Magnetization Switching Research Articles

Spin‐Orbit Torque Switching of Magnetization in Ultra‐Thick Ferromagnetic Layers

AbstractCurrent‐induced magnetization switching via spin‐orbit torque (SOT) holds great potential for applications in high‐speed and energy‐efficient magnetic memory and logic devices. In the extensively studied heavy metal/ferromagnet (HM/FM) SOT heterostructures, the thickness of the FM layer is typically restricted to a few nanometers or less due to the rapid spin dephasing, making it challenging to implement thermally stable memory cells with high density. In this study, it is demonstrated that this thickness constraint can be significantly alleviated by utilizing an oxide ferromagnet La0.67Sr0.33MnO3 (LSMO). Through electrical transport and magnetic optical measurements, it is found that the SOT can switch the magnetization in Pt/LSMO heterostructures even at an LSMO thickness of 35 nm, which is one order of magnitude larger than that for metallic FMs, such as CoFeB. Furthermore, based on the FM thickness dependence of the switching current and the domain switching type revealed by magnetic optical Kerr effect imaging (MOKE), a possible picture is proposed to describe the SOT switching in Pt/LSMO, which highlights the critical role of the domain wall propagation in the vertical direction. The work provides valuable insights into the behavior of SOT switching in ultra‐thick FM films, offering new possibilities for their practical applications.

Neuromorphic Computing in Synthetic Antiferromagnets by Spin‐Orbit Torque Induced Magnetic‐Field‐Free Magnetization Switching

AbstractSynthetic antiferromagnet (SAF) with high thermal stability, ultra‐fast spin dynamics, and highly efficient spin‐orbit torque switching has great application potential in neuromorphic computing hardware. However, two challenges, the weakening of Hall signal in the remanent state and the need for a large auxiliary magnetic field for perpendicular magnetization switching, greatly limit the advantages of SAF in neuromorphic computing. In this work, both the enhanced anomalous Hall resistance and magnetic‐field‐free perpendicular magnetization switching are achieved by using oblique sputtering to fabricate the Pt/CoPt/Ru/CoTb SAF with strong interlayer exchange coupling and magnetic moment compensation. The fabricated SAF as synapse shows nearly linear, nonvolatile multistate plasticity, and as neuron exhibits a nonlinear sigmoid activation function, which are used to construct a fully connected neural network with a remarkable 97.0–98.1% recognition rate for the handwritten digits. Additionally, SAF serving as spike‐timing‐dependent plasticity synapse is used to construct an adaptive, unsupervised learning spiking neural network, and achieve an 87.0% accuracy in handwritten digit recognition. The findings exhibit the promise of SAFs as specialized hardware for high‐performance neuromorphic computing, offering high recognition rates and low power consumption.

Ultrafast all-optical toggle writing of magnetic bits without relying on heat

Ultrafast excitation of matter can violate Curie’s principle that the symmetry of the cause must be found in the symmetry of the effect. For instance, heating alone cannot result in a deterministic reversal of magnetization. However, if the heating is ultrafast, it facilitates toggle switching of magnetization between stable bit-states without any magnetic field. Here we show that the regime of ultrafast toggle switching can be also realized via a mechanism without relying on heat. Ultrafast laser excitation of iron-garnet with linearly polarized light modifies magnetic anisotropy and thus causes toggling magnetization between two stable bit states. This new regime of ‘cold’ toggle switching can be observed in ferrimagnets without a compensation point and over an exceptionally broad temperature range. The control of magnetic anisotropy required for the toggle switching exhibits reduced dissipation compared to laser-induced-heating mechanism, however the dissipation and the switching-time are shown to be competing parameters.

Unraveling the Correlation between the Interface Structures and Tunable Magnetic Properties of La1-xSrxCoO3-δ/La1-xSrxMnO3-δ Bilayers Using Deep Learning Models.

