Current-induced Magnetization Research Articles

Spin-Orbit Torque Vector Quantification in Nanoscale Magnetic Tunnel Junctions.

Spin-orbit torques (SOT) allow ultrafast, energy-efficient toggling of magnetization state by an in-plane charge current for applications such as magnetic random-access memory (SOT-MRAM). Tailoring the SOT vector comprising of antidamping (TAD) and fieldlike (TFL) torques could lead to faster, more reliable, and low-power SOT-MRAM. Here, we establish a method to quantify the longitudinal (TAD) and transverse (TFL) components of the SOT vector and its efficiency χAD and χFL, respectively, in nanoscale three-terminal SOT magnetic tunnel junctions (SOT-MTJ). Modulation of nucleation or switching field (BSF) for magnetization reversal by SOT effective fields (BSOT) leads to the modification of SOT-MTJ hysteresis loop behavior from which χAD and χFL are quantified. Surprisingly, in nanoscale W/CoFeB SOT-MTJ, we find χFL to be (i) twice as large as χAD and (ii) 6 times as large as χFL in micrometer-sized W/CoFeB Hall-bar devices. Our quantification is supported by micromagnetic and macrospin simulations which reproduce experimental SOT-MTJ Stoner-Wohlfarth astroid behavior only for χFL > χAD. Additionally, from the threshold current for current-induced magnetization switching with a transverse magnetic field, we show that in SOT-MTJ, TFL plays a more prominent role in magnetization dynamics than TAD. Due to SOT-MRAM geometry and nanodimensionality, the potential role of nonlocal spin Hall spin current accumulated adjacent to the SOT-MTJ in the mediation of TFL and χFL amplification merits to be explored.

Magnetic vortex control with current-induced axial magnetization in centrosymmetric Weyl materials

We consider magnetic Weyl metals as a platform to achieve current control of magnetization textures with transport currents utilizing their underlying band geometry. We show that the transport current in a Weyl semimetal produces an axial magnetization due to orbital magnetic moments of the Weyl electrons. The associated axial magnetization can generate a torque acting on the localized magnetic moments. For the case of a magnetic vortex in a nanodisk of Weyl materials, this current-induced torque can be used to reverse its circulation and polarity. We discuss the axial magnetization torques in Weyl metals on general symmetry grounds and compare their strength to current-induced torques in more conventional materials.

Influence of ferromagnetic interlayer exchange coupling on current-induced magnetization switching and Dzyaloshinskii-Moriya interaction in Co/Pt/Co multilayer system.

This paper investigates the relationship among interlayer exchange coupling (IEC), Dzyaloshinskii-Moriya interaction (DMI), and multilevel magnetization switching within a Co/Pt/Co heterostructure, where varying Pt thicknesses enable control over the coupling strength. Employing Brillouin Light Scattering to quantify the effective DMI, we explore its potential role in magnetization dynamics and multilevel magnetization switching. Experimental findings show four distinct resistance states under an external magnetic field and spin Hall effect related spin current. We explain this phenomenon based on the asymmetry between Pt/Co and Co/Pt interfaces and the interlayer coupling, which, in turn, influences the DMI and subsequently impacts the magnetization dynamics. Numerical simulations, including macrospin, 1D domain wall, and simple spin wave models, further support the experimental observations of multilevel switching and help uncover the underlying mechanisms. Our proposed explanation, supported by magnetic domain observation using polar-magnetooptical Kerr microscopy, offers insights into both the spatial distribution of magnetization and its dynamics for different IECs, thereby shedding light on its interplay with DMI, which may lead to potential applications in storage devices.

Current-Induced Magnetization in Metal Without Space-Inversion Symmetry

Current-Induced Magnetization in Metal Without Space-Inversion Symmetry

Perpendicular magnetic anisotropy in permalloy ultrathin film grown on RuO2(101) surface

Permalloy (Py) films are commonly regarded as soft magnetic materials, wherein the magnetization aligns within the film plane. Our studies reveal the presence of perpendicular magnetic anisotropy in Py thin films deposited on the collinear antiferromagnetic RuO2(101) surface. By employing both the magneto-optical Kerr effect and the anomalous Hall effect, we identified the interfacial origin of the observed perpendicular anisotropy, quantifying it with an interfacial anisotropy energy of approximately 0.77 erg/cm2. Current-induced magnetization switching in Py/RuO2(101) has been achieved under an in-plane field, with the current applied along both [010] and [10 1¯] directions. Py films exhibiting perpendicular magnetic anisotropy offer an innovative material platform for investigating the spin–orbit effect, holding significant potential for spintronics applications.

