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.

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.

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

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.

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.

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.

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.

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.

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).

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.

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.

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 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).

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.

In a topological insulator (TI), spin-charge conversion (SCC) is expected to occur at the surface through the spin-momentum locking of the metallic surface states. MBE-grown TIs however are oftentimes unintentionally doped resulting in bulk states that can convert spin to charge through the inverse spin hall effect. In this work, we investigate the SCC mechanisms of the TI Bi2Se3 using time-resolved THz emission spectroscopy. Here, we demonstrate THz emission in NiFe/Bi2Se3 heterostructures using bulk-insulating Bi2Se3 and bulk-conducting Bi2Se3. We also present results comparing NiFe/Bi2Se3 to W/NiFe/Pt, a heavy metal spintronic THz emitter.

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.

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)

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.

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.

In the recent past, the area associated with generation and manipulation of orbital currents and orbital magnetism, and their utilization for magnetization dynamics, has gained considerable attention. However, in the ultrafast domain, it is only till recently that the orbital degree of freedom started gaining attention in THz emission experiments. In my talk, I will report on our understanding of ultrafast generation of orbital magnetism and orbital currents by light in various light materials, i.e. metallic Rashba surfaces, antiferromagnets and altermagnets.

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. In this presentation, we introduce the successful demonstration of detection of the ferroic BCD at RT in a topological crystalline insulator, PbSnTe. 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.

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.

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.

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.

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 two) 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.

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. 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.

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.

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.

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.

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.

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.

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/cm²) 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.

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. We identify two complementary, comprehensive classifications of crystals, based on five categories of electric and magnetic multipole order (parapolar, electropolar, magnetopolar, antimagnetopolar, and multipolar) and five categories of chirality (parachiral, electrochiral, magnetochiral, antimagnetochiral, and multichiral). Each category of chirality arises from distinct superpositions of electric and magnetic multipole densities. We provide a complete theory of minimal effective models characterizing the different categories of chirality in different systems. Jointly these two schemes yield a classification of all 122 magnetic crystallographic point groups into 15 types that treat space inversion, time inversion and their combined inversion on the same footing.

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.

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.

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.

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.

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)

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).

The spin Seebeck effect (SSE)1 describes the spin current generation in a magnetic material parallel to the temperature gradient with polarization along the magnetization axis. The SSE is typically detected by injecting the generated spin current into a heavy metal layer with a large spin Hall effect (e.g., Pt) and measuring the transverse voltage.2 The SSE has been, thus far, predominantly studied in magnetic insulator/heavy metal systems since, in all-metallic structures the SSE-generated voltage becomes indistinguishable by symmetry from the anomalous Nernst effect (ANE) in the magnetic layer.3 In this talk, we will present an extensive characterization of the ANE and the SSE in a wide variety of Fe-based heterostructures. We will show clear regimes where either the ANE or the SSE is dominant, or else they equally contribute and (nearly) cancel each other. Interestingly, both effects have nonlinear contributions due to the increased magnon density and reduced saturation magnetization at elevated temperatures, which increase the SSE and decrease the ANE, respectively.4 Finally, we will present strategies to engineer micrometer-scale spin thermoelectric devices based on the ANE in ferrimagnetic multilayers. We will show giant thermoelectric voltage generation nearing 4 mV, which is more than 10 times larger with respect to the ones reported in similar devices.5 References [1] K. Uchida et al. Nature, 455, 2008, 778 [2] K. Uchida et al. Appl. Phys. Lett. 97, 172505 (2010) [3] D. Yang et al. Mater. Res. Bul. 136, 111153 (2021) [4] J. A. de Sousa et al., submitted (2024) [5] Y. Sakuraba et al. Appl. Phys. Exp., 6, 033003, (2013)

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.