We report the terahertz (THz) emission from Bi1-xSbx/Co and Bi2SnTe4/Co bilayers including nanometer-thin ferromagnetic layer as spin-injectors and Bi1-xSbx and Bi2SnTe4 topological insulators (TI) from the Bi family grown by molecular beam epitaxy. Using THz emission spectroscopy, an efficient dynamical spin-to-charge conversion in the sub-picosecond timescale is demonstrated in these heterostructures with an output THz amplitude sizeable compared to reference metallic spintronic THz emitters. We investigate TI thickness dependence and azimuthal crystalline orientation dependence on the THz emission which are both in line with interfacially-mediated interconversion. We show that a strong reminiscent THz signal at the limit of small TI thickness is explained by a spin-charge interconversion occurring at the level of the first planes of their interface in contact with Co. This strongly suggests a spin-charge interconversion via inverse Rashba-Edelstein effect (IREE) onto spin-locked TI’s interface states.

Three-dimensional (3D) topological insulators (TI) with large spin Hall conductivity have emerged as potential candidates for spintronic applications. Here, we report spin to charge conversion in bilayers of amorphous ferromagnet Fe78Ga13B9 (FeGaB) and 3D TI Bi85Sb15 (BiSb) activated by two complementary techniques: spin pumping and ultrafast spin-current injection. The spin pumping parameters derived from inverse spin Hall effect (ISHE) measurements are consistent with the results of femtosecond light-pulse induced THz emission. These measurements are successfully verified using theoretical calculations of thickness-dependent spin Hall conductivity of BiSb thin films based on a tight-binding model.

Materials with Dirac and Weyl fermions have highly unusual magnetooptical properties which can be utilized in compact optoelectronic devices and circuits. Moreover, magnetooptical spectroscopy can provide a cleaner way of studying topological properties of their electron states as compared to transport measurements. I will discuss several examples illustrating these points: an extremely high optical nonlinearity in Landau-quantized graphene and its device applications; optical anisotropy and optical Hall effect in magnetic Weyl semimetals; inverse Faraday effect in graphene and topological materials; and unique properties of magnetopolaritons in Dirac and Weyl semimetals in a strong magnetic field.

Spin-orbit effects appearing in topological insulators (TI) and at Rashba interfaces are currently revolutionizing how we can manipulate spins and have led to several newly discovered effects, from spin-charge interconversion and spin-orbit torques to novel magnetoresistance phenomena. In particular, a puzzling magnetoresistance has been evidenced, bilinear in electric and magnetic fields. Here, we report the observation of bilinear magnetoresistance (BMR) in strained HgTe, a prototypical TI. We show that both the amplitude and sign of this BMR can be tuned by controling, with an electric gate, the relative proportions of the opposite contributions of opposite surfaces. At magnetic fields of 1 T, the magnetoresistance is of the order of 1 % and has a larger figure of merit than previously measured TIs. We propose a theoretical model giving a quantitative account of our experimental data. This phenomenon, unique to TI, offer novel opportunities to tune the electrical response of surface states for spintronics.

Current-generated spin in topological insulators (TIs) has been shown to efficiently switch FM magnetization via spin-orbit torque (SOT) with much lower critical currently densities. However, TI bulk are often degenerately doped and can shunt current from the surface states. Here we demonstrate SOT switching from bulk-insulating Bi2Se3, obtained by growth on BiInSe/In2Se3 buffer layers by MBE, with significantly reduced critical current density than conventional “bulk-conducting” Bi2Se3. We further grew epitaxial In2Se3 tunnel barriers on Bi2Se3, and demonstrate its spin sensitivity, towards further minimize current shunting through the FM metal and overall power consumption for magnetization switching.

Majorana states hosted in vortex cores of topological insulator/superconductor heterostructures present a promising alternative to nanowire-based approaches. Vortices can be pinned to anti-dots pre-fabricated in the superconductor; large anti-dots are relatively simple to fabricate and ensure that weak magnetic fields are sufficient to induce a required quantum of flux. However, it has thus far been unclear whether current topological insulator materials are clean enough to sustain Majorana vortex modes with a sizable gap to excitations. Also, if the anti-dot is too large, the level spacing of subgap vortex states may become too small even in a clean case. In this work, we study numerically the vortex subgap spectrum as a function of disorder, chemical potential, and the anti-dot size. We employ a two-dimensional low-energy description of the topological insulator surface, which allows as to simulate large quantum systems with vortices up to 1 micron in diameter. We connect our results to existing mobility measurement data to translate the level of disorder in existing materials to our simulated disorder model.

