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Abstract Direct light-spin interactions, induced by spin-orbit coupling (SOC), enable ultrafast spin manipulation through coherent spin excitation, which is universal in magnetic materials due to the ubiquity of SOC. Despite earlier reports, coherent spin excitation has been overlooked, because of the challenges of interpreting magneto-optical Kerr effect (MOKE) signals and the assumption of optical dipole transitions. By combining time-resolved MOKE measurements with density functional theory calculations, we demonstrate that a transient MOKE peak during laser pulse includes coherent spin excitation. We observe a pronounced transient MOKE peak in Co/Pt bilayer and a weaker one in Co single layer across different probe beam wavelengths. Theoretical calculations reveal that the Co/Pt peak includes contributions from both coherent spin excitation and nonlocal transport whereas the Co peak contains the contribution from the coherent spin excitation exclusively, which is governed by anti-crossing physics. Our findings highlight the SOC-mediated coherent spin dynamics, advancing ultrafast spintronic technology.

Abstract Spin-orbit torque (SOT), arising from spin-orbit coupling-induced spin currents, provides efficient control of magnetization. SOT characterization involving harmonic Hall resistances is typically done in low-current regimes, distinct from high-current regimes, where SOT-induced magnetization switching occurs. In this study, we investigate azimuthal angle (ϕ)-dependent harmonic Hall resistances of a Pt/yttrium iron garnet (YIG) layer across a wide range of measurement currents. Under low-current conditions, conventional ϕ-dependent Hall resistances are observed; the first harmonic Hall resistance exhibits sin 2ϕ behavior and the second harmonic Hall resistance comprises cos ϕ and cos 3ϕ terms. Interestingly, with increasing current, higher-order angular-dependent terms become non-negligible, referring to the sin 4ϕ and sin 6ϕ terms for the first harmonic and the cos 5ϕ and cos 7ϕ terms for the second harmonic Hall resistances. We attribute this unconventional angular dependence to the nonlinear response of magnetization direction to SOT, emphasizing its relevance to understanding the magnetization dynamics during SOT-induced switching under large currents.

Abstract Neuromorphic computing offers energy‐efficient computations for large‐scale data processing compared to conventional von Neumann computing. The artificial synapse, a key element for learning and memory operations in neuromorphic computing, requires multi‐state characteristics and the capability to change and store its states. The implementation of hardware‐based artificial synapses using nonvolatile memory provides significant advantages in terms of energy consumption and circuit area compared to their currently employed CMOS‐based counterparts. In this regard, spintronic devices have emerged as a promising candidate due to their desirable properties for artificial synapses, including multilevel formability, non‐volatility, and outstanding writing performance. In this study, spintronic artificial synapses utilizing voltage‐controlled multilevel magnetic states and energy‐ and area‐efficient artificial neural network architectures associated with them are demonstrated. The multilevel states are created by gradually modulating the magnetic easy‐axis orientation from perpendicular to in‐plane and vice versa, which is achieved either by sequentially applying gate voltage pulses or by adjusting the pulse width of the gate voltage. Based on these spintronic artificial synapses, convolutional neural network (CNN) and spiking neural network (SNN) architectures are constructed, demonstrating high recognition accuracy for the MNIST dataset with improved energy efficiency and a reduced circuit area.

Chirality-induced spin selectivity (CISS) allows for the generation of spin currents without the need for ferromagnets or external magnetic fields, enabling innovative spintronic device designs. One example is a chiral spin valve composed of ferromagnetic and chiral materials, in which the resistance depends on both the magnetization direction of the ferromagnet and the chirality of the chiral material. So far, chiral spin valves have predominately employed chiral organic molecules, which have limited device applications. Chiral perovskites, which combine the properties of inorganic perovskites with chiral organic molecules, provide an excellent platform for exploring CISS-based devices. However, previous chiral perovskite-based spin valves exhibited magnetoresistance (MR) only at low temperatures. Here, we report room temperature MR in a chiral spin valve consisting of chiral perovskites/AlOx/perpendicular ferromagnet structures. It is observed that the chiral MR increases with rising temperature, suggesting the crucial role of phonon-induced enhancement of spin–orbit coupling in CISS in our device. Furthermore, we enhanced the chiral MR by introducing chiral molecules with amplified chirality. This highlights the potential of chirality engineering to improve CISS and the associated chiral MR, thereby opening possibilities for chiral spin valves tailored for cutting-edge spintronic applications.

