Spin Accumulation Research Articles

Thermally robust electrically reversible spin selection in a quantum dot qubit with double reservoir spin accumulations

Thermally robust electrically reversible spin selection in a quantum dot qubit with double reservoir spin accumulations

Néel Vector Auto-Oscillations and Reorientations Induced by Spin-Polarized Electric Currents in Antiferromagnetic Mn2Au Nanolayer

The electrical excitation of spin dynamics in antiferromagnets is important for the development of high-speed spintronic devices with a low power consumption. Here, we present a theoretical study of the spin dynamics generated in the Ni/Cu/Mn2Au/Cu/Ni nanostructure by a perpendicular-to-plane electric current. Our investigation includes atomistic simulations of spin reorientations in a few monolayer antiferromagnetic Mn2Au film and numerical calculations of nonequilibrium spin accumulation in the nanostructure comprising ferromagnetic Ni polarizers with antiparallel magnetizations. This combined approach enables us to quantify the dynamics of Mn magnetic moments induced by the spin-transfer torque created by the spin-polarized charge flow. The calculations show that the direct electric current with the density exceeding some temperature-dependent threshold value gives rise to steady-state auto-oscillations of the Néel vector in the [Formula: see text]-oriented Mn2Au nanolayer. As the precession of Mn magnetic moments occurs around an out-of-plane direction strongly deflected from their initial in-plane orientations, the emergence of auto-oscillations should be regarded as the realization of a dynamic spin reorientation transition in the antiferromagnetic crystal. Remarkably, the precession frequency rises with increasing current density J and exceeds 1[Formula: see text]THz at [Formula: see text][Formula: see text]A[Formula: see text]m[Formula: see text], which makes the described antiferromagnetic spin transfer nano-oscillator attractive for device applications. Furthermore, our simulations demonstrate that a picosecond current pulse injected into the Ni/Cu/Mn2Au/Cu/Ni nanostructure can induce a precessional switching of the Néel vector. Depending on the current density and pulse duration, the Néel vector rotates by either 90° or 180° and attains stable orientation along the corresponding [Formula: see text] easy axis in the plane of the Mn2Au nanolayer. This feature shows that the Ni/Cu/Mn2Au/Cu/Ni nanostructure could be used as a nonvolatile memory cell with the electrical writing and readout.

Temperature dependence of spin transport behavior in Heusler alloy CPP-GMR

In this study, we investigate the effect of temperature on the performance of a read sensor by utilizing an atomistic model coupled with a spin transport model. Specifically, we study the temperature dependence of spin transport behavior and MR outputs in a Co2FeAl0.5Si0.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {Co}_2\ ext {FeAl}_{0.5}\ ext {Si}_{0.5}$$\\end{document} (CFAS)(5nm)/Cu(5nm)/CFAS(5nm) trilayer with diffusive interfaces. Initially, the two-channel model of spin-dependent resistivity is used to calculate the temperature dependence of spin transport parameters which serves as essential input for the spin accumulation model. Our findings demonstrate that as the temperature increases, the spin transport parameters and magnetic properties decrease due to the influence of thermal fluctuation. At a critical temperature, where the ferromagnet transitions to a paramagnetic state, we observe zero spin polarization. Furthermore, at elevated temperatures, the spin accumulation deviates from the equilibrium value, leading to a reduction in the magnitude of spin current and spin transport parameters due to thermal effects. As a consequence, the MR ratio decreases from 65% to 20% with increasing temperature from 0 to 400 K. Our results are consistent with previous experimental measurements. This study allows to deeply understand the physical mechanism in the reader stack which can significantly benefit reader design.

Atomistic spin dynamics simulations of magnonic spin Seebeck and spin Nernst effects in altermagnets

Magnon band structures in altermagnets are characterized by an energy splitting of modes with opposite chirality, even in the absence of applied external fields and relativistic effects, because of an anisotropy in the Heisenberg exchange interactions. We perform quantitative atomistic spin dynamics simulations based on electronic structure calculations on rutile RuO2, a prototypical “d-wave” altermagnet, to study magnon currents generated by thermal gradients. We report substantial spin Seebeck and spin Nernst effects, i.e., longitudinal or transverse spin currents, depending on the propagation direction of the magnons with respect to the crystal, together with a finite spin accumulation associated with nonlinearities in the temperature profile. Our findings are consistent with the altermagnetic spin-group symmetry, as well as predictions from linear spin-wave theory and semiclassical Boltzmann transport theory. Published by the American Physical Society 2024