Perovskite oxides are gaining significant attention for use in next-generation magnetic and ferroelectric devices due to their exceptional charge transport properties and the opportunity to tune the charge, spin, lattice, and orbital degrees of freedom. Interfaces between perovskite oxides, exemplified by La1-xSrxCoO3-δ/La1-xSrxMnO3-δ (LSCO/LSMO) bilayers, exhibit unconventional magnetic exchange switching behavior, offering a pathway for innovative designs in perovskite oxide-based devices. However, the precise atomic-level stoichiometric compositions and chemophysical properties of these interfaces remain elusive, hindering the establishment of surrogate design principles. We leverage first-principles simulations, evolutionary algorithms, and neural network searches with on-the-fly uncertainty quantification to design deep learning model ensembles to investigate over 50,000 LSCO/LSMO bilayer structures as a function of oxygen deficiency (δ) and strontium concentration (x). Structural analysis of the low-energy interface structures reveals that preferential segregation of oxygen vacancies toward the interfacial La0.7Sr0.3CoO3-δ layers causes distortion of the CoOx polyhedra and the emergence of magnetically active Co2+ ions. At the same time, an increase in the Sr concentration and a decrease in oxygen vacancies in the La0.7Sr0.3MnO3-δ layers tend to retain MnO6 octahedra and promote the formation of Mn4+ ions. Electronic structure analysis reveals that the nonuniform distributions of Sr ions and oxygen vacancies on both sides of the interface can alter the local magnetization at the interface, showing a transition from ferromagnetic (FM) to local antiferromagnetic (AFM) or ferrimagnetic regions. Therefore, the exotic properties of La1-xSrxCoO3-δ/La1-xSrxMnO3-δ are strongly coupled to the presence of hard/soft magnetic layers, as well as the FM to AFM transition at the interface, and can be tuned by changing the Sr concentration and oxygen partial pressure during growth. These insights provide valuable guidance for the precise design of perovskite oxide multilayers, enabling tailoring of their functional properties to meet specific requirements for various device applications.

Field-free magnetization switching through large out-of-plane spin–orbit torque in the ferromagnetic CoPt single layers

Spin–orbit torque (SOT) induced magnetization switching in an energy-efficient and fast way has exhibited great application potential in next generation magnetic memories. However, a complicated layer structure is usually needed to break the mirror symmetry for achieving SOT induced field-free magnetization switching. Here, we report a sizeable field-free magnetization switching through large out-of-plane SOT in the chemically disordered A1-CoxPt100−x single layers within a Co composition range from 40 to 70. The largest absolute out-of-plane SOT efficiency is found at its equiatomic concentration (Co50Pt50), in which the absolute in-plane SOT efficiency also reaches the maximum value, 22.7 Oe/107 A cm−2. We further demonstrate that the symmetry dependence of field-free magnetization switching might arise from the chemically ordered L11-CoPt nano-scaled platelets formed during the sample deposition. We expect that the experimental identification of the field-free magnetization switching in the ferromagnetic CoPt single layer is desirable to simplify the applications of spin logic devices.

Directional switching of surface plasmons in a parity-time-symmetric insulator-metal-insulator structure

In the present work, we study the generation and propagation of surface plasmon polaritons (SPPs) in a geometrically flat insulator-metal-insulator (IMI) structure with parity-time (PT) symmetric modulation on the dielectric layers. Unidirectional SPPs are generated by PT-symmetric modulation. Moreover, magnetic field switching is obtained between two metal-dielectric interfaces. It has been noticed that the metal thickness affects the oscillation frequency of excited SPPs along the direction of propagation. Also, we report that the field at the upper interface is manipulated by solely maneuvering the permittivity of the bottom dielectric. The magnetic field distribution of the present structure is studied using COMSOL Multiphysics software. To ensure the accuracy and reliability of the simulation results, comprehensive analytical investigations have also been conducted.

