Magnetization Switching Dynamics Research Articles

Magnetic Switching Dynamics and Tunnel Magnetoresistance Effect Based on Spin-Splitting Noncollinear Antiferromagnet Mn3Pt

In comparison to ferromagnets, antiferromagnets are believed to have superior advantages for applications in next-generation magnetic storage devices, including fast spin dynamics, vanishing stray fields and robust against external magnetic field, etc. However, unlike ferromagnetic orders, which could be detected through tunneling magnetoresistance effect in magnetic tunnel junctions, the antiferromagnetic order (i.e., Néel vector) cannot be effectively detected by the similar mechanism due to the spin degeneracy of conventional antiferromagnets. Recently discovered spin-splitting noncollinear antiferromagnets, such as Mn3Pt with momentum-dependent spin polarization due to their special magnetic space group, make it possible to achieve remarkable tunneling magnetoresistance effects in noncollinear antiferromagnetic tunnel junctions. Through first-principles calculations, we demonstrate that the tunneling magnetoresistance ratio can reach more than 800% in Mn3Pt/perovskite oxides/Mn3Pt antiferromagnetic tunnel junctions. We also reveal the switching dynamics of Mn3Pt thin film under magnetic fields using atomistic spin dynamic simulation. Our study provides a reliable method for detecting Néel vector of noncollinear antiferromagnets through the tunnel magnetoresistance effect and may pave its way for potential applications in antiferromagnetic memory devices.

Micromagnetic simulations of emergent monopole defects and magnetization reversal in connected and dipolar Square Artificial Spin Ice

Artificial spin ice (ASI) structures have garnered significant attention due to their unique properties including emergent magnetic monopoles, phase transitions, and collective switching dynamics. In this study, we employ micromagnetic simulations to investigate the magnetic microstates and magnetization reversal dynamics of square ASI with varying connectivity. In particular, we explore an intermediate or a quasi-connected system where the connectivity is altered without significantly changing the strength of dipolar interactions through the introduction of narrow gaps at the vertices. Our results reveal distinct differences in the populations of different vertex types during magnetic reversal for lattices with varying connectivity. These results suggest the possibility of engineering ASI systems in which specific vertex types can be repeatably accessed through adjustments in lattice connectivity.

Picosecond spin-orbit torque-induced coherent magnetization switching in a ferromagnet.

Electrically controllable nonvolatile magnetic memories show great potential for the replacement of conventional semiconductor-based memory technologies. Here, we experimentally demonstrate ultrafast spin-orbit torque (SOT)-induced coherent magnetization switching dynamics in a ferromagnet. We use an ultrafast photoconducting switch and a coplanar strip line to generate and guide a ~9-picosecond electrical pulse into a heavy metal/ferromagnet multilayer to induce ultrafast SOT. We then use magneto-optical probing to investigate the magnetization dynamics with sub-picosecond resolution. Ultrafast heating by the approximately 9 picosecond current pulse induces a thermal anisotropy torque which, in combination with the damping-like torque, coherently rotates the magnetization to obtain zero-crossing of magnetization in ~70 picoseconds. A macro-magnetic simulation coupled with an ultrafast heating model agrees well with the experiment and suggests coherent magnetization switching without any incubation delay on an unprecedented time scale. Our work proposes a unique magnetization switching mechanism toward markedly increasing the writing speed of SOT magnetic random-access memory devices.

Nonuniform Current-Driven Formation and Displacement of the Magnetic Compensation Point in Variable-Width Nanoscale Ferrimagnets.