Abstract Current-induced spin torques enable the electrical control of the magnetization with low energy consumption. Conventional magnetic random access memory (MRAM) devices rely on spin-transfer torque (STT), this however limits MRAM applications because of the nanoseconds incubation delay and associated endurance issues. A potential alternative to STT is spin-orbit torque (SOT). However, for practical, high-speed SOT devices, it must satisfy three conditions simultaneously, i.e., field-free switching at short current pulses, short incubation delay, and low switching current. Here, we demonstrate field-free SOT switching at sub-ns timescales in a CoFeB/Ti/CoFeB ferromagnetic trilayer, which satisfies all three conditions. In this trilayer, the bottom magnetic layer or its interface generates spin currents with polarizations in both in-plane and out-of-plane components. The in-plane component reduces the incubation time, while the out-of-plane component realizes field-free switching at a low current. Our results offer a field-free SOT solution for energy-efficient scalable MRAM applications.

Abstract Spin Seebeck effect (SSE) refers to the generation of an electric voltage transverse to a temperature gradient via a magnon current. SSE offers the potential for efficient thermoelectric devices because the transverse geometry of SSE enables to utilize waste heat from a large-area source by greatly simplifying the device structure. However, SSE suffers from a low thermoelectric conversion efficiency that must be improved for widespread application. Here we show that the SSE substantially enhances by oxidizing a ferromagnet in normal metal/ferromagnet/oxide structures. In W/CoFeB/AlOx structures, voltage-induced interfacial oxidation of CoFeB modifies the SSE, resulting in the enhancement of thermoelectric signal by an order of magnitude. We describe a mechanism for the enhancement that results from a reduced exchange interaction of the oxidized region of ferromagnet, which in turn increases a temperature difference between magnons in the ferromagnet and electrons in the normal metal and/or a gradient of magnon chemical potential in the ferromagnet. Our result will invigorate research for thermoelectric conversion by suggesting a promising way of improving the SSE efficiency.

In a ferrimagnet, there are two magnetic sublattices coupled antiferromagnetically. The dynamics of the two magnetic sublattices isn’t well understood, as the magnetic moments for each reside at different energy levels. Here, Park et al show that the magnetic moments at deeper energy levels show spin-glass like characteristics.

Spin-hall nano-oscillators are a variation on spin-torque nano-oscillators, where the spin-Hall effect is used to drive oscillations, however past examples have had limited frequency tunability. Here, Choi et al demonstrate spin-hall oscillators with wide voltage controlled frequency tunability.

Abstract Physical unclonable functions (PUFs), which exploit uncontrollable and unpredictable randomness of materials or devices, have been investigated as a hardware‐based security primitive owing to their robustness against adversarial attacks. Spin–orbit torque (SOT) switching is one of the promising techniques for PUF applications because it can provide randomness by the stochastic switching distribution of perpendicular magnetization. In this study, the improvement in the reliability of SOT‐based PUFs against external magnetic fields with write‐back operation (WBO) is demonstrated. A PUF consisting of 8 × 4 array Hall‐bar devices with a Ta/CoFeB/MgO structure is fabricated, where the random distribution of the SOT switching current serves as an entropy source. However, the information stored in the PUF is easily modified by the application of an external magnetic field. To improve the robustness against magnetic fields, a WBO is introduced that applies an additional current to saturate the magnetization in either the upward or downward direction depending on the magnetic state. As a result, the SOT‐based PUF maintains an entropy value close to unity under a magnetic field of up to the coercive field of the CoFeB layer. Furthermore, the WBO provides a digitalized output, which potentially reduces peripheral circuitry such as analog‐to‐digital converters.