The recent invention of spintronic THz emitters has attracted a lot of attention due to their high power, broadband emission, and easy fabrication. They commonly consist of nanometer-thin bilayers of a magnetic (M) and a nonmagnetic (NM) metal film and rely on the inverse spin Hall effect. In my talk, I will present our recent results on bilayer M/NM spintronic emitters based on magnetic alloys, such as GdFe or CoFe, and Pt as the NM layer. Furthermore, functional multilayer spintronic THz emitter systems, which can be conveniently switched on and off, will be presented.

The ultimate speed limit for all-optical spin manipulation requires control schemes on the timescale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast and coherent spin transfer between distinct magnetic subsystems in ferromagnetic Ni-Fe alloys and Heusler compounds. Our experimental findings are fully supported by time dependent density functional theory simulations and, hence, suggests the possibility of coherently controlling spin dynamics on sub-femtosecond timescales.

Big data generated by Internet-of-Things (IoT) sensors and devices along with AI is changing the society we live in, allowing us to better understand and manage the world. Both IoT and AI require low energy information handling. For example, IoT devices are much better if it does not require a battery (i.e. energized by energy harvesting) and AI is known to be power-hungry calling for a means to drastically reduce the power for its ubiquitous use. Spintronics nonvolatile device, magnetic tunnel junction (MTJ), has been shown to reduce the operation power of a CMOS-based microprocessor by up to two orders of magnitude, suitable for IoT purposes and other applications. MTJs have also been shown to be scalable down to 2.3 nm without resorting to new materials, a significant advantage along with their high thermal stability, high endurance, and low voltage operation. Moreover, new computing schemes are emerging, where one expects to have higher performance than relying solely on silicon-based digital processing. One of the approaches is neuromorphic computing; we have made proof-of-concept spintronics devices for artificial synapses as well as neurons for neuromorphic applications. Another approach utilizes less stable MTJs for a novel form of computing, probabilistic computing, to address optimization problems: One can formulate integer factorization as an optimization problem in such a way that the most preferred state in terms of energy gives the factorized result. The time scale involved in these probabilistic MTJs is also discussed. If time allows, I will touch upon synthetic antiferromagnetic skyrmions for information carriers

We present an overview of Everspin’s proprietary Spin-Transfer Torque (STT) MRAM technology. We have optimized our perpendicular MTJ STT-MRAM technology to achieve high-speed reliable switching over the temperature range of -40°C to +85°C with data retention of more than 10 years at +105°C and endurance of greater than 1e15 cycles at -40°C. This advanced STT-MRAM technology has been deployed in commercially available 1Gb ST-DDR4 stand-alone memory and low-latency 64Mb serial peripheral interface (SPI) STT-MRAM products, both integrated on 28 nm CMOS technology.

Magnetic tunnel junctions with perpendicular magnetic anisotropy for Magnetic Random Access Memory need to combine high speed and low critical switching current. Higher spin transfer torque (STT) write efficiency is required. This can be achieved introducing a switchable assistance layer, which can be designed to maximize the STT efficiency independently of the switching direction. At the same time, the assistance layer also increases the retention in standby. The reversal process was confirmed with time-resolved measurements. The outlook for scaling to the sub-20 nm diameter range will also be reviewed looking at STT driven switching in perpendicular shape anisotropy cells.

Spin Transfer Torque (STT) MRAM is a very attractive enabling technology for space and defense applications due to its versatility (NVM to SRAM like performance), wide operating temperature range, high density and radiation tolerance. In this presentation we will review the promise of state-of-the-art STT-MRAM and its potential impact on current and future space applications. Included will be a thorough review of recent rad-effects characterization results for STT-MRAM. More exotic spintronic technology such as Spin Orbit Torque (SOT) MRAM and Voltage Controlled Magnetic Anisotropy (VCMA) MRAM will be discussed and considered for their specific benefits in rad-hard applications.

