcome this issue with a ‘task-adaptive’ approach to physical reservoir computing. By leveraging a thermodynamical phase space to reconfigure key reservoir properties, we optimize computational performance across a diverse task set. We use the spin-wave spectra of the chiral magnet Cu2OSeO3 that hosts skyrmion, conical and helical magnetic phases, providing on-demand access to different computational reservoir responses. The task-adaptive approach is applicable to a wide variety of physical systems, which we show in other chiral magnets via above (and near) room-temperature demonstrations in Co8.5Zn8.5Mn3 (and FeGe).

Weyl semimetals exhibit quantum phenomena like the anomalous Hall effect and the chiral anomaly, with tangible macroscopic effects. These arise from breaking inverse or time-reversal symmetry, segregating Weyl points based on momentum or energy. Near these points, fermionic quasiparticles show opposing chirality, leading to non-trivial topological phases that persist even at temperatures above 77 Kelvin. Experimental evidence supports these phenomena, such as anomalous Hall conductivity in materials like Co3Sn2S2 and magneto-optical measurements in Cd3As2. We investigate the magneto-optically induced chiral anomaly effect in Co3Sn2S2, with potential applications in chiral-sensitive sensors akin to FETs. Unlike conventional ARPES techniques, our method operates at higher temperatures and tracks behavioral changes under external magnetic fields. Furthermore, Weyl semimetals exhibit broad responses to low-energy photons, particularly in the infrared spectrum, effectively exciting electrons within the Weyl cone structure.

In solid-state systems spin-orbit coupling (SOC) is a friend and foe[1]. While SOC is responsible for the loss of information and spin depolarization, it is also crucial for transferring spin between different systems[2]. We focus on two SOC manifestations. (i) Electric dipole spin resonance in proximitized Dirac material where we reveal an overlooked resonant spin-pseudospin coupling responsible for a huge increase of THz absorption, explained by coupled spin-pseudospin torques[3], (ii) Spin-orbit torque magnetization switching, which allows to electrically reverse the helicity of the emitted light from III-V quantum dots at 300 K and zero applied magnetic field[4]. We discuss the implications of these findings, from elucidating hidden proximity effects to establishing a missing link between photonics, electronics, and spintronics[5,6]. 1. I. Žutić, H. Dery, Nat. Mater. 10, 647 (2011) 2. I. Žutić, J. Fabian, S. Das Sarma, RMP 76, 323 (2004) 3. K. S. Denisov, I. V. Rozhansky, S. O. Valenzuela, I. Žutić, preprint 4. P. A. Dainone et al., Nature 627, 783 (2024) 5. I. Žutić et al., Proximitized Materials, Mater. Today 22, 85 (2019) 6. M. Lindemann et al., Nature 568, 212 (2019)

We have recently shown that the emission spectrum of spintronic THz emitters (STE) is largely influenced by charge dynamics that result from charging and discharging effects. This effect is present in any large area STE. In recent experiments, we have demonstrated that the current backflow can be controlled by using arrays of patterned STE with each of the emitters being smaller than the laser spot. We show that by using differently sized respective emitters we can modify especially the lower part of the spectrum and are able to suppress the emission of spectral components lower than 4 THz while elongated emitters allow us to extend the spectrum even to lower frequencies. The experimental results can be reproduced by an analytical model based on the emission of a large area emitter. Without any fitting parameters, we can model the emission of STE of various sizes with surprising accuracy. On the other hand, arrays of linear emitters can be used to emit a sequence of pulses as a kind of grating in time. These allow us to emit narrow band THz radiation as a THz burst with an arbitrary fundamental frequency, in our case up to 20 THz.

The booming fields of antiferromagnetic spintronics and THz magnonics urge to understand the ultrafast dynamics triggered in antiferromagnets by ultrashort stimuli. The interest in ultrafast magnetism of antiferromagnets has led to new and vastly counter-intuitive findings in experimental and theoretical research. We report on the ultrafast dynamics in the poorly explored, non-collinear phase of rutile antiferromagnets where sublattice symmetry is broken and spins are canted. In particular, we show that by tuning the spin canting, one can create the Fermi resonance condition in CoF2 between otherwise non-interacting magnon fm and phonon fph modes at fph=2fm. This new regime of strongly coupled dynamics is accompanied by a mutual, nontrivial energy exchange and represents an important milestone for coherent control in magnonics and phononics.

Spin-charge conversion (SCC) in topological insulators (TI) is expected to be more efficient than SCC in heavy metals. Here, we demonstrate enhanced THz emission in NiFe/Bi2Se3 heterostructures. Compared to W/NiFe/Pt films, we report an order of magnitude increase in THz amplitude, suggesting that spin-momentum locking is more efficient than the inverse spin hall effect in converting spin to charge. Furthermore, comparing bulk-insulating Bi2Se3 and bulk-conducting Bi2Se3, we find that the majority of the SCC occurs at the surface of the TI rather than in the bulk.

Over the past decade, terahertz electromagnetic waves with sub-millimeter wavelength (30 µm to 3 mm) have attracted much attention due to the wide range of applications in medical and industrial fields. Magnetic thin film heterostructures (spintronic emitters) consisting of ferromagnet and nonmagnetic metal layers have recently emerged as broadband THz sources that may have advantages over conventional sources such as photoconductive antennas (PCA) and nonlinear crystals. Here, we demonstrate how to use spintronic emitters patterned into microstructures of different geometries and dimensions to modify the characteristic electromagnetic spectrum of the emitted THz waves. Furthermore, we theoretically explore the possibility of combining emergent spintronic emitters with conventional PCA as hybrid emitters for THz pulse shaping and controlling the handedness of the polarization of THz waves. This new method takes advantage of the broad bandwidth of spintronic emitters and high-intensity THz emission at the low frequency of PCAs. These modelings are confirmed by preliminary experiments. Our results are beneficial to the production of functional THz devices with tunable parameters.

The capability of magnons to hybridize and strongly couple with diverse excitations offers a promising avenue for realizing and controlling emergent properties that hold significant potential for applications in devices, circuits, and information processing. In this talk, we present recent theoretical developments in magnon-based hybrid systems, focusing on the combination of magnon excitation in an antiferromagnet with other excitations, namely plasmons in a topological insulator, phonons in a 2D AFM. We explore several directions to advance magnon hybrid systems, including strong coupling between a surface plasmon and magnon polariton in a TI/AFM bilayer and a giant spin Nernst effect induced by magnon phonon coupling in 2D AFMs. These examples highlight the potential of magnon-based hybrid systems for advancing device and information processing technologies and the importance of both understanding and controlling material properties and interactions to realize such technologies. References: [1] Phys. Rev. B 108, 085435 (2023) [2] Phys. Rev. Materials 7, 045201 (2023) [3] Phys. Rev. Materials 6, 085201 (2022) [4] Appl. Phys. Lett. 124, 000000 (2024)

The development of ultrafast spintronic devices calls for approaches to generate ultrafast spin current. The currently available approaches rely on the bulk properties and require specific crystal orientations or large magnetic fields for antiferromagnets. Here, we show a picosecond spin current generation from the vicinal metal|Cr2O3 interfaces by an interfacial nonlinear magnetic-dipole (MD) transition at zero field. Our theoretical model reveals our findings caused by the in-plane inversion symmetry breaking. Our work demonstrates that ultrafast spin current can be generated via the nonlinear MD transition by manipulating the symmetry and opens new opportunities in antiferromagnetic and ultrafast spintronics.

Traditionally, magnetic solids are divided into two major classes – ferro and antiferromagnets. Recently, it was realized that this division is incomplete and needs to be complemented with the third class called altermagnets. Owing to their unique properties, combining antiferromagnetic order with phenomena typical for ferromagnets, altermagnets are believed to hold a great potential for spintronics and magnonics. Here we demonstrate a new functionality of altermagnets for magnonics operating at THz clock-rates.