A major challenge for spin based electronics is the electrical control of magnetization. Here, Kang et al demonstrate how electric field control of the Rashba effect in a Pt/Co/AlOx can enable control of the spin-orbit torque and allow for field free switching of the magnetization.

Antiferromagnets have great promise for spin-based information processing, offering both high operation speed, and an immunity to stray fields. Here, Kang et al demonstrate electrical manipulation of the exchange-bias, without the need for a heavy metal layer.

The orbital Hall effect involves the transport of orbital angular momentum perpendicular to an applied charge current, analogous to the spin Hall effect. Here, Lee et al examine magnetic torques present in a Nickel/Tantalum bilayer, clearly demonstrating the contribution of the orbital Hall effect.

Manipulation of the magnetization is of major importance in spintronics. The authors demonstrate that an electric field triggers a transverse flow of orbital moment: the so-called orbital Hall effect. This enables the efficient magnetization control, holding the promise for fast and miniaturized memories and sensors.

Unidirectional spin Hall magnetoresistance (USMR) is a directionally dependent feature of a ferromagnetic/normal metal bilayer for which the underlying mechanisms are still under debate. Here, the authors investigate the crystallographic dependence of USMR in epitaxial Cr/Fe bilayers finding that electron-magnon scattering plays an important role.

Abstract Electrical conduction in magnetic materials depends on their magnetization configuration, resulting in various magnetoresistances (MRs). The microscopic mechanisms of MR have so far been attributed to either an intrinsic or extrinsic origin, yet the contribution and temperature dependence of either origin has remained elusive due to experimental limitations. In this study, we independently probed the intrinsic and extrinsic contributions to the anisotropic MR (AMR) of a permalloy film at varying temperatures using temperature-variable terahertz time-domain spectroscopy. The AMR induced by the scattering-independent intrinsic origin was observed to be approximately 1.5% at T = 16 K and is virtually independent of temperature. In contrast, the AMR induced by the scattering-dependent extrinsic contribution was approximately 3% at T = 16 K but decreased to 1.5% at T = 155 K, which is the maximum temperature at which the AMR can be resolved using THz measurements. Our results experimentally quantify the temperature-dependent intrinsic and extrinsic contributions to AMR, which can stimulate further theoretical research to aid the fundamental understanding of AMR.

Abstract Interface‐generated spin currents in ferromagnet (FM)/nonmagnet (NM) structures provide both in‐plane and out‐of‐plane spin–orbit torques (SOTs), enabling the field‐free switching of perpendicular magnetization of the other FM layer in magnetic trilayers. In this study, the NM thickness dependence of interface‐generated spin currents and associated SOTs in FM/Ti/CoFeB trilayers is investigated. In such magnetic trilayers, it is known that the in‐plane SOT results from the spin–orbit filtering of the interface‐generated spin current, while the out‐of‐plane SOT is due to the spin–orbit precession. These results show that the polarity of current‐induced magnetization switching under an in‐plane magnetic field reverses with increasing Ti thickness. This indicates that the sign of the in‐plane SOT depends on the current distribution between the bottom FM and Ti layers. On the other hand, field‐free switching occurs only for a Ti thickness of up to ≈4 nm, and the same polarity is retained, demonstrating that out‐of‐plane SOT is governed by the charge current flowing near the interface. These results suggest that field‐free switching efficiency can be enhanced by engineering the relative conductance of the FM/NM bilayers to constructively combine in‐plane and out‐of‐plane SOTs caused by interface‐generated spin currents.