We have developed new manufacturing processes based on ion irradiation to tailor the structural properties of ultra-thin magnetic films and spintronic devices at atomic level and improve their performances. The key feature of the technology is the post-growth control at the atomic scale of structural properties and the corresponding magnetic properties. When realized through a mask this technology allows lateral modulation of magnetic properties without any physical etching. In this talk, we will show a few important results that suggest a pathway to optimize the performances of future generation of spintronic devices using ion irradiation.

I will present a novel concept that blends spintronics and quantum thermodynamics to generate electricity. This concept is invoked to explain our experimental observations of electrical generation across oxide and molecular spintronic devices that comprise paramagnetic centers sandwiched between electrodes with full transport spin polarization. The presence of so-called quantum resources, leading to a source of work of quantum origin called ergotropy, appears to be manifest in sub-kBT spectral features, as well in an apparent signature of a phase transition of the spin fluctuations on the paramagnetic centers. I will discuss our present research tracks to better understand this spintronic quantum engine. General info may also be found at www.spinengine.tech.

Spin optoelectronics has the ability to interconvert photon spins to electrical charges and may revolutionize information processing. Circularly-polarized photon sources rely on spin lasers whereas reciprocal devices are solid-state helicity detectors, the fabrication of which has remained a challenge for decades. Experimental results obtained on a ferromagnetic CoFeB/MgO/III-V semiconductor tunnel diode will be presented. We show that the helicity-dependent photocurrent is mostly determined by a dynamical factor resulting from the competition between carrier recombination in the metal and in the semiconductor. This constitutes a radical shift in the description of these emerging spin devices.

Transition metal dichalcogenides (TMD) can be used to enhance the spin-orbit coupling of graphene, leading to new spin transport channels with unprecedented spin textures. We have optimized bilayer graphene/WSe2 van der Waals heterostructures to achieve magnetic-field-free spin precession. Remarkably, the sign of the precessing spin polarization can be tuned electrically by backgate voltage and drift current, being the first realization of a spin field-effect transistor at room temperature in a diffusive system. The spin-orbit proximity in graphene/TMD van der Waals heterostructures also leads to spin Hall effect (SHE), first observed by our group using MoS2 as the TMD. The combination of long-distance spin transport and SHE in the same material gives rise to an unprecedented figure of merit (product of spin Hall angle and spin diffusion length) of 40 nm in graphene proximitized with WSe2, which is also gate tunable.

Magnetically doped topological insulators enable the quantum anomalous Hall effect. Stoichiometric systems of AB2C4 stoichiometry are considered promising but MnBi2Te4 is antiferromagnetic. MnSb2Te4 has been predicted to be antiferromagnetic as well and topologically trivial but we show that spin- and angle-resolved photoemission reveal a clear Dirac cone as sign of a topological insulator. Moreover, it is ferromagnetic with a high Curie temperature of up to 50 K, i. e., two times larger than the Néel temperature of the related MnBi2Te4. The reason for these properties is found in a 5% excess Mn as detailed calculations show.

Magnetic tunnel junctions (MTJs) provide an attractive platform for implementing neural networks because of their simplicity, nonvolatility and scalability. In a hardware realization, however, device variations, write errors, and parasitic resistance will generally degrade performance. To quantify such effects, we perform experiments on a 2-layer perceptron constructed from a 15 × 15 passive array of MTJs, examining classification accuracy and write fidelity. Despite imperfections, we achieve accuracy of up to 95.3 % with proper tuning of network parameters. The success of this tuning process shows that new metrics are needed to characterize and optimize networks reproduced in mixed signal hardware.

Prevention of integrated circuit counterfeiting through logic locking faces the fundamental challenge of securing an obfuscation key against physical and algorithmic threats. Previous work has focused on strengthening the logic encryption to protect the key against algorithmic attacks but failed to provide adequate physical security. In this work, we propose a logic locking scheme that leverages the non-volatility of the nanomagnet logic (NML) family to achieve both physical and algorithmic security. Polymorphic NML minority gates protect the obfuscation key against algorithmic attacks, while a strain-inducing shield surrounding the nanomagnets provides physical security via a self-destruction mechanism, securing against invasive attacks. We experimentally demonstrate that shielded magnetic domains are indistinguishable, securing against imaging attacks. As NML suffers from low speeds, we propose a hybrid CMOS logic scheme with embedded obfuscated NML “islands”. The NML secures the functionality of sensitive logic while CMOS drives the timing-critical paths.