Coherent spin wave excitation with radio-frequency spin–orbit torque

Spin waves, collective perturbations of magnetic moments, are both fundamental probes for magnetic physics and promising candidates for energy-efficient signal processing and computation. Traditionally, coherent propagating spin waves have been generated by radio frequency (RF) inductive Oersted fields from current-carrying electrodes. An alternative mechanism, spin–orbit torque (SOT), offers more localized excitation through interfacial spin accumulation but has been mostly limited to DC to kHz frequencies. SOT driven by RF currents, with potentially enhanced pumping efficiency and unique spin dynamics, remains largely unexplored, especially in magnetic insulators. Here, we conduct a comprehensive theoretical and computational investigation into the generation of coherent spin waves via RF-SOT in the prototypical yttrium iron garnet. We characterize the excitation of forward volume, backward volume, and surface modes in both linear and nonlinear regimes, employing single and interdigitated electrode configurations. We reveal and explain several unique and surprising features of RF-SOT compared to inductive excitation, including higher efficiency, distinct mode selectivity, and directional symmetry, a ∼3π/4 phase offset, reduced anharmonic distortion in the nonlinear regime, and the absence of second harmonic generation. These insights position RF-SOT as a promising new mechanism for future magnonic and spintronic applications.

Spin Hall-induced bilinear magnetoelectric resistance.

Magnetoresistance is a fundamental transport phenomenon that is essential for reading the magnetic states for various information storage, innovative computing and sensor devices. Recent studies have expanded the scope of magnetoresistances to the nonlinear regime, such as a bilinear magnetoelectric resistance (BMER), which is proportional to both electric field and magnetic field. Here we demonstrate that the BMER is a general phenomenon that arises even in three-dimensional systems without explicit momentum-space spin textures. Our theory suggests that the spin Hall effect enables the BMER provided that the magnitudes of spin accumulation at the top and bottom interfaces are not identical. The sign of the BMER follows the sign of the spin Hall effect of heavy metals, thereby evidencing that the BMER originates from the bulk spin Hall effect. Our observation suggests that the BMER serves as a general nonlinear transport characteristic in three-dimensional systems, especially playing a crucial role in antiferromagnetic spintronics.

Non-Hermitian Mott Skin Effect.

We propose a novel type of skin effects in non-Hermitian quantum many-body systems that we dub a "non-Hermitian Mott skin effect." This phenomenon is induced by the interplay between strong correlations and the non-Hermitian point-gap topology. The Mott skin effect induces extreme sensitivity to the boundary conditions only in the spin degree of freedom (i.e., the charge distribution is not sensitive to boundary conditions), which is in sharp contrast to the ordinary non-Hermitian skin effect in noninteracting systems. Concretely, we elucidate that a bosonic non-Hermitian chain exhibits the Mott skin effect in the strongly correlated regime by closely examining an effective Hamiltonian. The emergence of the Mott skin effect is also supported by numerical diagonalization of the bosonic chain. The difference between the ordinary non-Hermitian skin effect and the Mott skin effect is also reflected in the time evolution of physical quantities; under the time evolution spin accumulation is observed while the charge distribution remains spatially uniform.

Chirality-Induced Magnet-Free Spin Generation in a Semiconductor.

Electrical generation and transduction of polarized electron spins in semiconductors (SCs) are of central interest in spintronics and quantum information science. While spin generation in SCs is frequently realized via electrical injection from a ferromagnet (FM), there are significant advantages in nonmagnetic pathways of creating spin polarization. One such pathway exploits the interplay of electron spin with chirality in electronic structures or real space. Here, utilizing chirality-induced spin selectivity (CISS), the efficient creation of spin accumulation in n-doped GaAs via electric current injection from a normal metal (Au) electrode through a self-assembled monolayer (SAM) of chiral molecules (α-helix l-polyalanine, AHPA-L), is demonstrated. The resulting spin polarization is detected as a Hanle effect in the n-GaAs, which is found to obey a distinct universal scaling with temperature and bias current consistent with chirality-induced spin accumulation. The experiment constitutes a definitive observation of CISS in a fully nonmagnetic device structure and demonstration of its ability to generate spin accumulation in a conventional SC. The results thus place key constraints on the physical mechanism of CISS and present a new scheme for magnet-free SC spintronics.