Electric fields operating at THz frequencies hold significant promise for inducing ultrafast coherent excitations in magnetic heterostructures. Through the utilization of ferromagnetic/heavy metal (FM/HM) heterostructures, we have demonstrated that THz radiation (0.1 – 30 THz) exhibits combined functionality of microwaves and visible light. 1) Similar to microwaves, THz fields can effectively generate spin currents through the spin-Hall effect (SHE), resulting in an excitation of THz-frequency magnon modes. 2) Akin to visible light excitation, THz fields deposit heat, leading to the demagnetization of FM layers. Harnessing the THz-induced demagnetization as a spin current source within FM/HM heterostructures, we exploit the half-cycle THz electric field to incite spin currents, which subsequently transformed into picosecond charge currents through the inverse SHE within the HM layer. This conversion process results in the emission of a THz second harmonic signal, offering the THz spintronic frequency conversion.

THz signals can be generated from photoconductive antennas, nonlinear crystals, air plasma, and spintronic devices. Among these various THz sources, only the spintronic emitter, typically constructed with ferromagnet and heavy metal heterostructures offers a way to generate a gapless broadband emission spectrum from 0.3 THz to 20 THz with low joule heating. We show that ferromagnet/ultrawide bandgap (UWBG) semiconductor heterostructures are superior to traditional ferromagnet/heavy metal heterostructures as the platform for spintronic THz sources, as they simultaneously allow broadband THz emission, tunability, and compatibility with existing integrated circuit (IC) platforms. Furthermore, by patterning the heterostructure into a photonic crystal with arrays of subwavelength pillars, transmission enhancement, wavelength selectivity, and radiation pattern engineering of emitted THz signal can be achieved. We believe this work offers an IC-compatible solution for broadband and tailorable THz generation.

We have studied the magnon-phonon coupling constant in magnetic thin film heterostructures. Surface acoustic waves (SAW) are used to excite acoustic phonons in the heterostructures. To determine the magnon-phonon coupling constant, transport and SAW transmission measurements are employed. The effect of the materials, film stacking and interface on the coupling constant will be discussed.

Moiré superlattices formed by stacking and twisting transition metal dichalcogenide semiconductor layers have shown a plethora of strongly correlated, topological and magnetic phenomena, including correlated insulators, integer and fractional quantum anomalous Hall effect, and generalized Wigner crystals. Almost all of these strongly correlated phases exhibit non-trivial magnetic properties, including ferromagnetism, antiferro-magnetism, and spin-density wave. Here we investigate the collective excitations of these strongly correlated magnetic phases. The collective spin(valley) excitations, i.e. magnons, exhibit rich topological structures in their bandstructure, which can be tuned directly using a perpendicular electric field. These tunability may find potential applications in future magnonic or spintronic devices.

Magnons can serve as information carriers and for signal transmission in nanoscale on-chip microwave electronics operating up to ultrahigh frequencies. They allow for in particular charge-free signal processing and logic in free-form 3D nanoarchitectures. However experiments on magnons in 3D nanoarchitectures are at their infancy due to a technology gap in large-scale nanofabrication. We have optimized the industrially relevant atomic layer deposition (ALD) of ferromagnetic metals and achieved conformal coatings consisting of either Ni or Ni80Fe20 with unprecedented qualities. They allow us to produce free-form 3D ferromagnetic nanostructures consisting of individual or interconnected tubular ferromagnetic shells which are curved in all three spatial directions. We produce 3D nanostructures with large footprints on either semiconductor nanotemplates or polymeric networks produced by two-photon lithography. We report on the local spectroscopy and micromagnetic simulations addressing the distinct magnon modes that we observe in tubes, screws and 3D woodpile nanoarchitectures consisting of either Ni or Ni80Fe20 shells.

Across many research fields, engineered local variations in physical properties have yielded groundbreaking material functionalities that play a crucial role in enabling future technologies. Using lithography tools initially developed for 2.5D patterning of photoresist surfaces, we have recently shown that direct-write laser annealing can be used to create arbitrarily shaped magnetic potential energy landscapes in numerous application-relevant magnetic thin-film systems. Particularly, we have created continuous variations in the magnetic compensation temperature of ferrimagnets, the interlayer coupling strength of synthetic antiferromagnets, and the magnetic anisotropy of ferromagnets and synthetic antiferromagnets. As this direct-write approach does not require patterned resist layers or ultrahigh vacuum environments, the process is significantly streamlined compared to other techniques used to locally modify materials. We envisage that the precise control of physical properties in complex patterns enabled by direct-write laser annealing is relevant not only for applications in magnetism but also for modifying any thin film whose properties change in response to heat treatment.

Quantum information science (QIS) represents an emerging paradigm with the potential to revolutionize a diverse range of scientific fields. To advance efforts within QIS, we have synthesized and tuned f-element polyoxometalate (POM) complexes so that they can serve as quantum bits (qubits) for QIS applications. Our specific focus is on systems featuring atomic clock transitions and here results will be presented detailing how changing first and second sphere metal cations affect the ground state wavefunctions as well as the frequency and accessibility of clock transitions of analogous complexes. Each system has been explored structurally using X-ray diffraction and vibrational and EPR spectroscopy results for lanthanide and actinide POM complexes will be highlighted as well.

In spin-encoded quantum information, a spin of magnitude s gives access to a (2s+1)-dimensional qudit. (In particular, s=1/2 yields a qubit). Quantum optics writ large as the quantum processing of boson fields has provided convenient experimental ways to generate effective, or "synthetic," spins by way of mathematical equivalences such as the Holstein-Primakoff transformation, which maps 1 boson field to 1 large, "linearized" spin, and the Schwinger transformation, which maps 2 boson fields to 1 spin. The latter is of particular interest as it also maps bijectively with two-mode Fock states, which have non-Gaussian Wigner functions and therefore enable, in principle, universal, fast-tolerant photonic quantum computing. The recent coming of age of high-efficiency photon-number-resolved detectors provided the missing experimental resource to work with Schwinger spins. In this talk, I will describe progress towards experimentally violating Mermin's inequality (the s>1/2 version of the Bell inequality) using superconducting transition edge sensors in our quantum optics laboratory.

Strongly-interacting nanomagnetic arrays are ideal systems for exploring the frontiers of magnonic control. They provide functional reconfigurable platforms and attractive technological solutions across storage, GHz communications and neuromorphic computing. Typically, these systems are primarily constrained by their range of accessible states and the strength of magnon coupling phenomena. Increasingly, nanomagnetism has explored the benefits of expanding into 3D. This has broadened the horizons of magnetic microstate spaces and functional behaviours, but precise control of 3D states and dynamics remains challenging. Here, we introduce a 3D magnonic metamaterial compatible with widely-available fabrication and characterisation techniques. By combining independently-programmable artificial spin-systems strongly coupled in the z-plane, we create a system with a rich 16^N microstate space and intense static and dynamic dipolar magnetic coupling. The system exhibits a broad range of emergent phenomena including ultrastrong magnon-magnon coupling with normalised coupling rates of Δω/γ=0.57, GHz mode shifts in zero applied field and reconfigurable generation of magnon frequency combs

Ranging from quantum-computing to sensing applications devices, the control of spin-degree of freedom has been highly desirable in semiconductors physics. In III-V semiconductors (e.g., in GaAs or InSb) this could be achieved with manganese impurities. Manganese forms a complex, where a hole from the host aligns antiferromagnetically with the five half spin of the 3d5 manganese core [1]. In our work, based on previous ESR measurements [1], we show a new pathway to treat the manganese core fully quantum-mechanically and, using an analytical treatment from the effective mass approximation [2], we suggest a coherent manipulation of the spatial structure of a single manganese in bulk III-V semiconductors. [1] J. Schneider, et al., Phys. Rev. Lett. 59, 240 (1997). [2] A. M. Yakunin, et al., Phys. Rev. Lett.92, 216806 (2004). This project has received funding from the European Union’sHorizon 2020 research and innovation programme under the MarieSkłodowska-Curie grant agreement No 956548.