We present initial experimental results and simulate a nanomagnet reservoir computer (NMRC) solving tasks requiring high memory content, with an area-energy-delay product ten-million times lower than CMOS systems. We manufactured a small nanomagnet reservoir demonstrating a frustrated state. We evaluated the performance on two novel tasks. Our results indicate the reservoir’s short-term memory capabilities and ability to integrate information from multiple concurrent inputs. In the end, our system saw a reduction in area by a factor of 50,000, in energy by a factor of 60, and in period by a factor of four as compared with an equivalent CMOS reservoir.

We present the first experimental demonstration of a neuromorphic network with magnetic tunnel junction (MTJ) synapses, which performs image recognition via vector-matrix multiplication. We also simulate a large MTJ network performing MNIST handwritten digit recognition, demonstrating that MTJ crossbars can match memristor accuracy while providing increased precision, stability, and endurance.

A semi-analytical method (data-driven model) is used to predict the dynamics of a Spin-Torque Vortex Oscillator (STVO). This model relies on an improved analytical model based on the Thiele equation approach and micromagnetic simulations. The improved analytical model shows that the Ampère-Oersted field cannot be neglected and it describes quantitatively the STVO dynamics only in the resonant regime when the data-driven model allows to describe it in the steady-state oscillating regime as well. In addition, the model is 2.1 million time faster than simulations. It can be used to simulate the spin-diode effect and functionalize the STVOs for neuromorphic applications.

This work introduces a methodology for modeling spintronic systems based on a deep learning approach. We show that using a limited amount of micromagnetic simulations or experimental data, we can train a specific type of neural network known as “Neural Ordinary Differential Equations” to predict the behavior of a spintronic system in new situations. On the simulation of a skyrmion-based system, our technique gives equivalent results to micromagnetic simulations but 200 times faster. We also show that based on five milliseconds of experimental data, our method predicted the results of weeks of measurements of spin-torque nanooscillators.

We develop an ultrasensitive magneto-optic method to quantify spin-orbit torques (SOT) based on a modified Sagnac magneto-optic Kerr effect (MOKE) interferometry (~ 5μRad/√Hz). The high sensitivity of Sagnac interferometry permits for the first time optical quantification of spin-orbit torque from small-angle magnetic tilting of samples with perpendicular magnetic anisotropy (PMA). We find significant disagreement between Sagnac measurements and simultaneously-performed harmonic Hall (HH) measurements of spin-orbit torque on Pt/Co/MgO and Pd/Co/MgO samples with PMA. This very surprising result demonstrates a flaw in the most-popular method for measuring spin-orbit torques in PMA samples, and represents an unsolved puzzle in understanding the planar Hall effect in magnetic thin films.

We investigated the magnetic proximity effect (MPE) as well as the unidirectional spin Hall magnetoresistance (USMR) and spin-orbit torques (SOTs) in heavy metal/Co/heavy metal trilayer systems compared to Pt/Co and Ta/Co bilayers. By using x-ray resonant magnetic reflectivity, we obtained magnetic depth profiles of the structural, optical and magnetic parameters, and studied the impact of the MPE in Pt on the total magnetic moment of the systems. A detailed analysis of harmonic longitudinal and Hall voltage measurements revealed enhanced SOTs and USMR in the trilayers that include a second heavy metal with the opposite sign of the spin Hall angle.