Nonlinear optical diode effect in a magnetic Weyl semimetal

Diode effects are of great interest for both fundamental physics and modern technologies. Electrical diode effects (nonreciprocal transport) have been observed in Weyl systems. Optical diode effects arising from the Weyl fermions have been theoretically considered but not probed experimentally. Here, we report the observation of a nonlinear optical diode effect (NODE) in the magnetic Weyl semimetal CeAlSi, where the magnetization introduces a pronounced directionality in the nonlinear optical second-harmonic generation (SHG). We demonstrate a six-fold change of the measured SHG intensity between opposite propagation directions over a bandwidth exceeding 250 meV. Supported by density-functional theory, we establish the linearly dispersive bands emerging from Weyl nodes as the origin of this broadband effect. We further demonstrate current-induced magnetization switching and thus electrical control of the NODE. Our results advance ongoing research to identify novel nonlinear optical/transport phenomena in magnetic topological materials and further opens new pathways for the unidirectional manipulation of light.

Interfacial Modulation of Spin-Orbit Torques Induced by Two-Dimensional van der Waals Material ZrSe3.

Two-dimensional van der Waals (2D vdW) materials are widely used in spin-orbit torque (SOT) devices. Recent studies have demonstrated the low crystal symmetry and large spin Hall conductivity of 2D vdW ZrSe3, indicating its potential applications in low-power SOT devices. Here, we study the interfacial contribution of SOTs and current-induced magnetization switching in the ZrSe3/Py (Ni80Fe20) and ZrSe3/Cu/Py heterostructures. SOT efficiencies of samples are detected by the spin-torque ferromagnetic resonance (ST-FMR), and out-of-plane damping-like torque (τB) is observed. The ratio between τB and the field-like torque (τA) decreases from 0.175 to 0.138 when inserting 1 nm Cu at the interface and then drops to 0.001 when the thickness of Cu intercalation is 2 nm, indicating that Cu intercalation inhibits the τB component of SOT. Moreover, the SOT efficiency is increased from 3.05 to 5.21, which may be attributed to the Cu intercalation being beneficial to improve the interface between Py and ZrSe3. Theoretical calculation has shown that the Cu spacer can change the conductivity of ZrSe3 from semiconductor to conductor, thereby decreasing the Schottky barrier and increasing the transmission efficiency of the spin current. Furthermore, magneto-optical Kerr effect (MOKE) microscopy is employed to verify the current-driven magnetization switching in these structures. In comparison to the ZrSe3/Py bilayer, the critical current density of ZrSe3/Cu/Py is reduced when inserting 1 nm Cu, demonstrating the higher SOT efficiency and lower power consumption in ZrSe3/Cu/Py structures.

Exact steady states of the impurity-doped XXZ spin chain coupled to dissipators

We give an exact matrix product steady state and matrix product forms of local observables for the bulk impurity-doped XXZ spin model coupled to dissipators at both ends, whose dynamics is described by the Lindblad quantum master equation. We find that local magnetization is induced at the impurity site when the spin current flows, which is contrary to the usual situation where the current suppresses magnetization due to heating. It is proved that this current-induced magnetization survives in the thermodynamic limit, and the spin current does not depend on the impurity strength. We discuss the role of bulk integrability by comparing the results with those of non-integrable models solved numerically by the quantum trajectory method.

Tunability of Microwave Frequency Using Spin Torque Nano Oscillator by the Generated Oersted Field with Tunable Free Layer