This study investigates the effects of annealing on the tunnel magnetoresistance (TMR) ratio in CoFeB/MgO/CoFeB-based magnetic tunnel junctions (MTJs) with different capping layers and correlates them with microstructural changes. It is found that the capping layer plays an important role in determining the maximum TMR ratio and the corresponding annealing temperature (Tann). For a Pt capping layer, the TMR reaches ~95% at a Tann of 350 °C, then decreases upon a further increase in Tann. A microstructural analysis reveals that the low TMR is due to severe intermixing in the Pt/CoFeB layers. On the other hand, when introducing a Ta capping layer with suppressed diffusion into the CoFeB layer, the TMR continues to increase with Tann up to 400 °C, reaching ~250%. Our findings indicate that the proper selection of a capping layer can increase the annealing temperature of MTJs so that it becomes compatible with the complementary metal-oxide-semiconductor backend process.

Magneto-optic Kerr effect microscopy is useful for dynamic magnetic studies, but is limited by the weak magneto-optical activity. Here, the authors show that extreme anti-reflection result in a Kerr amplitude as large as 20° and enables real-time detection of sub-wavelength magnetic domain reversals.

In normal metal/ferromagnet bilayers, the spin Hall magnetoresistance originating from the spin Hall effect of normal metal possesses positive sign regardless of the sign of spin Hall conductivity of normal metal. Here, the authors report negative spin Hall magnetoresistance of Ta/NiFe bilayers due to interfacial spin orbit coupling at interfaces.

We investigate the Pt thickness dependence of spin–orbit torques (SOTs) in Co/Pt layers grown on single crystalline SrTiO3 and LaAlO3 and amorphous SiO2 substrates. We measure the SOT-induced effective damping-like fields and spin Hall magnetoresistances of the Co/Pt (tPt) samples, where tPt varies from 0.5 to 5.5 nm. We find that the Co/Pt layers grown on the single crystalline substrates show weaker thickness dependence of the SOT than the samples on the SiO2 substrate, which cannot be explained by the conventional bulk spin Hall effect in the Pt layer. This indicates that there is a non-negligible interfacial SOT originating from the Pt/substrate interface, which is more pronounced for thinner Pt. These results provide a way to design SOT material structures with enhanced SOT efficiency by incorporating interfacial SOT.

Pure spin current generated thermally in nonmagnetic materials known as spin Nernst effect has not been demonstrated experimentally. Here, the authors report the observation of spin Nernst effect by studying the thermally-induced transverse magnetoresistance in ferromagnet/non-magnet heavy metal bilayers

Electronic skins and health monitoring devices rely on integrated tactile sensors, which often require tailored degrees of sensitivity in specific pressure ranges. Here, the authors fabricate a versatile matrix array of pressure-sensitive graphene transistors operating in the wide 250 Pa to 3 MPa pressure range.

Spin–orbit torque facilitates efficient magnetisation switching via an in-plane current in perpendicularly magnetised heavy-metal/ferromagnet heterostructures. The efficiency of spin–orbit-torque-induced switching is determined by the charge-to-spin conversion arising from either bulk or interfacial spin–orbit interactions or both. Here, we demonstrate that the spin–orbit torque and the resultant switching efficiency in Pt/CoFeB systems are significantly enhanced by an interfacial modification involving Ti insertion between the Pt and CoFeB layers. Spin pumping and X-ray magnetic circular dichroism experiments reveal that this enhancement is due to an additional interface-generated spin current of the non-magnetic interface and/or improved spin transparency achieved by suppressing the proximity-induced moment in the Pt layer. Our results demonstrate that interface engineering affords an effective approach to improve spin–orbit torque and thereby magnetisation switching efficiency.

The original version of this Article contained an error in ref. 27, which was incorrectly given with the wrong journal name as: Meyer, S. et al. Observation of the spin Nernst effect. Nat. Phys. 16, 977–981 (2017). The correct form of ref. 27 is: Meyer, S. et al. Observation of the spin Nernst effect. Nat. Mater. 16, 977–981 (2017). This has now been corrected in the PDF and HTML versions of the Article.