The stationary state of the spin-Hall bar is studied in the framework of a variational approach that includes non-equilibrium screening effects at the lateral edges. The minimization of the power dissipated in the system is performed with taking into account the spin-flip relaxation and the global constrains due to the electric generator and global charge conservation. The calculation is performed within both the approximations of negligible spin-flip scattering and strong spin-flip scattering. In both cases, simple expressions the longitudinal and the transverse pure spin-currents are derived analytically. In the first case, the spin-accumulation is linear across the Spin-Hall bar, symmetric from one edge to other, and the transverse spin-current vanishes. With strong spin-flip scattering, a transverse pure spin-current flows across the sample together with with longitudinal pure spin-current. Surprisingly, due to the small values of the screening Debye length, the effect of the spin-flip scattering seems not to change significantly the profile and the amplitude of the spin-accumulation. A quantitative analysis predicts a spin accumulation of the order of $1\%$ of the density of carriers for an applied electric field of $5 mV/\m m^2$ at $30K$, with a spin-Hall angle of $\theta_{SH} = 10^{-4}$.

I will introduce the fundamental requirements for practically linking quantum objects into large-scale coherent quantum systems as well as the advantages of coherent magnonics for next-generation quantum coherent systems (i.e., spin-entangling quantum gates). Other critical challenges for quantum information science then will motivate the development of coherent magnonics for quantum transduction from “stationary” spin systems to “flying” magnons and for quantum memory. Finally, the advantages of all-magnon quantum information technologies that rely on manipulating and encoding quantum information in superpositions of fixed magnon number states will highlight the potential of new magnetic materials, devices, and systems.

Ballistic hot electrons, whose energy is lower than 2.5 eV, are extracted from a magnetic tunnel junction and injected into a metallic base. After passing through the base, the electrons are energy filtered by a Schottky barrier. The use of a low height Si/Cu Schottky barrier allows to disentangle the different contributions to the scattering. The hot electrons transport is interpreted as being mainly influenced by inelastic diffusion. A transport model reproduces our measurements and explains them as resulting directly from a diffusion process related to the d-band of the ferromagnetic material involved.

Electrical field control of magnetization in nanoscale magnets has the potential to be extremely energy efficient. We will discuss voltage-induced strain and acoustic wave switching of the magnetization as well as direct voltage control of magnetic anisotropy (VCMA) to control magnetic skyrmion states in nanomagnets with confined geometry. We will then discuss various voltage-controlled nanoscale magnetic device proposals towards implementation of energy efficient, memory, deep neural networks whose synaptic weights can be reprogrammed online and reservoir computing devices that are amenable to online training. This could be a key enabling technology for edge computing in IOT devices.

Advanced voltage-controlled MRAM devices for low-power memory and machine learning was recorded at SPIE Optics + Photonics held in San Diego, California, United States 2022.

Proximity effects at superconductor(SC)/ferromagnet(FM) interfaces provide novel functionality in the field of superconducting spintronics. We investigate the spin current transport in SC/Permalloy (Py) heterostructures with a Pt spin sink layer. To this end, we excite ferromagnetic resonance in the Py-layer via a coplanar waveguide (CPW) at different microwave frequencies. A phase sensitive detection of the microwave transmission signal is used to quantitatively extract the inductive coupling strength between sample and CPW as a function of temperature. This approach enables the analysis of ac charge currents excited by the magnetization precession in SC/FM hybrids, like inverse spin-orbit torque induced charge currents.

This talk will review my group’s recent progress in generating pure spin supercurrents in superconducting proximity structures and the optimization of magnetic proximity effects in nearly a nm-thick s-wave. In the first part of the talk I will demonstrate spin-triplet pair generation via spin-orbit coupling and magnetic exchange fields and in the second part I will demonstrate infinite magnetoresistance in superconducting Nb sandwiched between insulating ferromagnetic layers. The results are key the development of superconducting spintronics.

Spin-controlled lasers are highly interesting photonic devices and have been shown to provide ultra-fast polarization dynamics in excess of 200 GHz. Another class of modern semiconductor lasers are high-beta emitters which benefit from enhanced light-matter interaction due to strong mode confinement in low-mode-volume microcavities. We combine the advantages of both laser types to demonstrate spin-lasing in high-beta microlasers for the first time. For this purpose, we realize bimodal high-beta quantum dot micropillar lasers for which the mode splitting and the polarization-oscillation frequency can be engineered via the pillar cross-section. The microlasers show very pronounced spin-lasing effects with polarization oscillation frequencies up to 15 GHz.