We show that a new analytical technique based primarily on the principles of electron paramagnetic resonance can be utilized to explore the physical and chemical nature of paramagnetic centers in semiconductors, insulators, and solid-state electronics. This new technique, near zero field magnetoresistance (NZFMR) spectroscopy, has substantial potential in various spintronics applications.

Cavity magnonic systems are ideally suited to explore the range of possibilities opened by tailoring the interactions between photons, phonons, and magnons. On the one hand, the radiation pressure-like coupling between magnons and phonons in magnets can modify the phonon frequency (magnomechanical spring effect) and decay rate (magnomechanical decay) via dynamical backaction. Such effects have been recently observed by coupling the uniform magnon mode of a magnetic sphere (the Kittel mode) to a microwave cavity. In particular, the ability to evade backaction effects has been recently demonstrated, which is a requisite for applications such as magnomechanical based thermometry. On the other hand, a magnomechanical system can be tailored such that a phonon and a magnon mode hybridize, forming a magnon-phonon polariton. This regime can be useful for transduction of information harnessing the tunability of the system and the characteristics of both collective excitations.

Magneto-resistive Random Access Memory (MRAM) serves as a reliable and robust non-volatile memory (NVM) for space data system memory and data storage applications. Such MRAMs use Magnetic Tunnel Junction (MTJ) bits in an MRAM Back-End-of-Line process integrated with radiation hardened CMOS transistors and metallization. High reliability and robust MRAMs for space applications result when holistic design practices are applied to technology developed holistically for space and radiation-hardened applications. Such MRAMs are radiation hardened for robust operation in radiation environments and also offer high levels of write and read cycling endurance, long data retention time, low error rate, wide operating temperature range, and long lifetimes over specified conditions. MRAMs have been qualified for space and QML applications including to MIL-PRF-38535 and MIL-STD-883 standards. Physical processes, principles of operation, and qualification methods and considerations will be described for high-reliability and robust MRAMs for space and radiation-hardened applications.

We will describe the latest spin-transfer torque magnetoresistive random access memory (STT-MRAM) product family from Everspin Technologies, which is aimed at industrial applications requiring low-latency, high-speed, low bit error rate, and high reliability. The STT-MRAM is built on GlobalFoundries’ 28nm CMOS, uses an expanded SPI (xSPI) interface, and is offered in densities from 4Mb to 128Mb. We describe how continuous process improvements to the magnetic tunnel junctions enabled the required performance in switching speed and bit error rate. We further demonstrate robust operation of the product dies over a full temperature range from -40°C to 105°C as well as their reliability performance in terms of the data retention against high temperature disturbs, the write cycling endurance, and the immunity to in-plane and perpendicular applied magnetic fields.

Spintronic nanodevices are a prime candidate for the implementation of neuromorphic computing systems. In particular magnetic tunnel junctions (MTJs) are multifunctional nanodevices compatible with CMOS processing are used as magnetic field sensors, non-volatile memories, non-linear oscillators, and rectifiers. We developed a multifunctional MTJ stack to fabricate various MTJs types in a single process. We present an experimental proof of principle of the weighted spin torque nano-oscillator (WSTNO ) consisting of three MTJ nanopillars. Two non-volatile memories weight the input, which is non-linearly transferred by one oscillator into an output signal. The STNOs have an output power above 1 µW and frequencies of 240 MHz. This WSTNO is a programmable building block for future neuromorphic computing systems. Other non-volatile effects in the MTJ are explored to enable for example non-volatile tuning of the resonance frequency. In this context, optical access to the nanodevices is a highly relevant and challenging topic which we address.

Berry curvature is the key physical trait in topological quantum materials (TQMs). Indeed, a wide variety of TQMs have been discovered and rise of the era of TQM was achieved. Among various topological features appearing in TQMs, the Berry curvature dipole (BCD) is a significant and intriguing phenomenon that requests inversion symmetry breaking of the topological systems. Meanwhile, despite the attractiveness of the BCD, the material stages for the BCD are still limited and the temperature range for it is far below room temperature (RT). Furthermore, nonvolatile, i.e., ferroic BCDs have not yet discovered in spite of its prediction in theory [1]. In this presentation, we introduce the successful demonstration of detection of the ferroic BCD at RT in a topological crystalline insulator, PbSnTe [2]. The nonlinear Hall effect is a good probe for the BCD. The magnitude of the BCD largely exceeds that of for example transition metal dichalcogenides. The detailed physics will be discussed in the presentation. References: [1] I. Sodemann and L. Fu, Phys. Rev. Lett. 115, 216806 (2015). [2] T. Nishijima, M. Shiraishi et al., Nano Lett. 23, 2247 (2023).

We report symmetry-breaking electronic orders tunable by an applied magnetic field in a model antiferromagnetic Kagome magnet FeSn, consisting of alternating stacks of 2D Fe3Sn Kagome and Sn2 honeycomb layers. On Fe3Sn terminated films epitaxially grown on SrTiO3(111) substrates, we observe trimerization of the Kagome lattice by scanning tunneling microscopy/spectroscopy, breaking its six-fold rotational symmetry while preserving the translational symmetry. Such a trimerized Kagome lattice shows an energy-dependent contrast reversal in dI/dV maps, as well as stripe modulations that are energy-dependent and tunable by an applied in-plane magnetic field. These results provide the first direct experimental evidence for symmetry-breaking nematicity arising from the entangled magnetic and charge degrees of freedom in the ground state of Kagome magnet FeSn.

Symmetry has been used for symmetry classification and material discovery across multiple fields, and enables the study of coupling effects among various degrees of freedom. Here, we introduce the studies of antiferromagnetic (AFM) Chern insulators and spin textures through symmetry design. We provide design principles by symmetry analysis for AFM Chern insulators and corresponding material candidates. For spin textures, we provide a comprehensive symmetry characterization of spin textures and present several novel spin textures. Our symmetry design provides the foundation for AFM Chern insulators and novel spin polarization effects, motivating the symmetry studies on other physical phases and effects.

We demonstrate the generation of orbital currents and orbital torque by the orbital Hall effect in metallic bilayers. We show that orbital currents propagate over longer distances than spin currents. Our results also show that the orbital current enables electric manipulation of magnetization through orbital torques. We also demonstrate the reciprocal effect of the orbital torque: the orbital pumping.

The transport of orbital current, characterizing the orbital properties of Bloch states in solids, exhibits superior ballistic travel capabilities with a larger coherence length across a diverse range of materials compared to its spin counterpart. This characteristic facilitates robust, high-density, and energy-efficient information transmission. Consequently, control of orbital transport becomes significant for quantum information technology. In contrast to spin angular momentum, orbital angular momentum efficiently couples to phonon angular momentum (PAM) through orbital-crystal momentum (L-k) coupling. This coupling mechanism provides an avenue for controlling orbital transport via angular momentum transfer mediated by crystal field potential. Leveraging the efficient L-k coupling dependent on orbital properties, our experimental endeavors have successfully demonstrated the active control of orbital current velocity using THz emission spectroscopy. Key findings of our study include the identification of a critical energy density essential for overcoming collisions in orbital transport, thereby enabling a more rapid flow of orbital current.

Recent works provide evidence for a large orbital Rashba-Edelstein effect at the interface between Cu and its oxide. Here, we experimentally demonstrate that a very large enhancement of both the net torque and the spin-pumping voltage (up to a factor of 2) can be obtained with the insertion of a Pt layer whose large spin-orbit coupling helps to convert a pure orbital current into a spin current. These two reciprocal phenomena, observed simultaneously for the first time in the same Co/Pt/Cu/CuOx samples, are in agreement and their orbital angular momentum nature associated to a charge-to-orbit (orbit-to-charge) conversion at the Cu/CuOx interface constitutes a robust interpretation. To disentangle spin and orbital currents in these systems, we also measure the ferromagnet thickness dependence of the net torques, and observe a clear increase of the corresponding dephasing length, indicating the contribution of pure orbital currents acting on the magnetization. From the Cu thickness dependence, we also verify that the conversion occurs at the Cu/CuOx interface through the orbital Rashba effect as observed in both torque and spin-pumping measurements.