The current-induced magnetization precession dynamics provoked by the spin transfer torque (STT) in a spin valve device i.e. tri-layer device (commonly spin torque nano oscillator (STNO)) is investigated numerically by solving the governing Landau–Lifshitz–Gilbert–Slonczewski (LLGS) equation. In this study, we have devised an STNO device made of EuO-based ferromagnetic alloy in free and fixed magnetic layers. The copper acts as a nonmagnetic spacer. Here, we have introduced the current induced Oesterd field (CIOF), which is generated when a spin-polarized current passes through the STNO device. In the device, we have tuned the free layer angle [Formula: see text] from [Formula: see text] to [Formula: see text] as an increment of [Formula: see text]. For every individual [Formula: see text] ranging from [Formula: see text] to [Formula: see text], the generated Oersted field’s strength can be altered by increasing the STNO device’s diameter. Henceforth, it is apparent that the frequency tunability is achieved in the device for all the values of [Formula: see text]. The frequency and power of the device depend entirely on the material’s saturation magnetization, which inherently reflects the current density and coherence of spin-polarized DC. From the results, it is apparent that for a particular [Formula: see text], the frequency keeps increasing with the eventual decrease in power when we increase the strength of the Oersted field from 10[Formula: see text]kA/m to 50[Formula: see text]kA/m. By doing so, the maximum frequency can be tuned up to 212[Formula: see text]GHz for [Formula: see text] with [Formula: see text] as 50[Formula: see text]kA/m. The high frequency emitted by the device acts as a linchpin ingredient, as well as a launch pad element in much of scientific and technological point of view. It paves way for a new route in the areas such as high capacity, high precision, high density as well as the sensing applications.

Magnetochiral tunneling in paramagnetic Co1/3NbS2

Electric currents have the intriguing ability to induce magnetization in nonmagnetic crystals with sufficiently low crystallographic symmetry. Some associated phenomena include the non-linear anomalous Hall effect in polar crystals and the nonreciprocal directional dichroism in chiral crystals when magnetic fields are applied. In this work, we demonstrate that the same underlying physics is also manifested in the electronic tunneling process between the surface of a nonmagnetic chiral material and a magnetized scanning probe. In the paramagnetic but chiral metallic compound Co1/3NbS2, the magnetization induced by the tunneling current is shown to become detectable by its coupling to the magnetization of the tip itself. This results in a contrast across different chiral domains, achieving atomic-scale spatial resolution of structural chirality. To support the proposed mechanism, we used first-principles theory to compute the chirality-dependent current-induced magnetization and Berry curvature in the bulk of the material. Our demonstration of this magnetochiral tunneling effect opens up an avenue for investigating atomic-scale variations in the local crystallographic symmetry and electronic structure across the structural domain boundaries of low-symmetry nonmagnetic crystals.

Magnon-mediated spin Hall magnetoresistance and unidirectional magnetoresistance in Pt/NiO/NiFe structures

Spin–orbit torque provides an efficient strategy for electric manipulation of magnetization. However, Joule heat accompanying with electron motion in the electron-mediated spin current result in unavoidable power dissipation. Moreover, the spin diffusion length in electron-mediated spin current is relatively short, preventing the transmission of spin information over long distances. Magnon-mediated spin current, without moving electrons, can be an excellent alternative to the conventional spin current. Magnon-mediated transfer torque effect has been reported in several previous works. Here, we report the magnon-mediated spin Hall magnetoresistance (SMR) and unidirectional magnetoresistance (UMR) in Pt/NiO/NiFe structures. The significant SMR and UMR were observed in the samples with the NiO thickness up to 60 nm, demonstrating the efficient transmission of magnon-mediated spin current over long distances in the NiO layer. In addition, we observed current-induced in-plane magnetization switching in the NiFe layer via the UMR measurement. These results demonstrated the possibility for developing the efficient spintronic devices operated by magnons.

Direct Observation of Room-Temperature Magnetic Skyrmion Motion Driven by Ultra-Low Current Density in Van Der Waals Ferromagnets.

The recent discovery of room-temperature ferromagnetism in 2D van der Waals (vdW) materials, such as Fe3 GaTe2 (FGaT), has garnered significant interest in offering a robust platform for 2D spintronic applications. Various fundamental operations essential for the realization of 2D spintronics devices are experimentally confirmed using these materials at room temperature, such as current-induced magnetization switching or tunneling magnetoresistance. Nevertheless, the potential applications of magnetic skyrmions in FGaT systems at room temperature remain unexplored. In this work, the current-induced generation of magnetic skyrmions in FGaT flakes employing high-resolution magnetic transmission soft X-ray microscopy is introduced, supported by a feasible mechanism based on thermal effects. Furthermore, direct observation of the current-induced magnetic skyrmion motion at room temperature in FGaT flakes is presented with ultra-low threshold current density. This work highlights the potential of FGaT as a foundation for room-temperature-operating 2D skyrmion device applications.