We report studies of spin textures including domain walls and skyrmions in synthetic antiferromagnetic multilayers. We have observed: phase coexistence of uniform antiferromagnetic regions with textured ferromagnetic regions during magnetisation; current-driven domain wall motion at lower current densities than in comparable ferromagnetic multilayers; and current-driven nucleation of a synthetic antiferromagnetic skyrmion.

Ferrimagnetic spintronics was recorded at SPIE Optics + Photonics held in San Diego, California, United States 2022.

Recent theories have predicted that unlike the spin Hall effect, the orbital Hall effect can be gigantic even in light metals because it does not require spin-orbit interaction [1]. This opens a novel route toward electric control of magnetic moments in spintronic devices by harnessing the angular momentum carried by orbital current [2]. As the choice of materials is not restricted to heavy elements, utilizing the orbital current offers great flexibility in choosing environment-friendly materials [3]. In recent years, so-called the orbital torque mechanism has been found in various magnetic heterostructures, demonstrating its potential for highly efficient spintronic devices [4-7]. So far, however, direct experimental evidence of the orbital current has still been missing. In this talk, we present a magneto-optical measurement of the orbital accumulation induced by the orbital Hall effect in a light metal Ti [8]. The orbital accumulation at the surface of Ti leads to ~ 50 nrad of Kerr rotation of a polarized light for current density ~107 A/cm2, which is of the same order of magnitude for the Kerr rotation induced by the spin Hall effect in Pt [9]. Our analysis based on semi-realistic calculations shows that this cannot be explained by the spin Hall effect, which is far too small in Ti, and the estimation of the Kerr angle caused by the spin accumulation is two orders of magnitude smaller than what we observe in experiment. Moreover, variations of the sign and magnitude of the Kerr angle agree with the theoretical model based on the orbital Hall effect while they significantly deviate from the model based on the spin Hall effect. As another evidence, we also measure the orbital torque in Ti/Co heterostructures. We find gigantic field-like component, which amounts to the orbital Hall angle of 0.3. Our experimental results, which are corroborated by theoretical analyses, provide a direct and strong evidence of the orbital Hall effect and serve as a solid ground for future directions of orbitronics research [10]. References: [1] D. Go et al., Phys. Rev. Lett. 121, 086602 (2018). [2] D. Go and H.-W. Lee, Phys. Rev. Res. 2, 013177 (2020). [3] D. Jo, D. Go, and H.-W. Lee, Phys. Rev. B 98, 214405 (2018). [4] S. Ding et al., Physical Review Letters 125, 177201 (2020). [5] J. Kim et al., Phys. Rev. B 103, L020407 (2021). [6] S. Lee et al., Commun. Phys. 4,234 (2021). [7] D. Lee et al., Nat. Commun. 12, 6710 (2021). [9] Y.-G. Choi et al., arXiv:2109.14847. [9] C. Stamm et al., Phys. Rev. Lett. 119, 087203 (2017). [10] D. Go et al., Europhys. Lett. 135, 37001 (2021).

The orbital Hall effect describes an electric-field-induced generation of the orbital current flowing in a perpendicular direction to the field, analogous to the spin Hall effect [1]. As the orbital current carries the angular momentum as the spin current does, injection of the orbital current into a ferromagnet can result in spin-torque on the magnetization, which provides a way to detect the orbital Hall effect [2]. With this motivation, we examine the current-induced spin-orbit torques in various ferromagnet/heavy metal bilayers [3]. Analysis of the spin-torque reveals the presence of the contribution from the orbital Hall effect in the heavy metal. In particular, we find that the net torque in Ni/Ta bilayers is opposite in sign to the spin Hall theory prediction but instead consistent with the orbital Hall theory, which unambiguously confirms the orbital torque generated by the orbital Hall effect. Our finding opens a possibility of utilizing the orbital current for spintronic device applications, and it will invigorate researches on spin-orbit-coupled phenomena based on orbital engineering. [1] Phys. Rev. B 77, 165117 (2008), Phys. Rev. Lett. 102, 016601 (2009). [2] Phys. Rev. Res. 2, 013177 (2020), arXiv: 2107.08478. [3] Nat. Comm. 12, 6710 (2021).