Nano- and microstructures based on ferrimagnets can demonstrate high efficiency and dynamics of current-induced magnetization switching combined with high stability of spin textures such as bubble domains and skyrmions, which are of practical importance for the development of spintronics and spin-orbitronics. This set of features is usually associated with magnetic momentum or angular momentum compensation states. Here, we experimentally show that the compensation state can be realized locally using nonuniform Joule heating. This effect is observed in the variable-width current guide made of the ferrimagnetic W/Co76Tb24/Ru thin films, where the position of a region heated to the compensation temperature depends linearly on the current pulse amplitude. This approach makes it possible to observe the simultaneous coexistence of Co-dominant and Tb-dominant regions, where current pulses induce spin-orbit torques in opposite directions, leading to local magnetization switching. It is found that the position of a Néel domain wall constraining the switched region lies in the vicinity of the coordinate corresponding to the compensation point but does not coincide with it due to high mobility under the action of spin current. Our findings open an alternative approach for engineering of ferrimagnetic nanodevices with advanced properties for future applications in spintronics, spin-orbitronics, and nanoelectronics.

Noise resilient leaky integrate-and-fire neurons based on multi-domain spintronic devices

The capability of emulating neural functionalities efficiently in hardware is crucial for building neuromorphic computing systems. While various types of neuro-mimetic devices have been investigated, it remains challenging to provide a compact device that can emulate spiking neurons. In this work, we propose a non-volatile spin-based device for efficiently emulating a leaky integrate-and-fire neuron. By incorporating an exchange-coupled composite free layer in spin-orbit torque magnetic tunnel junctions, multi-domain magnetization switching dynamics is exploited to realize gradual accumulation of membrane potential for a leaky integrate-and-fire neuron with compact footprints. The proposed device offers significantly improved scalability compared with previously proposed spin-based neuro-mimetic implementations while exhibiting high energy efficiency and good controllability. Moreover, the proposed neuron device exhibits a varying leak constant and a varying membrane resistance that are both dependent on the magnitude of the membrane potential. Interestingly, we demonstrate that such device-inspired dynamic behaviors can be incorporated to construct more robust spiking neural network models, and find improved resiliency against various types of noise injection scenarios. The proposed spintronic neuro-mimetic devices may potentially open up exciting opportunities for the development of efficient and robust neuro-inspired computational hardware.

Magnetization switching in the inertial regime

We have numerically solved the Landau-Lifshitz-Gilbert (LLG) equation in its standard and inertial forms to study the magnetization switching dynamics in a $3d$ thin film ferromagnet. The dynamics is triggered by ultrashort magnetic field pulses of varying width and amplitude in the picosecond and Tesla range. We have compared the solutions of the two equations in terms of switching characteristic, speed and energy analysis. Both equations return qualitatively similar switching dynamics, characterized by regions of slower precessional behavior and faster ballistic motion. In case of inertial dynamics, ballistic switching is found in a 25 % wider region in the parameter space given by the magnetic field amplitude and width. The energy analysis of the dynamics is qualitatively different for the standard and inertial LLG equations. In the latter case, an extra energy channel, interpreted as the kinetic energy of the system, is available. Such extra channel is responsible for a resonant energy absorption at THz frequencies, consistent with the occurence of spin nutation.

Magnetism, symmetry and spin transport in van der Waals layered systems

The discovery of an ever-increasing family of atomic layered magnetic materials, together with the already established vast catalogue of strong spin–orbit coupling and topological systems, calls for some guiding principles to tailor and optimize novel spin transport and optical properties at their interfaces. Here, we focus on the latest developments in both fields that have brought them closer together and make them ripe for future fruitful synergy. After outlining fundamentals on van der Waals magnetism and spin–orbit coupling effects, we discuss how their coexistence, manipulation and competition could ultimately establish new ways to engineer robust spin textures and drive the generation and dynamics of spin current and magnetization switching in 2D-materials-based van der Waals heterostructures. Grounding our analysis on existing experimental results and theoretical considerations, we draw a prospective analysis about how intertwined magnetism and spin–orbit torque phenomena combine at interfaces with well-defined symmetries and how this dictates the nature and figures of merit of spin–orbit torque and angular momentum transfer. This will serve as a guiding role in designing future non-volatile memory devices that utilize the unique properties of 2D materials with the spin degree of freedom.