Enhancing thermal stability of optimal magnetization reversal in nanoparticles

Energy-efficient switching of nanoscale magnets requires application of a time-varying magnetic field characterized by microwave frequency. At finite temperatures, even weak thermal fluctuations induce perturbations in the magnetization that can accumulate in time, disrupt the phase locking between the magnetization and the applied field, and eventually compromise magnetization switching. It is demonstrated here that the magnetization reversal is mostly disturbed by unstable perturbations arising in a certain domain of the configuration space of a nanomagnet. The instabilities can be suppressed and the probability of magnetization switching enhanced by applying an additional stimulus such as a weak longitudinal magnetic field that ensures bounded dynamics of the perturbations. Application of the stabilizing longitudinal field to a uniaxial nanomagnet makes it possible to reach a desired probability of magnetization switching even at elevated temperatures. The principle of suppressing instabilities provides a general approach to enhancing thermal stability of magnetization dynamics. Published by the American Physical Society 2024

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.

Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy

The development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 Å demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism.

Efficient magnetization switching induced by spin–orbit torque in perpendicularly magnetized Mn1+xCo2−xAl Heusler alloys

Perpendicularly magnetized Co-based Heusler alloys are promising candidates in high-performance spintronic devices. However, there is a contradiction between thermal stability and damping-like spin–orbit torque (SOT) efficiency in Co-based Heusler alloys with interface-induced perpendicular magnetic anisotropy (PMA). Here, we present epitaxially grown perpendicularly magnetized Mn1+xCo2−xAl (MCA) Heusler alloys through tetragonal distortion. Ferrimagnetism and distortion-induced PMA are obtained in MCA Heusler alloys. A large effective spin Hall angle (θeff) up to 0.33 is experimentally demonstrated in Pt/MCA bilayers, markedly surpassing that in conventional Pt/FM bilayers. Consequently, SOT-induced efficient magnetization switching is realized in Pt/MCA bilayers, with a critical switching current density (Jc) as low as 4 × 107 A/cm2. The large θeff and high SOT efficiency would be attributed to the staggered magnetic exchange torques in the ferrimagnetic MCA Heusler alloys. These findings demonstrate that perpendicularly magnetized ferrimagnetic MCA Heusler alloys are highly promising for high-density SOT-magnetic random-access memory with superior thermal stability and low power consumption.

First-principles study on the coexistence of different valence states of Zn vacancy, Cu1+/2+ doping, and Hi in ZnO magnetic switch

In this study, we used first principles to investigate the magnetic sources and switches of Zn34Cu1+HiO36(VZn0), Zn34Cu1+HiO36(VZn2−), Zn34Cu2+HiO36(VZn0), and Zn34Cu2+HiO36(VZn2−) systems. On the basis of formation energy, phonon spectrum, and temporal variation of energy, we concluded that the above systems were all in a stable state. The doped system can be magnetized by adjusting the valence state of Zn vacancy, and the Zn34Cu1+HiO36(VZn0) and Zn34Cu2+HiO36(VZn2−) systems demonstrated magnetism. We found that the Zn34Cu1+HiO36(VZn0) system acquired magnetism through (O.↓↑+VZn")x⇔(↑O.+VZn"+↑O.)x⇔(↑O.+VZn"+↓O.)x super- and double-exchange interactions, whereas the magnetism of the Zn34Cu2+HiO36(VZn2−) system originated mainly from the action of the O1−–VZn2−–Cu2+-bound magnetic polaron and double exchange. Our theory provides a new approach for the use of magnetic switches in magnetic semiconductors.

Hidden Real Topology and Unusual Magnetoelectric Responses in Two-Dimensional Antiferromagnets.

Recently, the real topology has been attracting widespread interest in two dimensions (2D). Here, based on first-principles calculations and theoretical analysis, the monolayer Cr2Se2O (ML-CrSeO) is revealed as the first material example of a 2D antiferromagnetic (AFM) real Chern insulator (RCI) with topologically protected corner states. Unlike previous RCIs, it is found that the real topology of the ML-CrSeO is rooted in one certain mirror subsystem of the two spin channels, and cannot be directly obtained from all the valence bands in each spin channel as commonly believed. In particular, due to antiferromagnetism, the corner modes in ML-CrSeO exhibit strong corner-contrasted spin polarization, leading to spin-corner coupling (SCC). This SCC enables a direct connection between spin space and real space. Consequently, large and switchable net magnetization can be induced in the ML-CrSeO nanodisk by electrostatic means, such as potential step and in-plane electric field, and the corresponding magnetoelectric responses behave like a sign function, distinguished from that of the conventional multiferroic materials. This work considerably broadens the candidate range of RCI materials, and opens up a new direction for topo-spintronics and 2D AFM materialsresearch.