Interfacial engineering to manipulate the effective spin Hall angle of perpendicularly magnetized systems

The spin current can be generated by passing an electric current through a heavy metal. The spin current generation depends on the spin Hall angle (θsh) of the material. To manipulate the effective θsh, a thin layer of Au has been introduced at the bottom Pt/Co and the top Co/Pt interfaces in Ta/Pt/Co/Pt based perpendicularly magnetic anisotropy systems, and current-induced magnetization reversal of Co has been studied to estimate Jc. The introduction of the Au layer at the top Co/Pt interface (Ta/Pt/Co/Au/Pt) did not produce any significant reduction in the Jc. However, a significant reduction of Jc (∼34%) has been observed, while the Au layer has been deposited at the bottom Pt/Co interface (Ta/Pt/Au/Co/Pt), indicating an enhancement in the value of θsh. We also performed a micromagnetic simulation to understand the qualitative change of the θsh. Micromagnetic simulation suggested that the θsh becomes 0.07 in Ta/Pt/Au/Co/Pt multilayer compared to θsh=0.05 of the Ta/Pt/Co/Pt system. Pt/Co/Au/Co/Au exhibits a reduction in Jc up to ∼30% and corresponds to θsh=0.09. A Ta capping layer has been introduced to inject more spin current into the Co layer since Pt and Ta have opposite spin Hall angles. The Jc lowers up to ∼58% in Ta/Pt/Au/Co/Pt/Ta multilayer, corresponding to θsh=0.23. We also achieved field-free switching at Jc=1.55×1011 by depositing an in-plane magnetized layer of Co in Ta/Pt/Au/Co/Pt/Ta/Co/Pt multilayer.

Field-free spin–orbit torque-induced switching of perpendicular magnetization at room temperature in WTe2/ferromagnet heterostructures

Spin–orbit torque provides an efficient way to achieve switching of perpendicular magnetization, which is essential for designing energy-efficient spintronic devices. An in-plane antidamping torque combined with an out-of-plane antidamping torque can often deterministically switch perpendicular magnetization without an external magnetic field. Encouragingly, field-free perpendicular magnetization switching of a two-dimensional (2D) material WTe2/ferromagnet bilayer has been reported recently, but the working temperature (< 200 K) is quite below room temperature. Here, we demonstrate field-free perpendicular magnetization switching in the Pt/Co/Pt/WTe2 multilayer films at room temperature, which is mainly attributed to the out-of-plane antidamping torque originating from the WTe2 layer. In addition, current-induced perpendicular magnetization switching at zero magnetic field is also accomplished in the [Co/Pt]2/WTe2 multilayer film with a very large perpendicular magnetic anisotropic field (∼13 600 Oe), which is very useful for practical applications. This work offers a potential way to develop spintronic devices based on 2D materials at room temperature.

Simultaneous multitone microwave emission by dc-driven spintronic nano-element

Current-induced self-sustained magnetization oscillations in spin-torque nano-oscillators (STNOs) are promising candidates for ultra-agile microwave sources or detectors. While usually STNOs behave as a monochromatic source, we report here clear bimodal simultaneous emission of incommensurate microwave oscillations in the frequency range of 6 to 10 gigahertz at femtowatt level power. These two tones correspond to two parametrically coupled eigenmodes with tunable splitting. The emission range is crucially sensitive to the change in hybridization of the eigenmodes of free and fixed layers, for instance, through a slight tilt of the applied magnetic field from the normal of the nanopillar. Our experimental findings are supported both analytically and by micromagnetic simulations, which ascribe the process to four-magnon scattering between a pair of radially symmetric magnon modes and a pair of magnon modes with opposite azimuthal index. Our findings pave the way for enhanced cognitive telecommunications and neuromorphic systems that use frequency multiplexing to improve communication performance.

Enhanced current-induced magnetization switching via antiferromagnetic coupled interface in Co/Ho heterostructures

Rare-earth ferromagnetic (RE–FM) heterostructures have attracted significant attention due to their intricate spin structures and physical phenomena. The antiferromagnetic coupled (AFC) interface formed by the distinctive interaction between the FM and RE elements has critical contributions to the magnetization reversal process. In this work, we investigate the enhancement of current-induced magnetization switching with the AFC interface at the Co/Ho heterostructure. The results shows that an increased spin–orbit torque (SOT) efficiency of up to 250% was achieved at a Ho thickness of 7 nm, with a critical switching current density of 2.7 × 1010 A/m2. When a Cu interlayer was introduced between the Co/Ho interface, a decreased SOT efficiency was observed, indicating that the SOT enhancement is primarily attributed to the AFC interfacial effect. At the AFC interface, the interaction between Co and Ho atoms generates an additional torque, enhancing the effective SOT efficiency.