Polarisation-dependent single-pulse ultrafast optical switching of an elementary ferromagnet

The ultimate control of magnetic states of matter at femtosecond (or even faster) timescales defines one of the most pursued paradigm shifts for future information technology. In this context, ultrafast laser pulses developed into extremely valuable stimuli for the all-optical magnetization reversal in ferrimagnetic and ferromagnetic alloys and multilayers, while this remains elusive in elementary ferromagnets. Here we demonstrate that a single laser pulse with sub-picosecond duration can lead to the reversal of the magnetization of bulk nickel, in tandem with the expected demagnetization. As revealed by realistic time-dependent electronic structure simulations, the central mechanism involves ultrafast light-induced torques that act on the magnetization. They are only effective if the laser pulse is circularly polarized on a plane that contains the initial orientation of the magnetization. We map the laser pulse parameter space enabling the magnetization switching and unveil rich intra-atomic orbital-dependent magnetization dynamics featuring transient inter-orbital non-collinear states. Our findings open further perspectives for the efficient implementation of optically-based spintronic devices.

Wide-field magneto-optical microscope to access quantitative magnetization dynamics with femtosecond temporal and sub-micrometer spatial resolution

We introduce a wide-field magneto-optical microscope to probe magnetization dynamics with femtosecond temporal and sub-micrometer spatial resolution. We carefully calibrate the non-linear dependency between the magnetization of the sample and the detected light intensity by determining the absolute values of the magneto-optical polarization rotation. With that, an analytical transfer function is defined to directly map the recorded intensity to the corresponding magnetization, which results in significantly reduced acquisition times and relaxed computational requirements. The performance of the instrument is characterized by probing the magnetic all-optical switching dynamics of GdFe in a pump-probe experiment. The high spatial resolution of the microscope allows for accurately subdividing the laser-excited area into different fluence-regions in order to capture the strongly non-linear magnetization dynamics as a function of the optical pump intensity in a single measurement.

Wide-field magneto-optical microscope to access quantitative magnetization dynamics with femtosecond temporal and sub-micrometer spatial resolution

We introduce a wide-field magneto-optical microscope to probe magnetization dynamics with femtosecond temporal and sub-micrometer spatial resolution. We carefully calibrate the non-linear dependency between the magnetization of the sample and the detected light intensity by determining the absolute values of the magneto-optical polarization rotation. With that, an analytical transfer function is defined to directly map the recorded intensity to the corresponding magnetization, which results in significantly reduced acquisition times and relaxed computational requirements. The performance of the instrument is characterized by probing the magnetic all-optical switching dynamics of GdFe in a pump–probe experiment. The high spatial resolution of the microscope allows for accurately subdividing the laser-excited area into different fluence-regions in order to capture the strongly non-linear magnetization dynamics as a function of the optical pump intensity in a single measurement.

General Regularities and Differences in the Behavior of the Dynamic Magnetization Switching of Ferrimagnetic (CoFe2O4) and Antiferromagnetic (NiO) Nanoparticles

In antiferromagnetic (AFM) nanoparticles, an additional ferromagnetic phase forms and leads to the appearance in AFM nanoparticles of a noncompensated magnetic moment and the magnetic properties typical of common FM nanoparticles. In this work, to reveal the regularities and differences of the dynamic magnetization switching in FM and AFM nanoparticles, the typical representatives of such materials are studied: CoFe2O4 and NiO nanoparticles with average sizes 6 and 8 nm, respectively. The high fields of the irreversible behavior of the magnetizations of these samples determine the necessity of using strong pulsed fields (amplitude to 130 kOe) to eliminate the effect of the partial hysteresis loop when studying the dynamic magnetic hysteresis. For both types of the samples, coercive force HC at the dynamic magnetization switching is markedly higher than HC at quasi-static conditions. HC increases as the pulse duration τP decreases and the maximum applied field H0 increases. The dependence of HC on field variation rate dH/dt = H0/2τP is a unambiguous function for CoFe2O4 nanoparticles, and it is precisely such a behavior is expected from a system of single-domain FM nanoparticles. At the same time, for AFM NiO nanoparticles, the coercive force is no longer an unambiguous function of dH/dt, and the value of applied field H0 influences more substantially. Such a difference in the behaviors of FM and AFM nanoparticles is caused by the interaction of the FM subsystem and the AFM “core” inside AFM nanoparticles. This circumstance should be taken into account when developing the theory of dynamic hysteresis of the AFM nanoparticles and also to take into account their practical application.