All the known Hall effects of magnons require certain kinds of spin-orbit coupling such as the Dzyaloshinskii-Moriya interaction. It is an open question of fundamental significance whether nature permits magnons to exhibit a Hall effect without spin-orbit coupling. By tackling this question, we predict a new type of an intrinsic Hall effect of magnons that requires no spin-orbit coupling, namely the magnon orbital Hall effect, in a honeycomb antiferromagnet, and thereby opens a new field of magnon orbitronics, where the hitherto-neglected magnon orbitals play central roles.

Orbital dynamics in time-reversal-symmetric centrosymmetric systems is examined theoretically. We demonstrate that many aspects of orbital dynamics are qualitatively different from spin dynamics because the algebraic properties of the orbital and spin angular momentum operators are different. We demonstrate the differences between the orbital and spin in terms of their Hamiltonians, Hall effects, relaxation dynamics, and pumping.

We introduce a tight-binding theory of spin-correlated dissipationless circulating currents in 2D systems. In host materials with strong spin-orbit coupling, the unquenching of the orbital angular momentum gives rise to finite ground state current densities induced by paramagnetic spin states. In contrast to the spin part, the orbital magnetic moment is still surprisingly not well understood. Here we use our real-space formalism of the discrete current operator to elucidate spatial anisotropies of the short-range orbital magnetic moments impacting spin-spin coupling, g-tensors and hyperfine interaction calculations of defects. The shape and spatial extent of these dissipationless circulating currents are greatly affected by the spatial symmetry of the spin-orbit fields, offering significant opportunities for manipulating nanoscale magnetic fields and coupling magnetic defects through electric gate control. Furthermore, the fringing magnetic field emerging from the current can provide a direct way to measure the spin-orbit fields of the host, as well as the defect spin orientation, through scanning nanoscale magnetometry

The orbital magnetic moment (OMM) of Bloch electrons has come under renewed scrutiny recently as part of a general effort to understand angular momentum dynamics in systems in which spin-orbit interactions are absent or negligible. I will present two recent results from our group. The first is related to the orbital Hall effect [1]. We have determined the full OHE in the presence of short-range disorder using 2D massive Dirac fermions as a prototype. We find that, in doped systems, extrinsic effects (skew scattering and side jump) provide ≈95% of the OHE. This suggests that, at experimentally relevant transport densities, the OHE is primarily extrinsic. In the second part I will show that the OMM is in general not conserved in an electric field. The force moment produces a torque on the OMM, which is determined by the quantum geometric tensor and the group velocities of Bloch bands. The torque vanishes in two-band systems with particle-hole symmetry, but is nonzero otherwise. For tilted massive Dirac fermions the torque is determined by the magnitude and direction of the tilt. 1. Hong Liu and D. Culcer, arXiv:2308.14878. 2. R. Burgos Atencia and D. Culcer, arXiv:2311.12108.

Electrical switching of magnetization is central to spintronic memory and computing. Despite the enormous investigations in the past two decades, the understanding of in-plane current-induced switching of perpendicular magnetization remains elusive as indicated by a number of remarkable long-standing puzzles (see [1,2] for reviews of this problem in thin-film and van der Waals systems). This talk will discuss the recent discoveries of two novel sin-orbit effects for magnetization manipulation, i.e., strong variation of the spin-orbit torque with the relative spin relaxation rate of the magnetic layer [3] and the long-range Dzyaloshinskii-Moriya interaction effect [4]. [1] L. Zhu, Adv. Mater. 35, 2300853 (2023). [2] X. Lin, L. Zhu, Materials Today Electronics 4, 100037 (2023). [3] L. Zhu, D. C. Ralph, Nat. Commun. 14, 1778 (2023). [4] Q. Liu, L. Zhu, et al., Nat. Commun. (under review).

The orbital Hall effect (OHE) has recently garnered tremendous attention as a promising approach to realize novel “orbitronic” devices that can be superior to spintronics ones. However, many fundamental questions concerning the physical picture behind apparently robust experimental results are still wide open. For example, the role of the omnipresent disorder, which is well-known to be very important for the spin Hall effect, has hardly been discussed yet. Here we assess the orbital Hall conductivity (OHC) in the presence of disorder by a quantum Boltzmann equation that takes into account electric field-induced interband coherence and defect scatterings on equal footing. We find that the OHC strongly depends on the details of the orbital character and the disorder. Depending on the specific orbital textures, diffuse scattering by an arbitrarily weak disorder can affect and even fully suppress an intrinsic orbital Hall current. Our work sheds doubt on whether most published theories of the OHE that completely neglect the disorder can explain experiments.

In quantum information science, superconducting quantum circuits, like qubits, have emerged as a useful platform for information processing. Significant progress has been made in extending the coherence time of these qubits, but further advances are required to achieve scalable quantum computing. Coherence time is often limited by loss from two-level systems and excess quasiparticles that arise at surfaces and interfaces as a result of materials’ defects, fabrication processes, and ambient exposure. Our recent efforts to address these loss sources have utilized a range of strategies, including surface encapsulation, substrate preparation methods, modification of the metal film growth, and new processes during fabrication. However, these methods typically address one surface or interface at a time. Here we examine the interplay of different surfaces and interfaces and the knock-on effects that these approaches have on the superconducting device as a whole. This work is supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under contract No. DE-AC02-07CH11359.

In this talk, I will present recent results where we demonstrate extreme magnon nonlinearities that couple to excitons in the two-dimensional magnet, CrSBr. I will show how we can reach the perturbative nonlinear regime and see low-order harmonic generation. I will extend these results by tuning a symmetry breaking magnetic field to induce nonlinear coupling of discrete magnon modes, resulting in sum- and difference frequency generations. I will further show the precise control we have by tuning these modes to induce parametric application of magnon signals. Finally, I will show how we can enter the non-perturbative nonlinear regime and generate high harmonic magnons past twentieth order.

Quantum technologies leverage the properties of quantum mechanics to solve grand challenges in STEM. For instance, single-photon detectors (SPD) are used to detect extremely faint light, like in the James Webb Space Telescope to observe the early universe. However, state-of-the-art SPDs function by the excitation of quasiparticles above an energy gap, setting a fundamental limit that severely limits single-photon detection of low energy photons (~mid-IR and beyond). In this talk, I will introduce a novel approach to SPD which detects the heat generated by a single telecom photon through the temperature rise of electrons in graphene. This was realized by utilizing a graphene Josephson junction SPD that marries the best of both worlds: the zero bandgap, weak electron-phonon coupling and vanishingly small heat capacity of monolayer graphene, and the electron thermometry provided by a superconductor-graphene-superconductor Josephson junction. I will conclude with a brief outlook on ways to extend the capabilities of the graphene Josephson junction SPD toward unprecedented low energy regimes with the goal of detecting single mid-IR, THz, and microwave photons.

We fabricated and characterized a vertical spin valve based on single-crystalline semiconductor nanomembranes engineered with 2D arrays of screw dislocations throughout their thickness. The device includes a bottom soft ferromagnetic contact (NiFe), semiconductors nanomembranes hosting screw dislocations, and a top hard ferromagnetic contact (Co). A change in the longitudinal magnetoresistance at the measured coercivity of NiFe was detected at 300 K solely in the devices embedding screw dislocations, suggesting that the line defects are responsible for the observed spin valve effect.

InAsxP1-x alloys are important for device applications since the band gaps can vary from 0.36 eV (InAs) to 1.35 eV (InP). In addition InAsxP1-x offers a wide tunability of g-factor ranging from 1.2 for InP to -14.75 for InAs. In this work, we experimentally and theoretically studied the magneto-optical properties of InAsxP1−x films at room temperature and ultrahigh magnetic fields (up to 120 T). We investigated how the effective masses and g-factors change with: 1) quantum confinement, 2) alloying, and 3) magnetic field. Our experimental and theoretical results showing that a g-factor close to zero, important for quantum communication devices, can be achieved in InAsxP1−x with the right alloy concentration x slightly less than 0.34. This work was supported by the AFOSR through grant FA9550-17-1-0341.