Interfacial Spintronic THz Emission

AbstractThe broken inversion symmetry at the ferromagnet (FM)/heavy‐metal (HM) interface leads to spin‐dependent degeneracy of the energy band, forming spin‐polarized surface states. As a result, the interface serves as an effective medium for converting spin accumulation into 2D charge current through the inverse Rashba–Edelstein effect. Exploring and assessing this spin‐to‐charge conversion (SCC) phenomenon at the FM/HM interface can offer a promising avenue to surpass the presumed limits of SCC in bulk HM layers. Spintronic heterostructures are utilized as a platform to measure the SCC experienced by photoexcited spin currents. Therefore, FM/HM heterostructures emitting terahertz electric field upon illumination by femtosecond laser pulses enable quantitative measure of the ultrafast SCC process. This results demonstrate a robust interfacial spin‐to‐charge conversion (iSCC) within a synthetic antiferromagnetic heterostructure, specifically for the NiFe/Ru/NiFe configuration, by isolating the SCC contribution originating from the interface and the bulk heavy‐metal (HM). Through the measurements of the emitted terahertz pulse, the iSCC at the NiFe/Ru interface is identified to be ≈27% of the strength as compared to SCC from the highest spin‐Hall conducting heavy‐metal, Pt. The results thus highlight the significance of interfacial engineering as a promising pathway for achieving efficient ultrafast spintronic devices.

Giant Spin‐Charge Conversion in Ultrathin Films of the MnPtSb Half‐Heusler Compound

AbstractHalf‐metallic half‐Heusler compounds with strong spin‐orbit‐coupling and broken inversion symmetry in their crystal structure are promising materials for generating and absorbing spin‐currents, thus enabling the electric manipulation of magnetization in energy‐efficient spintronic devices. In this work, the spin‐to‐charge conversion in the sputtered half‐Heusler MnPtSb within thickness (t) range from 1 to 6 nm is reported. A combination of X‐ray and transmission electron microscopy measurements evidence the epitaxial nature of these ultrathin non‐centrosymmetric layers, with a clear (111)‐orientation on top of (0001) single‐crystal sapphire. By broadband ferromagnetic resonance (FMR), a four orders of magnitude tunable spin accumulation in the MnPtSb‐based heterostructures, within t = 1–6 nm range, is observed. By using spin pumping FMR, a remarkable t‐dependent spin‐charge conversion in the MnPtSb layers is measured, which clearly demonstrates the interfacial origin of the conversion. When interpreted within the inverse Edelstein effect (IEE), the spin‐charge conversion efficiency extracted at room temperature for the thinnest MnPtSb layer reaches λIEE≈3 nm, representing an extremely high conversion. The still never explored ultrathin regime of the MnPtSb films studied in this work and the discovery of their outstanding functionality are two ingredients that demonstrate the potentiality of such materials for future applications in spintronics.

Room-temperature spin injection across a chiral perovskite/III-V interface.

Spin accumulation in semiconductor structures at room temperature and without magnetic fields is key to enable a broader range of optoelectronic functionality1. Current efforts are limited owing to inherent inefficiencies associated with spin injection across semiconductor interfaces2. Here we demonstrate spin injection across chiral halide perovskite/III-V interfaces achieving spin accumulation in a standard semiconductor III-V (AlxGa1-x)0.5In0.5P multiple quantum well light-emitting diode. The spin accumulation in the multiple quantum well is detected through emission of circularly polarized light with a degree of polarization of up to 15 ± 4%. The chiral perovskite/III-V interface was characterized with X-ray photoelectron spectroscopy, cross-sectional scanning Kelvin probe force microscopy and cross-sectional transmission electron microscopy imaging, showing a clean semiconductor/semiconductor interface at which the Fermi level can equilibrate. These findings demonstrate that chiral perovskite semiconductors can transform well-developed semiconductor platforms into ones that can also control spin.