Very recently, a dramatic increase in magnetic torque has been observed in ferrimagnetic insulators interfaced with CuOx and the results are explained in the framework of orbital-current generation and emission from CuOx. The conversion of charge-current into the orbital current is governed via the orbital Hall or orbital Rashba effect, a similar analogy to the spin Hall and Rashba effect. In this talk, we will present our latest results on the observation of damping-like torque in Co/CuOx without any heavy-metal layer due to orbital Rashba effect at Co/CuOx interface. Furthermore, we observe a two-fold increase in damping-like torques in Co(2)/Pt(4)/Cu/CuOx(3) that further validates our hypothesis.

5d transition metal Pt is one of the classical spin Hall materials for efficient generation of spin-orbit torques (SOTs) in Pt/ferromagnetic layer (FM) heterostructures. However, for a long while with tremendous engineering endeavors, the damping-like SOT efficiencies (ξDL) of Pt and Pt alloys are still limited to ξDL<0.5. Here we present that with proper alloying elements, particularly 3d transition metals V and Cr, the strength of the high spin Hall conductivity of Pt (σSH∼6.45×105(ℏ/2e)Ω−1⋅m−1) can be developed. Especially for the Cr-doped case, an extremely high ξDL∼0.9 in a Pt0.69Cr0.31/Co device can be achieved with a moderate Pt0.69Cr0.31 resistivity of ρxx∼133μΩ⋅cm. A low critical SOT-driven switching current density of Jc∼3.16×106A⋅cm−2 is also demonstrated. The damping constant (α) of Pt0.69Cr0.31/FM structure is also found to be reduced to 0.052 from the pure Pt/FM case of 0.078. The overall high σSH, giant ξDL, moderate ρxx, and reduced α of such Pt-Cr/FM heterostructure makes it promising for versatile extremely low power consumption SOT memory applications.

We report thermoelectric generation caused by nuclear spins in a solid: nuclear-spin Seebeck effect. The sample is a magnetically ordered material MnCO3 having a large nuclear spin (I = 5/2) of 55Mn nuclei and strong hyperfine coupling, with a Pt contact. In the system, we observe low-temperature thermoelectric signals down to 100 mK due to nuclear-spin excitation. The results were quantitatively reproduced by a theoretical calculation in which interfacial Korringa process is taken into consideration. The nuclear thermoelectric effect demonstrated here offers a new way for exploring thermoelectric science and technologies at ultralow temperatures.

Superluminal propagation of antiferromagnetic magnons in NiO was recorded at SPIE Optics + Photonics held in San Diego, California, United States 2022.

High-resolution scanning transmission X-ray microscopy (STXM) can provide quantitative access to the tilt angles of the local magnetization with respect to the surface. We use this state-of-the-art microscopy technique to probe the magnetic texture of the weak stripes texture hosted by a 180 nm thick Co40Fe40B20 layer. We report a comprehensive set of measurements of the weak stripes texture tilt angle, as well as a method that uses only a single direction x-ray beam to extract the angle. This method also extracts the spatial profile with 30 nm resolution and measures the tilt angle as a function of the applied field. Combining these informations, the macroscopic loop of magnetization as a function of applied field on a complex spin texture can be reconstructed in an unprecedented way and shows without doubt that flux closure domains exist. Beyond the characterization of the weak stripe angle, this quantitative magnetic X-ray microscopy technique can be used to study the local textures present in all materials exhibiting some circular magnetic X-ray dichroism.

The quantum diamond microscope (QDM) is a recently-developed technology used for magnetic imaging with high spatial resolution. The QDM offers micron-scale resolution over a few-mm field of view, high-reliability operation in ambient conditions, and high signal-to-noise readout of nano- and micro-scale magnetic sources, which have enabled scientific results that were previously impractical or impossible. I will present recent results applying QDM techniques to magnetically interrogating behavior and phenomena in commercial electronic devices, with applications to nanomagnetism and electrical engineering.

In the quest for ultra-low power electronic devices beyond the CMOS platform, ferroelectric Rashba semiconductors offer intriguing possibilities. After an overview of the main findings in this context, I will show that the ferroelectric polarization of epitaxial thin films of GeTe can be reliably switched by electrical gating and used to control spin-to-charge conversion by spin Hall effect. Ferroelectricity allows for efficient switching and stable state retention, while spin currents provides an effective read-out of the memory state. Doping, allowing and dimensionality can be used to tailor the properties of these compounds towards logic-in-memory devices with monolithic integrability with silicon.