Features of the quasi-static and dynamic magnetization switching in NiO nanoparticles: Manifestation of the interaction between magnetic subsystems in antiferromagnetic nanoparticles

We report on the investigations of a system of 8-nm NiO particles representing antiferromagnetic (AFM) materials, which are weak magnetic in the form of submicron particles, but can be considered to be magnetoactive in the form of nanoparticles due to the formation of the uncompensated magnetic moment in them. The regularities of the behavior of magnetization switching in AFM nanoparticles are established by studying the magnetic hysteresis loops under standard quasi-static conditions and in a quasi-sinusoidal pulsed field of up to 130 kOe with pulse lengths of 4–16 ms. The magnetic hysteresis loops are characterized by the strong fields of the irreversible magnetization behavior, which is especially pronounced upon pulsed field-induced magnetization switching. Under the pulsed field-induced magnetization switching conditions, which are analogous to the dynamic magnetic hysteresis, the coercivity increases with an increase in the maximum applied field H0 and a decrease in the pulse length. This behavior is explained by considering the flipping of magnetic moments of particles in an external ac magnetic field; however, in contrast to the case of single-domain ferro- and ferrimagnetic particles, the external field variation rate dH/dt is not a universal parameter uniquely determining the coercivity. At the dynamic magnetization switching in AFM nanoparticles, the H0 value plays a much more important role. The results obtained are indicative of the complex dynamics of the interaction between magnetic subsystems formed in AFM nanoparticles.

Threshold Current Density for Perpendicular Magnetization Switching Through Spin-Orbit Torque

We theoretically investigate the threshold current density for spin-orbit-torque- (SOT) induced perpendicular magnetization switching. According to the relative sign of fieldlike torque (FLT) and dampinglike torque (DLT), we derive the corresponding analytical formulas of the threshold current density ${J}_{\mathrm{th}}$ required to switch magnetization from equilibrium states to nearly in-plane states. These formulas, which agree well with numerical results among a wide range of material parameters, indicate that SOT current density can be significantly reduced in the presence of a large FLT. The conditions are then explored to achieve complete magnetization reversal after removing the SOT currents. When the ratio of FLT to DLT \ensuremath{\eta} is negative in our sign convention, we find that SOT pulses with long enough fall time, for example, 0.2 ns in our simulations, can guarantee stable switching. In contrast, a small in-plane magnetic field or a large damping is necessary when \ensuremath{\eta} g 0. Based on the conditions, the measured threshold current densities in several reported experiments, which deviate from previous models, can be well reproduced by our derived ${J}_{\mathrm{th}}$. We further study the possible incubation delay induced by SOT, clarifying the recent contradiction on incubation time during SOT switching. Our work sheds insights on the SOT material optimization and the comprehension of magnetization switching dynamics induced by SOT.

Size dependent chaotic spin–orbit torque induced magnetization switching of a ferromagnetic layer with in-plane anisotropy

Understanding the magnetization switching dynamics induced by the spin–orbit torque (SOT) in a ferromagnetic layer is crucial to the design of the ultrafast and energy-saving spin–orbit torque magnetic random access memory. Here, we investigate the SOT switching dynamics of a ferromagnetic layer with in-plane anisotropy with various elliptic sizes in different easy-axis orientations using micro-magnetic simulations. The reliable and ultrafast magnetization switching can be realized by tilting the easy axis to an optimum angle with respect to the current injecting direction. The switching time, in general, decreases smoothly with an increasing current density, and the optimum tilting angle is determined for small device sizes with width smaller than 100 nm. This optimum angle is a small angle deviating from a case when the in-plane easy axis is orthogonal to the current direction. It depends on the size, the current density, and also the damping constant. However, with the device increasing to a certain size (e.g., 250 nm), especially at small tilting angles, we observe chaotic switching behavior where the switching times fluctuate locally with the current density. We attribute this size dependent chaotic switching phenomenon to the nucleation and formulation of complex multi-domains during switching. This chaotic phenomenon can be alleviated by enhancing the field-like torque in the device and thus decreasing the switching times. Consequently, the shape and size of the devices should be carefully taken into account while designing a practical fast switching and low power SOT device with in-plane anisotropy.