Single heavy hole spin in semiconductor quantum dots can be controlled and manipulated by applying electric fields, magnetic fields, and the lateral dot size for designing next-generation solid-state optoelectronic devices. In this presentation and manuscript, we report our findings on the variation of Landé g-factor and spin-flip behavior of III-V semiconductor heavy hole spins in quantum dots mediated by phonon. More precisely, we calculate the influence of spin-orbit coupling on the spin-flip rate of heavy holes in both isotropic and anisotropic quantum dots. In III-V semiconductor dots, spin-orbit coupling is dominated by the Rashba spin-orbit coupling due to structural inversion symmetry and the Dresselhaus spin-orbit coupling due to bulk inversion symmetry. Here we discuss the influence of the Rashba and Dresselhaus spin-orbit coupling effect on the isotropic and anisotropic behaviors of heavy hole spin hot-spot in quantum dots. Funding supports: Northwest Missouri State University (SP) and National Science Foundation PHY-2110318 (HSC)

GaN-based semiconductors attract much attention owing to the theoretically high Curie temperature of GaN-based diluted magnetic semiconductors and novel spin-orbit coupling(SOC) properties due to the strong polarization electric field. To overcome the conductance mismatch issue in a two-dimensional electron gas (2DEG) system, we take an ultrathin AlN layer at the hetero-interface as a barrier to form high-quality 2DEG in the triangular quantum well and a tunneling barrier for the spin injection. As for spin relaxation, owing to the canceled spin–orbit coupling (SOC), the spin relaxation time as long as 311 ps in InGaN/GaN multiple quantum wells is obtained at room temperature, being much longer than that in bulk GaN. Further, spin-polarized carrier transfer and spin relaxation processes in 2DEG of the InGaN/GaN QW were investigated by photon-energy dependent TRKR measurements.

3D topological insulators (TIs) such as Bi2Se3 can be utilized to switch ferromagnets via spin-orbit torque (SOT). Here we compare magnetization switching of NiFe from MBE-grown bulk-insulating Bi2Se3 on novel BiInSe/In2Se3 buffer layers, where surface states dominate transport up to RT, to that of conventional bulk-conducting Bi2Se3, and found significant reduction in critical current density and enhancement in SOT efficiency. We further fabricate vdW heterostructures with exfoliated 2D ferromagnet Fe3GeTe2 with perpendicular anisotropy (PMA), and demonstrate magnetization switching with record low critical current density and a large SOT efficiency. DFT calculations reveal weak interlayer interactions lead to a weakened interfacial dipole, suppressing proximity induced magnetic moment on Bi2Se3, preserving its spin texture. Our results highlight the clear advantage of bulk-insulating TIs and all-vdW heterostructures in enhancing SOT efficiency and minimize critical current density, an important step towards realizing next generation low-power non-volatile memory and spintronic devices.

Breaking of spatial inversion symmetry has profound effects on the electronic properties of materials. One prominent manifestation of such effects of much recent interest is chirality-induced spin selectivity (CISS), where real-space structural chirality induces spin polarization of electrons injected from a nonmagnetic normal metal electrode. CISS has been reported in a variety of chiral molecules and inorganic chiral crystals, however, definitive understanding of its physical origin remains elusive. We have studied the CISS effect through measurements of spin-selective transport in chiral molecular junctions on magnetic (GaMnAs) and nonmagnetic (n-GaAs) semiconductors. The robust semiconductor-based device platform enabled a rigorous examination of the bias-dependence of the CISS effect and revealed a key role of the spin-orbit coupling in the normal metal electrode. Our experiments have provided new insights on CISS and demonstrated its potential for enabling semiconductor spintronics free of any magnetic materials.

Topological insulators are quantum materials characterized by Time-reversal protected surface states (TSS) which make them appealing candidates for the design of next generation of highly efficient spintronic devices. The very recent demonstration of large transient spin-charge conversion (SCC) and subsequent powerful THz emission from Co|Bi1-xSbx bilayers clearly demonstrate such potentiality and feasibility for the near future. Amongst the exotic properties appearing in and at the surface of such quantum materials, spin-momentum locking (SML) remains as a key ingredient to effectively convert the spin degree of freedom into a charge or a voltage signal. In that sense, in this work we will provide some clear theoretical and numerical insights implemented by multiorbital and multi-layered tight-binding methods (TB) to clarify our recent experimental results obtained by THz-TDS spectroscopy.

X-ray detected ferromagnetic resonance (XFMR) spectroscopy can measure ferromagnetic resonance with elemental resolution and provides a unique tool for measuring transport and coupling within magnetic thin films. Until now, XFMR has been shown only at synchrotron sources. We present here a laboratory-scale instrument that uses ultrafast, extreme ultraviolet (EUV) light to perform XFMR spectroscopy, in principle extensible to frequencies exceeding 100 GHz. We applied this to three samples on opaque Si substrates: permalloy at 8.5 GHz, a Co-Fe alloy at 17 GHz, and a Ni/TaOx/Fe multilayer at 8.5 GHz to highlight the ability to detect element- and layer-resolved XFMR. The high bandwidth enables measurements of materials with high magnetic anisotropy, as well as ferrimagnets, antiferromagnets, and high-wavevector spinwaves in nanodevices. Finally, the coherence of the source will enable us to use dynamic, nanoscale, lensless imaging techniques such as ptychography and holography to measure transport in active devices.

Magnons, the elementary excitations of a ferromagnet, are promising for several quantum applications, including magnetic field sensing, transduction, and solid state bosonic qubits. While there are several theoretical proposals and experimental developments in quantum state generation of magnons, there has been comparatively less research on their quantum measurements. Brillouin light scattering (BLS) is a well-established classical technique for probing magnons known for its high sensitivity and temporal resolution. We explore the efficacy of BLS for quantum tomography of magnons. We find that in the state-of-the-art setups, it is feasible to reconstruct the 'classical component' of magnons, i.e. the regions of positive Wigner function. I will also discuss techniques to overcome this limitation to achieve a faithful quantum tomography.

Ultrafast optical control of electron spin is an exciting method for writing magnetic memory with write times on the order of hundreds of femtoseconds. However, most ultrafast optical writing of magnetization has been conducted with high laser fluence (> 1 mJ/cm^2) which leads to excessive heating required for deterministic high-power magnetization switching. We present time-resolved magneto-optical Kerr effect observations with laser fluence 1000 times lower than previous studies, to demonstrate low-power optical control of spin in Co/Pd ultrathin multilayers. We observe clear trends between the ferromagnetic layer thickness and significant enhancement in the amplitude of optically excited magnetization precession, and demonstrate the importance of large perpendicular magnetic anisotropy for achieving high sensitivity of electron spin to low power optical excitation. Lastly, we observe a magnetization precession cone angle double of those previously reported which is an important step to achieve a 90-degree precession angle, needed for magnetization switching.

For certain applications such as in Artificial Intelligence and neuromorphic computing, modern CMOS-based computing schemes can require prohibitively large circuit- and energy-footprints. Probabilistic computing offers an alternative approach that seeks to exploit its inherently probabilistic nature to act as low-cost natural hardware accelerators for solving such complex problems ranging from large-scale combinatorial optimization to Bayesian inference, and invertible Boolean logic. The base unit of probabilistic computing is known as the probabilistic bit, or p-bit, and requires tunable stochasticity; low-barrier Magnetic Tunnel Junctions (MTJs), in which the magnetization of the free layer fluctuates at room-temperature, are a natural spintronics-based solution for such high-quality random number generation and p-bit purposes. In this work, we present the experimental realization of a scaled p-bit core, integrating a stochastic in-plane MTJ with a high-performance novel multi-finger 2D-MoS2 transistor to achieve a compact spintronics-based p-bit platform that displays true randomness and the desired voltage-tunable stochasticity.