This work reports remanent electric-control of spin-orbit torques (SOT) in a perpendicular ferromagnet-SrTiO3 system. Non-volatile electric-control of the sheet resistance is achieved with 1150% contrast, and two remanent resistivity states. A remanent electric-control of the SOT efficiency is demonstrated using second harmonic Hall methods, with sign inversion of the anti-damping-like effective field. These results are consistent with a combination of both intrinsic modulation of the SOT efficiency and extrinsic modulation due to the non-volatile electric-control of the current injection in the 2DEG. The non-volatile control of the SOT effective field is evidenced by reproducible inversion of the SOTs after voltage pulses initialization, opening the way to reconfigurable SOT memories and logic-gate architectures.

What is quantum spin torque: spintronics meets strongly correlated and long-range entangled quantum matter was recorded at SPIE Optics + Photonics held in San Diego, California, United States 2022.

In chiral crystals, the absence of inversion and mirror symmetry is responsible for unconventional spin properties and the emergence of exotic topological fermions [1]. Trigonal tellurium is one of the simplest chiral crystals, and by using spin-and-angle-resolved photoemission spectroscopy (srARPES) we have recently reported signatures of composite, accordionlike and Kramers-Weyl (KW) fermions in its band structure [2]. In contrast to conventional Weyl fermions that arise from band inversion, and are thereby degenerate in energy, KW points with opposite topological charges are pinned at different energies and at different time-reversal invariant momenta (TRIM) by the action of time-reversal symmetry. In KW fermions the spin is predicted to lie parallel to the wavevector, thus realizing a hedgehog texture [1]. In our study we clarify that the radial spin texture is not a prerogative of the KW fermions, but it can be observed also at non-TRIM point [2], as confirmed by an independent srARPES study of the spin properties around the H point of Te [3]. This is made possible by the presence of multiple rotational axes, combined with the breaking of mirror symmetry. Our results illustrate how the arrangement of spin in the reciprocal space is a consequence of the local point group symmetry, and it does not reflect only global properties of the crystal. Spin texture more complex than hedgehog-like can be stabilized, sharing common features with the arrangement taken in Skyrmions by magnetic momenta in real space [3, 4]. These spin textures are important ingredient to explain the chiral induced spin selectivity (CISS) effect that might find application in spintronics devices [5]; hence a complete classification of the spin texture for all local point group symmetries is highly demanded. [1] G. Chang et al., Nat. Mater. 17, 978 (2018) [2] G. Gatti et al., Phys. Rev. Lett. 125, 216402 (2020) [3] M. Sakano et al., Phys. Rev. Lett. 124, 136404 (2020) [4] C.M. Acosta et al., Phys. Rev. B 104, 104408 (2021) [5] S. Dalum and P. Hedegård, Nano Lett. 19, 5253 (2019)

The combination of spin-orbit coupling with broken spatial inversion symmetry in semiconductors (e.g. zinc-blende quantum-wells and surfaces) and localized spin states originated from a single magnetic defect is a promising system to realize future semiconductor spintronics devices [1]. We present a theory of dissipationless circulating current induced by a magnetic defect in a two-dimensional electron gas with both Bychkov-Rashba and Dresselhaus spin-orbit coupling [1]. The shape and spatial extent of these dissipationless circulating currents depend dramatically on the relative strengths of spin-orbit fields with differing spatial symmetry, offering the potential to use an electric gate to manipulate nanoscale magnetic fields and couple magnetic defects. The spatial structure of the fringing magnetic field emerging from the current is calculated and provides a direct way to measure the spin-orbit fields of the host, as well as the defect spin orientation, through scanning nanoscale magnetometry [3]. [1] Wolfowicz, G., Heremans, F.J., Anderson, C.P. et al. Nat RevMater 6, 906–925 (2021). [2] Da Cruz, A.R. and Flatté, M. E., arXiv:2111.06770 [3] Casola, F. and van der Sar, T. and Yacoby, A. Nature Reviews Materials, 3 (1), 17088 (2018).