Unconventional magnetism mediated by spin-phonon-photon coupling

Magnetic order typically emerges due to the short-range exchange interaction between the constituent electronic spins. Recent discoveries have found a crucial role for spin-phonon coupling in various phenomena from optical ultrafast magnetization switching to dynamical control of the magnetic state. Here, we demonstrate theoretically the emergence of a biquadratic long-range interaction between spins mediated by their coupling to phonons hybridized with vacuum photons into polaritons. The resulting ordered state enabled by the exchange of virtual polaritons between spins is reminiscent of superconductivity mediated by the exchange of virtual phonons. The biquadratic nature of the spin-spin interaction promotes ordering without favoring ferro- or antiferromagnetism. It further makes the phase transition to magnetic order a first-order transition, unlike in conventional magnets. Consequently, a large magnetization develops abruptly on lowering the temperature which could enable magnetic memories admitting ultralow-power thermally-assisted writing while maintaining a high data stability. The role of photons in the phenomenon further enables an in-situ static control over the magnetism. These unique features make our predicted spin-spin interaction and magnetism highly unconventional paving the way for novel scientific and technological opportunities.

Enhanced Spin–Orbit Torques in Graphene by Pt Adatoms Decoration

Abstract Graphene (Gr) with widely acclaimed characteristics, such as exceptionally long spin diffusion length at room temperature, provides an outstanding platform for spintronics. However, its inherent weak spin–orbit coupling (SOC) has limited its efficiency for generating the spin currents in order to control the magnetization switching process for applications in spintronics memories. Following the theoretical prediction on the enhancement of SOC in Gr by heavy atoms adsorption, here we experimentally observe a sizeable spin–orbit torques (SOTs) in Gr by the decoration of its surface with Pt adatoms in Gr/Pt(t Pt)/FeNi trilayers with the optimal damping-like SOT efficiency around 0.55 by 0.6-nm-thick Pt layer adsorption. The value is nearly four times larger than that of the Pt/FeNi sample without Gr and nearly twice the value of the Gr/FeNi sample without Pt adsorption. The efficiency of the enhanced SOT in Gr by Pt adatoms is also demonstrated by the field-free SOT magnetization switching process with a relatively low critical current density around 5.4 MA/cm2 in Gr/Pt/FeNi trilayers with the in-plane magnetic anisotropy. These findings pave the way for Gr spintronics applications, offering solutions for future low power consumption memories.

Switching of magnetization in quantum antiferromagnets with time-dependent control fields

Switching of magnetization in quantum antiferromagnets with time-dependent control fields

Fabrication and Magnetic Behavior of Jellyfish-Like Ni Nanowires Synthesized Using Bilayered Nanoporous Anodic Alumina Templates.

The potential to produce nanostructures with intricate shapes in large quantities holds promise for a range of applications in the fields of nanoelectronics and biomedicine. Here a method for fabricating jellyfish-like Ni nanowires (JFNWs) using bilayered nanoporous anodic alumina templates with through pores of varying diameters in each layer is presented. To assess the capabilities of this method,samples are created with different voltages during the second step of anodization, resulting in distinct geometrical characteristics of the second layer of the template,and subsequently synthesizeNiJFNWs. By employing magnetometry and first-order reversal curve (FORC)method, the magnetic properties are examined and a significant alteration in their magnetic behavior,attributed to the differing shapes of theJFNWsand the magnetostatic interactions within the array,is observed.The study utilizes magnetic force microscopy to evaluate the stray magnetic fields generated by the individualJFNWsand unveils their unusual and asymmetric distribution. Through simulations based on the experimental data, the study analyzes the field- andcurrent-induced domain wall movement in a singleJFNWand their array. The findings revealnon-trivial micromagnetic configurations in these structures, including a remarkable 'corkscrew' state, and allowfor an examination of the process of magnetization switching.