Domain-wall (DW) logic holds promise for compact and energy-efficient logic circuits. Indeed, fast DW motion driven by spin-orbit torque (SOT) and the Dzyaloshinskii-Moriya interaction (DMI) in magnetic/heavy-metal multilayers led to the experimental demonstration of current-driven DW logic circuits by magnetic imaging. Advancing towards applications, we present DW devices with electrical write/read using a magnetic tunnel junction (MTJ) stack with a hybrid free layer on 300-mm wafers. The first layer provides efficient spin-transfer torque (STT) and high tunneling magnetoresistance (TMR), while the second layer enables fast SOT-driven DW motion. We showcase full electrical control of nanoscale DW devices, involving write/read at input/output MTJs and SOT-driven propagation between them. Notably, we demonstrate cross-shaped DW devices allowing for the implementation of majority operations. Finally, to alleviate challenges in current-driven DW motion, we present a concept of chirally coupled MTJs through DMI. This enables current-free information processing in compact inverter and minority gates.

When an intense laser interacts with a gas of atoms, high-order harmonics are generated. In the time domain, this radiation forms a train of extremely short light pulses, of the order of 100 attoseconds. Attosecond pulses allow the study of the dynamics of electrons in atoms and molecules, using pump-probe techniques. This presentation will highlight some of the key steps of the field of attosecond science.

Theoretical simulations based on the density functional theory (DFT) provide a framework with predictive power that can be used to describe hybrid materials in a realistic manner. In this talk I will show how first principles studies can be successfully employed to fundamentally elucidate and explain a large variety of surface-science experiments. For example, the interaction between the π-like electronic cloud of organic materials or the lone electron pairs of the 2D systems with the magnetic states of a metal influences the (i) spin-polarization, (ii) magnetic exchange coupling, (iii) magnetic moments and (iv) their orientation at the hybrid interfaces. Furthermore, I will demonstrate that due to its predictive power, the DFT practically (i) represents a fundamental theoretical engine that essentially guides the experiments and (ii) is a pathfinder in designing novel materials.

Electric and magnetic order in solids is the basis of modern electronics; but it is also the foundation of future technologies in magnetotronics and spintronics. Remarkably, a systematic theory of electric and magnetic order in crystals has been missing so far. Here we present such a theory, and we show how the electric and magnetic order shapes the electronic band structure that is directly relevant for many applications. Our comprehensive theory is applicable to all crystalline materials including insulators and metals.

Quantum well structures lacking inversion symmetry host a plethora of exciting physical phenomena. In this talk, we show the design, creation, and electron-spin-lattice interactions of quantum well polar materials for spintronics and energy transduction. First, we show the role of dimensionality on the phonon and polarization dynamics of free-standing two-dimensional membranes [1]. Then, we show the discovery of the persistent spin helix in a hybrid ferroelectric perovskite with a natural quantum well structure [2]. We demonstrate that the spin-polarized band structure is switchable at room temperature via an intrinsic ferroelectric field. The favored short spin helix wavelength (three orders of magnitude shorter than in III–V materials), room-temperature operation and non-volatility make the hybrid perovskite an ideal platform for understanding symmetry-tuned spin dynamics, towards designing spintronic or spin-orbit qubit materials and devices that can resolve the control-dephasing dilemma. References: [1] Nature, 607, 480, 2022. [2] Nature Photonics 16, 529, 2022.

Quantization of spin and orbital angular momenta in standard quantum mechanics is rooted in the fundamental rotational symmetry of space in conjunction with the results of experiments conducted with the aid of the Stern-Gerlach apparatus on atomic and sub-atomic particles. In this presentation, we explain some of the most important results pertaining to spin and orbital angular momenta as inescapable consequences of certain elementary Stern-Gerlach experiments viewed in light of the rotational symmetry of space.

Strong magneto-elastic coupling in Yittrium Iron Garnet (YIG) - Gadolinium Gallium Garnet (GGG) structures allows for effective hybridization and transduction between acoustic and spin waves and resonance. It allows to design hybrid magneto-acoustic devices for signal processing and computing that combine the advantages of both fields. We design magneto-acoustic oscillators operating in low-phase noise, chaotic, and burst regimes. The low-phase noise YIG-GGG oscillator can be tuned between high overtone bulk acoustic wave resonance with an applied external magnetic field that changes the ferromagnetic resonance of YIG film and demonstrates exceptional frequency stability due high-Q factor of an acoustic subsystem. The chaotic magneto-acoustic oscillator operates in a chaotic, modulated, and single-frequency regime which can be controlled by an external low-frequency modulation signal. We also demonstrate a time-multiplexed magneto-acoustic oscillator array design based on a single YIG-GGG structure that can be exploited for large-scale Ising machines.

Spintronics might offer ultrawide bandwidth, nonvolatile data storage, low energy consumption and in-memory computing for data-intensive AI applications. Spin wave phenomena are emerging spin-spin interactions under an effective magnetic anisotropy field driven by intrinsic magnetism (saturation magnetism, exchange, damping, anisotropy, Dzyaloshinskii-Moriya), extrinsic effects (geometry, stacking, temperature), magnetic field or voltage bias. These interactions are described using Landau-Lifshitz-Gilbert equation, which is a nonlinear partial differential equation solved using a discretized mesh and electromagnetic boundary conditions under iterative total energy minimization for each time step. Despite LLG equation’s success in capturing spin wave phenomena, designing, or optimizing spin logic devices under competing constraints is not possible due to the prohibitively large computational resource and compute time requirements. Here, we use iterative solutions of micromagnetic device solutions and genetic algorithms to design and optimize spin logic devices (AND gate) to demonstrate that our approach can enable near-ideal figures of merit which would not otherwise be feasible.

Many probabilistic computing frameworks have been developed in recent years due to their potential as faster, energy-efficient alternatives to von Neumann computers for combinatorial optimization problems. In this work, we study the dynamics of a two-spin analog Ising computer implemented with superparamagnetic tunnel junctions (SMTJs). The operational-amplifier-based circuit features a polarity selection and a programmable gain parameter, allowing us to achieve both positive and negative coupling and perform simulated annealing if the gain is treated as inverse temperature. Experiments show that correlation between coupled SMTJs approaches 1 in the high-gain limit. Scaling of this design requires only trivial modifications to the circuit; however, scaling up to large networks of spins requires the development of SMTJs with enhanced properties, suggesting that a co-design approach between devices, architectures and algorithms is necessary.

Recently, the determination of the topological Hall effect has caused some controversy in the field of spintronics due to the validity of subtracting a magnetization-scaled anomalous Hall component from an experimentally measured Hall resistance. Often the underlying magnetic texture and topology are determined ex-situ of the Hall measurement which can lead to experimental error. Here we report the simultaneous determination of the magnetic texture and Hall effect in a ferrimagnetic FeGd thin film using resonant soft X-ray scattering with in-situ magneto-electrical transport. We find the largest departure of the Hall component at the 6-fold symmetric scattering of the dipole skyrmion state but also an additional smaller feature of opposite sign at higher fields where we observe diffuse scattering just before saturation. We attribute this additional feature to a skew scattering term arising from isolated skyrmions close to saturation.

Time-Resolved Scanning Transmission X-ray Microscopy (TR-STXM) is a powerful element-specific spectromicroscopy tool to study dynamic magnetic processes for such as spintronics and magnonics applications. It provides sub-50 nm spatial resolution and picosecond, phase-resolved time sampling. In this presentation, I will introduce the TR-STXM technique, and describe the setup developed at the Advanced Light Source at Berkeley Lab [1]. TR-STXM on samples ranging from Py/Co microstructures [2], to magnetosomes [3,4] will be shown together with supplementing ferromagnetic resonance spectroscopy and micromagnetic simulations. The talk will be concluded with an outlook on Time-Resolved X-ray ptychography. Research performed in collaboration with Helmholtz Center Berlin, Johannes Kepler University Linz, Research Center Jülich, SLAC National Accel. Laboratory, University of Duisburg-Essen, Université Grenoble Alpes - Institut Néel CNRS. [1] T. Feggeler, et al. J. Electron Spectrosc. Relat. Phenom. 2023. 267: 147381. [2] T. Feggeler, et al. Sci. Rep. 2022, 12: 18724. [3] T. Feggeler, et al. Phys. Rev. Res. 2021, 3(3): 033036. [4] T. Feggeler, et al. New J. Phys. 2023, 25(4): 043010.