Features of the Pulsed Magnetization Switching in a High-Coercivity Material Based on ε-Fe2O3 Nanoparticles

The magnetic structure of the e-Fe2O3 iron oxide polymorphic modification is collinear ferrimagnetic in the range from room temperature to ~150 K. As the temperature decreases, e-Fe2O3 undergoes a magnetic transition accompanied by a significant decrease in the coercivity Hc and, in the low-temperature range, the compound has a complex incommensurate magnetic structure. We experimentally investigated the dynamic magnetization switching of the e-Fe2O3 nanoparticles with an average size of 8 nm in the temperature range of 80–300 K, which covers different types of the magnetic structure of this iron oxide. A bulk material consisting of xerogel SiO2 with the e-Fe2O3 nanoparticles embedded in its pores was examined. The magnetic hysteresis loops under dynamic magnetization switching were measured using pulsed magnetic fields Hmax of up to 130 kOe by discharging a capacitor bank through a solenoid. The coercivity Нс upon the dynamic magnetization switching noticeably exceeds the Нс value under the quasi-static conditions. This is caused by the superparamagnetic relaxation of magnetic moments of particles upon the pulsed magnetization switching. In the range from room temperature to ~ 150 K, the external field variation rate dH/dt is the main parameter that determines the behavior of the coercivity under the dynamic magnetization switching. It is the behavior that is expected for a system of single-domain ferro- and ferrimagnetic particles. Under external conditions (at a temperature of 80 K) when the e-Fe2O3 magnetic structure is incommensurate, the coercivity during the pulsed magnetization switching depends already on the parameter dH/dt and is determined, to a great extent, by the maximum applied field Hmax. Such a behavior atypical of systems of ferrimagnetic particles is caused already by the dynamic spin processes inside the e-Fe2O3 particles during fast magnetization switching.

Field-sweep-rate and time dependence of transverse resistivity anomalies in ultrathin SrRuO3 films

We investigated the field-sweep-rate and time dependence of the transverse resistivity anomalies in ultrathin $\mathrm{SrRu}{\mathrm{O}}_{3}$ epitaxial thin films the behaviors of which resemble what has been claimed to be the topological Hall effect. We find that the magnitudes of the anomalies are independent of the field sweep rate, while the magnetic field at which the anomalies appear $({H}_{\mathrm{hump}})$ and the coercive field $({H}_{c})$ exhibit identical sweep-rate dependence. We also show that the transverse resistivity under a fixed external magnetic field exhibits transient behavior, eventually (\ensuremath{\approx}5000 s) approximating the value of the anomalous Hall resistivity that is obtained after the magnetization reversal processes are completed. These results indicate that ${H}_{c}$ is inhomogeneous within the films and activation energies of the magnetization reversal at ${H}_{\mathrm{hump}}$ and ${H}_{c}$ are identical. The observed field-sweep-rate and time dependence of the transverse resistivity anomalies can be understood in the framework of conventional magnetic switching dynamics, and topological interpretation would not be necessarily taken into account. Our results are in line with a model in which inhomogeneity of ${H}_{c}$ and anomalous Hall resistivity are responsible for the transverse resistivity anomalies in $\mathrm{SrRu}{\mathrm{O}}_{3}$.