Magnetization switching driven by spin current in a T-type ferromagnetic trilayer

The T-type CoFeB/spacer/CoFeB structure is a promising candidate for the development of perpendicular spin–orbit torque (SOT) magnetic random-access memory and other SOT devices. It consists of an in-plane magnetized layer, a perpendicularly magnetized layer, and a non-magnetic metal spacer that induces interlayer exchange coupling. By engineering the W spacer, this system achieves field-free SOT switching with a nearly 100% switching ratio. Furthermore, it realizes a high exchange coupling field of 255 Oe using a relatively thinner spacer thickness, enhancing the reliability and energy efficiency of SOT switching. Measurement of current switching probability suggests that this perpendicular magnetic anisotropy system may enable the implementation of probability-adjustable true random number generators in future applications. The T-type structures with strong interlayer coupling exhibit great potential for spintronic device applications.

Perpendicular magnetic properties and field-free current-induced switching in W/CoFeB/MgO trilayers with a compensating oblique W underlayer

Zero-field magnetization switching (ZFS) driven by current-induced spin–orbit torque (SOT) holds significant importance in spintronic applications. The introduction of a lateral asymmetric structure (LAS) through oblique deposition proves to be an effective strategy for breaking inversion symmetry, thereby enabling SOT-driven ZFS. However, the coexistence of wedge thickness structure and slanted columnar microstructure in the obliquely deposited films poses challenges in distinguishing their respective effects. In this study, we conducted a comparative investigation of the perpendicular magnetic properties and current-induced switching in W/Co40Fe40B20/MgO films by oblique sputtering of the W underlayer at a fixed tilting angle and at two opposite tilting angles with its wedge thickness compensated. We have found that the perpendicular magnetic properties of the Co40Fe40B20 layer are significantly altered at large tilting angles, irrespective of whether the W wedge thickness is compensated. Notably, at a tilting angle of 50°, ZFS is realized for both the conventional oblique sample and the compensating oblique sample, with the switching polarity contingent on the final tilting direction of the W layer. We have identified a gradient in perpendicular magnetic anisotropy in these samples, attributed to the laterally varying roughness associated with the slanted columnar microstructure of the W underlayer. This study underscores the dominant role of microscopic LAS in obliquely deposited films in breaking SOT symmetry. Our research sheds light on the impact of the slanted columnar microstructure on the magnetic and magneto-transport properties of films, offering valuable insights for advancing spintronic device research.

Write-error reduction in voltage-driven magnetization switching using a recording layer with low magnetic damping

Write-error reduction in voltage-driven magnetization switching using a recording layer with low magnetic damping

High‐performance Pyroelectric Property Accompanied by Spin Crossover in a Single Crystal of Fe(II) Complex

AbstractPyroelectric materials hold significant potential for energy harvesting, sensing, and imaging applications. However, achieving high‐performance pyroelectricity across a wide temperature range near room temperature remains a significant challenge. Herein, we demonstrate a single crystal of Fe(II) spin‐crossover compound shows remarkable pyroelectric properties accompanied by a thermally controlled spin transition. In this material, the uniaxial alignment of polar molecules results in a polarization of the lattice. As the molecular geometry is modulated during a gradual spin transition, the polar axis experiences a colossal thermal expansion with a coefficient of 796×10−6 K−1. Consequently, the material's polarization undergoes significant modulation as a secondary pyroelectric effect. The considerable shift in polarization (pyroelectric coefficient, p=3.7–22 nC K−1cm−2), coupled with a low dielectric constant (ϵ′=4.4–5.4) over a remarkably wide temperature range of 298 to 400 K, suggests this material is a high‐performance pyroelectric. The demonstration of pyroelectricity combined with magnetic switching in this study will inspire further investigations in the field of molecular electronics and magnetism.