The study of magnetism in reduced dimensions is not only of significant fundamental interest but also offers potential applications for materials with unique functionalities. Here, we will address the direct imaging of spin textures and spin dynamics in two-dimensional (2D) magnets using x-ray imaging. Using scanning transmission x-ray microscopy (STXM), we investigated the formation and stability of skyrmions and higher order topological states in Fe3GeTe2 flakes. Moreover, the propagation of spin waves was directly observed in Fe5GeTe2 flakes using time-resolved (TR)-STXM, revealing spin-wave characteristics of 2D magnets that are distinct from their 3D counterparts.

We present a theory of the inverse Edelstein effect induced by spin pumping for a junction system composed of a ferromagnetic insulator (FI) and a two-dimensional electron gas (2DEG) with Rashba and Dresselhaus spin-orbit couplings. Applying the Boltzmann equation to a microscopic model, we formulate an induced current in 2DEG under ferromagnetic resonance. We clarify the dependence of the generated current on the magnetization orientation in the ferromagnetic insulator, the frequency of the microwave irradiated from outside, and the ratio of the sizes of the Rashba and Dresselhaus spin-orbit couplings. [1] M. Yama, M. Matsuo, and T. Kato, Phys. Rev. B 108, 144430 (2023)

The integration of ferromagnetic resonance (FMR) with X-ray absorption spectroscopy (XAS) as the underlying detection mechanism marks an important achievement for the exploration of magnetic interactions, as it extends the scope of X-ray magnetic circular dichroism (XMCD) to the dynamic range. This enables the direct study of magnetization dynamics with element, site, and valence state specificity and may even be employed to disentangle spin and orbital contributions to the magnetic excitations. Here, I will present an overview of our work utilizing these tools to directly probe the generation and propagation of AC spin currents in different multilayer structures, to individually survey the spin dynamics of coherently coupled cations in a ferrimagnetic insulator, and to quantify the orbital-to-spin ratio in the magnetization dynamics of Ni in a permalloy/Ho heterostructure. Further, I will present the first realization of time-resolved dynamic X-ray magnetic linear dichroism (XMLD) as a new tool towards the study of GHz spin dynamics in systems beyond ferromagnetic order.

Magnetic bubbles and bubble lattices are of great importance for investigations in magnonics because of their non-trivial magnon band structures and skew scattering predicted between spin waves and magnetic bubbles. However, those systems host bubbles and skyrmions either typically at cryogenic temperature or the damping parameters are significantly increased. In this work, magnetic bubbles down to 250 nm-diameter in low-damping 20 nm-thick Bi-doped YIG were stabilized by controlling the anisotropy. We utilize the Maxymus time-resolved scanning transmission X-ray microscope (STXM) at Bessy II to image the spin dynamic behavior of the bubbles. Below 100 MHz, multiple types of magnetic bubble resonance were observed, depending on the shape and the textures of the bubbles. Above 100 MHz, spin waves excited from the antenna propagate through the area of resonating magnetic bubbles and interact with them. Isolated bubble resonances were also observed at different applied fields. Our observations offer insight into the manipulation of magnetic bubbles using spin waves and pave the way for bubble-based magnonic device designs.

Antiferromagnetic (AFM) materials are a pathway to spintronic memory and computing devices with unprecedented speed, energy efficiency, and bit density [1-3]. Realizing this potential requires AFM devices with simultaneous electrical writing and reading of information, which are also compatible with established silicon-based manufacturing. Recent experiments have shown tunneling magnetoresistance (TMR) readout in epitaxial AFM tunnel junctions. However, these TMR structures were not grown using a silicon-compatible deposition process, and controlling their AFM order required external magnetic fields. Here we show three-terminal AFM tunnel junctions based on the noncollinear antiferromagnet PtMn3, sputter-deposited on silicon [1]. The devices simultaneously exhibit electrical switching using current-induced torque, and electrical readout by a room-temperature TMR effect as large as 110%. [1] J. Shi, S. Arpaci et al., arXiv:2311.13828 (2023) [2] S. Arpaci, V. Lopez-Dominguez et al., Nature Communications 12, 4555 (2021) [3] J. Shi, V. Lopez-Dominguez et al., Nature Electronics 3, 92 (2020)

I will review our computational developments during the past decade to explore several open questions and provide guiding rules for the design of ultra-low energy spintronic devices.1-6 (1) Exploit the large spin orbit coupling and emergence of magnetism in ultrathin heavy-metal-based ferromagnetic (FM) or antiferromagnetic (AFM) heterostructures to achieve large perpendicular magnetic anisotropy and high voltage-controlled magnetic anisotropy efficiency, the two major challenges for ultralow-power and high density nonvolatile MeRAM devices; (2) Employ the effect of alloying or phonons to enhance the charge-to-spin current conversion efficiency in nonmagnetic heavy metals; (3) Dynamically control both the direction and amount of current-induced spin accumulation at heavy metal/FM interface using an electric field in an oxide capped spin orbit torque device; and (4) Search and identify novel two-dimensional van der Waals Dirac half-metal magnets characterized by a band structure with a large gap in one spin channel and a Dirac cone in the other with carrier mobilities comparable to those in graphene. This research was supported by NSF-PREM Grant No. DMR-1205734 and NSF Grant No. ERC

Perovskite strontium ferrite, SrFeO3, hosts a variety of spiral magnetic phases at low temperature including multi-q states of different proper screw and/or cycloid ordering. Among them is a phase believed to support topologically-protected magnetic structures and may explain an observed finite-field anomaly in Hall effect. Interestingly, SrFeO3 is centrosymmetric. Instead of a Dzyaloshinskii-Moriya interaction, the helimagnetism has been suggested to arise due to an interplay of electronic interactions. Using resonant soft x-ray scattering and neutron diffraction, we study how these complex magnetic orderings depend on the biaxial strain state, which potentially influences the electronic structure.

Chiral antiferromagnets such as Mn3Sn provides a unique platform for research and development of correlated topological physics and ultrafast spintronic functionalities as they exhibit the large transverse responses due to the combination of magnetic octupoles without magnetization and Weyl fermions. Here we present our recent work on three different ways of manipulation of correlated Weyl fermions in Mn3Sn; 1) by using strain through piezomagnetic effect, 2) by using electrical current through spin-orbit torque, and 3) by using intense pulse laser through doping photocarrier. Our work clarifies the energy hierarchy of spin and charge degrees of freedom of correlated Weyl fermions and paves the way for developing the ultrafast nonvolatile memory.

In this talk I present our recent progress in probing angular momentum transport in antiferromagnetic insulators via magnons by all-electrical measurements [1,2,3,4]. I will show that the quantized spin excitations of an ordered antiferromagnet with opposite chirality represent pairs of spin-up and -down magnons. A magnonic pseudospin can characterize this two-level nature. Over the last years, we studied the associated dynamics of antiferromagnetic pseudospin and observed the magnon Hanle effect in hematite thin films [1,2,3,4]. Its realization via electrically injected and detected spin transport in an antiferromagnetic insulator demonstrates its high potential for devices and as a convenient probe for magnon eigenmodes and the underlying spin interactions in the antiferromagnet [2,3]. Here, we observe a nonreciprocity in the Hanle signal measured in hematite using two spatially separated platinum electrodes as spin injector/ detector [4]. [1] T. Wimmer et al., Phys. Rev. Lett. 125, 247204 (2020). [2] A. Kamra et al., Phys. Rev. B 102, 174445 (2020). [3] J. Gückelhorn et al., Phys. Rev. B 105, 094440 (2022). [4] J. Gückelhorn et al., Phys. Rev. Lett. 130, 247204 (2023).