Ultra-high spin emission from antiferromagnetic FeRh

An antiferromagnet emits spin currents when time-reversal symmetry is broken. This is typically achieved by applying an external magnetic field below and above the spin-flop transition or by optical pumping. In this work we apply optical pump-THz emission spectroscopy to study picosecond spin pumping from metallic FeRh as a function of temperature. Intriguingly we find that in the low-temperature antiferromagnetic phase the laser pulse induces a large and coherent spin pumping, while not crossing into the ferromagnetic phase. With temperature and magnetic field dependent measurements combined with atomistic spin dynamics simulations we show that the antiferromagnetic spin-lattice is destabilised by the combined action of optical pumping and picosecond spin-biasing by the conduction electron population, which results in spin accumulation. We propose that the amplitude of the effect is inherent to the nature of FeRh, particularly the Rh atoms and their high spin susceptibility. We believe that the principles shown here could be used to produce more effective spin current emitters. Our results also corroborate the work of others showing that the magnetic phase transition begins on a very fast picosecond timescale, but this timescale is often hidden by measurements which are confounded by the slower domain dynamics.

Second-order charge and spin transport in LaO/STO system in the presence of cubic Rashba spin orbit couplings

Under an applied electric field, certain non-centrosymmetric materials with broken time-reversal symmetry may exhibit non-reciprocal transport behavior in which the charge and spin currents contain components that are second order in the electric field. In this study, we investigate the second-order spin accumulation and charge and spin responses in the LaAlO3/SrTiO3 (LaO/STO) system with magnetic dopants under the influence of linear and cubic Rashba spin–orbit coupling (RSOC) terms. We explain the physical origin of the second-order response and perform a symmetry analysis of the first- and second-order responses for different dopant magnetization directions relative to the applied electric field. We then numerically solve the Boltzmann transport equation by extending the approach of Schliemann and Loss (2003 Phys. Rev. B 68 165311) to higher orders in the electric field. We show that the sign of the second-order responses can be switched by varying the magnetization direction of the magnetic dopants or relative strengths of the two cubic RSOC terms and explain these trends by considering the Fermi surfaces of the respective systems. These findings provide insights into the interplay of multiple SOC effects in the LaO/STO system and how the resulting first- and second-order charge and spin responses can be engineered by exploiting the symmetries of the system.

Inverse Spin-Hall Effect and Spin Swapping in Spin-Split Superconductors.

When a spin-splitting field is introduced to a thin film superconductor, the spin currents polarized along the field couples to energy currents that can only decay via inelastic scattering. We study spin and energy injection into such a superconductor where spin-orbit impurity scattering yields inverse spin-Hall and spin-swapping currents. We show that the combined presence of a spin-splitting field, superconductivity, and inelastic scattering gives rise to a strong enhancement of the ordinary inverse spin-Hall effect, as well as unique inverse spin-Hall and spin-swapping signals orders of magnitude stronger than the ordinary inverse spin-Hall signal. These can be completely controlled by the orientation of the spin-splitting field, resulting in a long-range charge and spin accumulations detectable much further from the injector than in the normal state. While the enhanced inverse spin-Hall signals offer a major improvement in spin detection sensitivity, the unique spin-swap signals can be utilized for designing devices where both the spin and current directions are controlled and altered throughout the geometry.

Exciton spin Hall effect in arc-shaped strained WSe2

Generating a pure spin current using electrons, which have degrees of freedom beyond spin, such as electric charge and valley index, presents challenges. In response, we propose a mechanism based on intervalley exciton dynamics in strained transition metal dichalcogenides (TMDs) to achieve the in an electrically insulating regime, without the need for an external electric field. The interplay between strain gradients and strain-induced pseudomagnetic fields results in a net Lorentz force on long-lived intervalley excitons in WSe2, carrying nonzero spin angular momentum. This process generates an exciton-mediated pure spin Hall current, resulting in opposite-sign spin accumulations and local magnetization on the two sides of the single-layer arc-shaped TMD. We demonstrate that the magnetic field induced by spin accumulation, at approximately ∼mT, can be detected using techniques such as superconducting quantum interference magnetometry or spatially resolved magneto-optical Faraday and Kerr rotations. Published by the American Physical Society 2024

Simulation study of the Gilbert damping in Ni80Fe20/Nd bilayers: comparison with experiments

We present an experimental and computational investigation the Neodymium thickness dependence of the effective damping constant ( αeff ) in Ni 80 Fe 20/Neodymium (Py/Nd) bilayers. The computational results show that the magnetic damping is strongly dependent on the thickness of Nd, which is in agreement with experimental data. Self consistent solutions of the spin accumulation model and the local magnetisation were used in the simulations. It was not possible to obtain agreement with experiment under the assumption of an enhanced damping in a single Py monolayer. Instead, it was found that the enhanced damping due to spin pumping needed to be spread across two monolayers of Py. This is suggested to arise from interface mixing. Subsequently, the temperature dependence of the effective damping was investigated. It is found that, with increasing temperature, the influence of thermally-induced spin fluctuations on magnetic damping becomes stronger with increasing Nd thickness.