Reducing switching current with Gilbert damping constant in nanomagnets with perpendicular anisotropy under Dzyaloshinskii–Moriya interaction

We report current-induced magnetization switching (j sw) with a nanosecond-duration pulse current (t p) in a perpendicularly magnetized nanomagnet under the Dzyaloshinskii–Moriya interaction (DMI) and investigate the effect of the Gilbert damping constant (α), t p, and DMI value (D) on j sw via micromagnetic simulations. When α is sufficiently small at t p = 1 ns, j sw for D = 1.0 erg cm−2 decreases by 42% compared to that for D = 0 erg cm−2. Further, j sw can be reduced under the DMI when α is small and t p is short, which is attributed to the twisted magnetization and increased initial magnetization angle.

Field-Free Spin-Orbit Torque Switching of Co/Pd Memory Layer in CPP-GMR With Perpendicularly Magnetized SAF Pinned Layer

For the development of high-capacity spin-orbit torque magnetic memory, a giant magneto-resistive (GMR) device with perpendicularly magnetized Co/Pd memory layer was fabricated, and current-induced magnetization reversal of the memory layer was investigated. A synthetic antiferromagnet (SAF) reference layer consisting of [Co/Pt]/Ru/[Co/Pt] multilayer was used for the GMR device to reduce the stray field in the pillar-shaped Co/Pd memory layer. Spin-transfer torque (STT) magnetization reversal of the memory layer as well as the field-assisted spin-orbit torque (SOT) magnetization reversal of the memory layer was confirmed, by applying a current pulse through the GMR or through the bottom electrode, respectively. Furthermore, by injecting SOT and STT simultaneously, deterministic SOT switching of the perpendicularly magnetized Co/Pd memory layer was achieved in the absence of an in-plane assisting magnetic field to break the symmetry. We confirmed that STT assisting current significantly reduced the SOT switching current. Moreover, the assist efficiency defined by Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SOT</sub> / Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">STT</sub> depended on the magnitude of in-plane magnetic field. Higher assist efficiency was obtained at lower in-plane magnetic field.

Field-free magnetization switching through modulation of zero-field spin–orbit torque efficacy

To make spin–orbit torque magnetic random access memory (SOT-MRAM) practical, current-induced magnetization switching without an external bias field is essential. Given that the CoFeB/MgO structure has already been used in typical spin-transfer torque-MRAM for its high tunneling magnetoresistance, leveraging a similar material system to achieve field-free SOT switching is of great importance. In this work, we systematically investigate the field-free switching mechanism in CoFeB/W/CoFeB T-type structures, where the two CoFeB layers are in-plane and perpendicularly magnetized, respectively. Initial SOT characterization shows a sizable zero-field SOT efficacy (χHx=0) for such T-type devices. Furthermore, field-free angle-dependent SOT measurement confirms the parallel relationship between the built-in bias field and the magnetization of the in-plane magnetized CoFeB. Based on thorough verification and exclusion of other potential mechanisms, the Néel orange-peel effect emerges as the dominant origin for such a built-in bias field, where a positive correlation between the deposited film surface roughness and χHx=0 is found. Based on this discovery, the field-free switching efficacy in T-type structures is further optimized via film roughness tuning and examined with pillar-shaped devices. Our results provide insights into the tentative approach to improve field-free switching using T-type devices and the feasibility of downscaling.

Fabrication of highly oriented Mn4N/Pt epitaxial bilayer structure on MgO(001) for spintronics applications

Magnetization control by electric current or spin current is an attractive technology with potential applications in memory and logic devices, and Mn4N has attracted much attention in recent years as a candidate material for current-induced magnetization switching and magnetic domain wall motion, as well as a skyrmion material. For these applications, a bilayer structure of Mn4N and heavy metals must be formed with high crystalline quality. In this study, we have achieved the epitaxial growth of Mn4N thin films on MgO(001) with a Pt underlayer. The reflection high-energy electron diffraction images and x-ray diffraction ω-rocking curves show that the Mn4N films fabricated on the Pt underlayers exhibit higher crystalline quality than the Mn4N films directly formed on the MgO. Furthermore, anomalous Hall effect loops with bump-like structures were observed. This is considered to be due to the topological spin texture. The Mn4N/Pt epitaxial bilayer structure has potential for use in spin–orbit torque applications and is a candidate configuration for topological spin textures.