Topological magnetic materials opened exciting possibilities to achieve efficient manipulation of magnetism via electrical means. Here we propose a hitherto unknown mechanism of spin-orbit torque, unique to topological magnetic insulators, as an inverse effect of the adiabatic Thouless pumping, which features zero heat production and the absence of interfaces. Through the predicted spin-orbit torque, magnetic dynamics exhibiting interesting layer-resolved patterns can be generated by an electric field with a theoretical energy conversion rate approaching 100%, leading to an unprecedentedly efficient operation that could enable transformative applications of magnetoresistive random-access memory.

Recently, long-distance transport of a spin current across antiferromagnetic (AFM) insulators has been observed[1]. However, most of the experiments report the transport of thermal magnon current; the electrical spin current transport has only been observed in three AFM insulators, alpha-Fe2O3[1], YFeO3[2], and CrPS4[3]. Here, we present our observation of the long-distance transport of electrical spin current in both Cr2O3 and alpha-Fe2O3 crystals, and our main findings include: (1) a non-zero spin current density exists even in the absence of a magnetic field, which is very different from the vanished thermal magnon signal[4, 5]; (2) the non-local spin current shows a sharp enhancement as increasing of the magnetic field along easy-axis; (3) magnon-polaron signatures are observed in the thermal magnon channel. The interaction between electron spin and antiferromagnetic magnons with different chirality will be discussed. [1] R. Lebrun et al., Nature 561, 222 (2018). [2] S. Das et al., Nat. Commun. 13, 6140 (2022). [3] D.K. de Wal et al., Phys. Rev. B 107, L180403 (2023). [4] J. X. Li et al., Nature (London) 578, 70 (2020). [5] J. X. Li et al., Phys. Rev. Lett. 125, 217201 (2020)

Topological and chiral quantum materials exhibit intriguing electronic, magnetic, and optical properties, holding great promise to shape future electronic and spintronic technologies. I will show three different approaches to manipulate quantum phases, based on physics models and density functional theory (DFT) calculations. First, exotic electronic band structures are realized through atomic lattice design, as exemplified by the design of perfectly flat bands based on the line graph theorem. I will present a system with two flat bands of opposite chirality that induces a giant circular dichroism effect. Secondly, topological states are manipulated by applying an external field. I will demonstrate a physical mechanism to remotely control the spin polarization of the topological corner states in a higher-order topological insulator. Thirdly, structural chirality engenders a chiral-induced spin selectivity effect, which enables the harnessing of electron spin to open promising opportunities in spintronics and quantum technologies. I will showcase the realization of higher-dimensional spin selectivity in chiral crystals for controlling phase transition and spin-flipping processes.

Spin waves, domain walls and other planar systems are some of the topological structures that can appear in magnetic systems. A magnetic system presents itself as a topological structure depending on its dimensionality. In one dimension we can have a network of spins and in two or 3 dimensions we can have zones and clusters of spins that present important properties in spintronics and other applications. Realistically these topological structures can be generated in magnetic thin films like LaSrMnO (LSMO) which has recently been enormously applied in materials science and spintronics. Paramagnetic materials (PM) and Ferromagnetic materials (FM) can be characterized topologically so that the regions where the spin sites are located behave and are identified with the bound states of the system. In this sense, in principle we could represent the magnetization of the system as a scalar field depending on the phase of the applied magnetic field.

Second-order topological insulators (SOTIs) support topological states beyond the usual bulk-boundary correspondence. To date, only a few 2D materials have been identified as HOTIs, and their experimental confirmations are still absent. Here, we propose three general methods for realizing 2D SOTIs: antidot engineering in 2D Dirac materials, 2D nonsymmorphic materials with obstructed and symmorphic charge centers, and boundary-obstructed 2D materials. An inherent connection is established for the existing various mechanisms of the SOTIs, including quadrupole polarization, filling anomaly, and generalized Su-Schrieffer-Heeger model. Manipulating corner states by using a magnetic exchange field was also demonstrated.

Spintronics offers promising routes for efficient memory, logic, and computing technologies. The central challenge in spintronics is electrically manipulating and detecting magnetic states in devices. The electrical control of magnetization via spin-orbit torques is effective in both conducting and insulating magnetic layers. However, the electrical readout of magnetization in the latter is inherently difficult, limiting its use in practical applications. In this work, we demonstrate magnetoresistive detection of perpendicular magnetization reversal in an electrically insulating ferrimagnet, terbium iron garnet (TbIG). To do so, we use TbIG|Cu|TbCo, where TbCo is a conducting ferrimagnet and serves as the reference layer, and Cu is a nonmagnetic spacer. Current injection through Cu|TbCo allows us to detect the magnetization reversal of TbIG with a simple resistance readout during an external magnetic field sweep. Technologically feasible magnetoresistive detection of perpendicular switching in a ferrimagnetic garnet is a breakthrough, as it opens broad avenues for novel insulating spintronic devices and concepts.

Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. While conventional ferromagnet/nonmagnetic-metal bilayers attain a larger SOT at the expense of higher damping, single-layer ferromagnets with bulk inversion asymmetry may enable both low damping and sizable SOTs. Here, we examine the impact of intentional compositional asymmetry on damping and SOTs in 10-nm-thick symmetric and vertically graded films consisting of two ferromagnetic elements: Fe with low intrinsic damping and Ni with sizable spin-orbit coupling. We confirm low intrinsic damping despite steep compositional gradients. Further, we find a sizable anti-damping SOT in each Fe-Ni structure. Remarkably, the magnitude of the torque does not correlate with the vertical compositional gradient; rather, asymmetry from a lattice strain gradient appears to play a key role. Our results provide intriguing insights into the mechanisms of damping and spin-orbit torques in single-layer ferromagnets.

Antiferromagnetic materials have emerged as a focal point within spintronics for advancing the next generation of picosecond and densely integrated information technologies. Their appeal stems from the absence of stray fields and the remarkable speed of spin dynamics they offer. Among these materials, the hexagonal D019-Mn3X (X = Ga, Ge, Sn) family stands out as exemplars of non-collinear spin structures. Notably, within this family, the magnetic Weyl semimetal Mn3Sn, distinguished by its unique Kagome lattice spin arrangement, showcases a spectrum of exotic properties. Despite its minimal magnetization, Mn3Sn manifests a notable anomalous Hall effect (AHE), attributed to a non-zero net Berry curvature in bands proximate to the Fermi level. Evidently, the magnetic characteristics of Mn3Sn are markedly influenced by diverse factors, including growth conditions and the presence of other phases. This presentation delves into an examination of various spin-orbit torque (SOT)-driven dynamic properties, such as current-induced switching and chiral rotation, within Mn3Sn/W heterostructures. Emphasis is particularly placed on elucidating the effects of interfacial structure.

Spin-torque manipulation of a spin system -- a key functionality of modern spintronics -- requires large spin current densities and therefore small device size. The development of antiferromagnetic spin-torque applications hinges upon reliable excitation and detection of coherent spin dynamics in a single antiferromagnetic nanodevice. It requires an electrical, magnetoresistive technique compatible with the device dimensions. Here, we develop a spin-torque antiferromagnetic resonance (ST-AFMR) experiment and study spin dynamics in an insulating antiferromagnetic crystal capped with a Platinum nanodevice. We observe antiferromagnetic magnons below and Goldstone excitations above the spin-flop field, from an area of only 200nm by 200nm -- a milestone for the development of antiferromagnetic spin-torque devices. By means of angle-dependent measurements, furthermore, we study the excitation and detection mechanisms and draw conclusions for antiferromagnet-based applications. This work was supported by the National Science Foundation through Grant No. ECCS-1810541.