Tunneling current-controlled spin states in few-layer van der Waals magnets.

Effective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in few-layer CrI3, depending on the polarity and amplitude of the current. We propose a mechanism involving nonequilibrium spin accumulation in the graphene electrodes in contact with the CrI3 layers. We further demonstrate tunneling current-tunable stochastic switching between multiple spin states of the CrI3 tunnel devices, which goes beyond conventional bi-stable stochastic magnetic tunnel junctions and has not been documented in two-dimensional magnets. Our findings not only address the existing knowledge gap concerning the influence of tunneling currents in controlling the magnetism in two-dimensional magnets, but also unlock possibilities for energy-efficient probabilistic and neuromorphic computing.

Giant magnetoimpedance effect and spin accumulation in conjugated polymeric networks with inhomogeneity

The magnetoimpedance effect allows us to estimate the extent of spin dependent scattering in disordered solids. The change in impedance with respect to applied magnetic field manifests through local change in permeability on the surface and it amplifies at defect sites. The local electrical inhomogeneities are expected to aid this effect through spin dependent scattering. The organic conjugated electrical networks provide scope for producing such inhomogeneities formed by path defects and protonic charge accumulation leading to spin dependent scattering. This hypothesis is investigated in the present work taking polyaniline as a prototype network. The electrical inhomogeneities in the network were controlled by selective oxidation and aging in polyaniline. The Giant Magnetoimpedance (GMI) was observed in the electrically inhomogeneous network with the change in electrical impedance of the order of 50%–60% for lower frequencies with prominent capacitive coupling and a change of the order of 200% at higher frequencies with prominent inductive coupling with the application of magnetic field. However, no spin accumulation was observed in the insulating networks formed by a modified oxidative process. This study is expected to serve as a tool to develop frequency selective spin accumulation based magnetic field sensors and oscillator networks.

The modulation of perpendicular magnetic anisotropy and spin–orbit toque in Pt/Co/Pt multilayers with interfacial decoration by insertion of a Bi layer

Bismuth is a well-known semimetal material with strong spin–orbit coupling and has been confirmed to exhibit a larger spin Hall angle in CuBi and PtBi alloys with low Bi-doping concentration. In this study, we investigated perpendicular magnetic anisotropy (PMA) and spin–orbit torques (SOTs) in Pt/Co/Pt multilayers by inserting a Bi layer with a thickness of 2 nm at the Co/Pt interface. Two types of PMA multilayers, Pt (3 nm)/Co (1 nm)/Pt (5 nm) and Pt (3 nm)/Co (1 nm)/Pt (1 nm), with different spin accumulations, were designed and prepared by varying the top Pt thickness. A significant enhancement of PMA and SOT efficiency is observed in the Pt (3 nm)/Co (1 nm)/Bi (2 nm)/Pt (5 nm) multilayer via a Bi-layer interfacial decoration. However, for the Pt (3 nm)/Co (1 nm)/Bi (2 nm)/Pt (1 nm) multilayers, both PMA and SOT efficiency decrease with decoration of the Bi layer at the Co/Pt interface. Meanwhile, the sign of SOTs changes in the Pt (3 nm)/Co (1 nm)/Pt (5 nm) and Pt (3 nm)/Co (1 nm)/Pt (1 nm) multilayers when introducing a Bi layer. This completely opposite behavior illustrates that the Bi/Pt interface plays an important role in modulating the PMA and SOT efficiency of Pt/Co/Bi/Pt multilayers. Optimizing the alloying of Bi/Pt could be an effective approach to increase the PMA and SOT efficiency of Pt/Co/Bi/Pt multilayers.

Superconducting phase diagram and spin diode effect via spin accumulation

Superconducting phase diagram and spin diode effect via spin accumulation