Общие закономерности и различия в поведении динамического перемагничивания ферримагнитных (CoFe-=SUB=-2-=/SUB=-O-=SUB=-4-=/SUB=-) и антиферромагнитных (NiO) наночастиц

Abstract In antiferromagnetic (AFM) nanoparticles, an additional ferromagnetic phase forms and leads to the appearance in AFM nanoparticles of a noncompensated magnetic moment and the magnetic properties typical of common FM nanoparticles. In this work, to reveal the regularities and differences of the dynamic magnetization switching in FM and AFM nanoparticles, the typical representatives of such materials are studied: CoFe_2O_4 and NiO nanoparticles with average sizes 6 and 8 nm, respectively. The high fields of the irreversible behavior of the magnetizations of these samples determine the necessity of using strong pulsed fields (amplitude to 130 kOe) to eliminate the effect of the partial hysteresis loop when studying the dynamic magnetic hysteresis. For both types of the samples, coercive force H _C at the dynamic magnetization switching is markedly higher than H _C at quasi-static conditions. H _C increases as the pulse duration τ_ P decreases and the maximum applied field H _0 increases. The dependence of H _C on field variation rate dH / dt = H _0/2τ_ P is a unambiguous function for CoFe_2O_4 nanoparticles, and it is precisely such a behavior is expected from a system of single-domain FM nanoparticles. At the same time, for AFM NiO nanoparticles, the coercive force is no longer an unambiguous function of dH / dt , and the value of applied field H _0 influences more substantially. Such a difference in the behaviors of FM and AFM nanoparticles is caused by the interaction of the FM subsystem and the AFM “core” inside AFM nanoparticles. This circumstance should be taken into account when developing the theory of dynamic hysteresis of the AFM nanoparticles and also to take into account their practical application.

Магнитоэлектроакустическая динамика в стрейнтронной ячейке памяти с произвольным доступом

Straintronic principle of nonvolatile magnetoelectric random access memory (MELRAM) draws much attention due to the prospects of ultra-low energy consumption in storage devices. The switching of magnetic moments by pulsed deformation of elastically coupled magnetic and piezoelectric subsystems is accompanied by excitation of acoustic oscillations in the memory cells. Oscillation period in nanoscale cells is comparable with the magnetic moment switching time, which can affect the information recording process. In the present paper the influence of acoustic excitations on the magnetic switching dynamics is studied by numerical modeling with application to a magnetostrictive cell 50×50×400 nm3 on a PMN-PT <011> substrate. Parameters of voltage control pulses that can insure reliable switching of magnetic states of the system are determined.

Effect of glass-removal on the magnetostriction and magnetic switching properties in amorphous FeSiB microwires

Abstract Results on the influence of glass-removal on the magnetostriction, magnetic switching properties and domain wall propagation in amorphous Fe77.5Si17.5B15 microwires are reported. Glass removal releases high thermoelastic stresses originated by the difference in thermal expansion coefficients of glass and metal, which reduces the coercivity, increases the domain wall mobility and decreases the local nucleation fields and their distributions. Evening the distribution of nucleation fields and eliminating the local surface defects due to glass removal increase the field range of single domain wall propagation regime. It was also found that the saturation magnetostriction coefficient significantly decreases after glass removal, which strengthens the impact of the reduced magnetoelastic anisotropy. The effect of glass removal on magnetostriction was previously observed only in Co-rich microwires with small negative magnetostriction. Understanding the factors that control the magnetic switching and domain wall dynamics is of interest for practical applications in sensing, storage and logic devices.

Magnetization switching dynamics in barium-ferrite nano-dot: dependence on magnetic damping constant

The Landau-Lifshift-Gilbert equation had been employed to study the effect of Gilbert damping on the dynamics of nano-Barium-Ferrite magnetization. This damping factor is necessary to investigate because of its impact on the dynamics of magnetization switching. In this study, Barium-Ferrite was chosen for study because it has large magnetic anisotropy, excellent chemical stability, and corrosion-free, potentially to be used as a magnetic storage medium with high-density. The nano-Barium-Ferrite magnetic parameters used were the anisotropy constants 3.0 × 106 erg/cm3, 4800 G saturation magnetization, 6.3 × 10−7 erg/cm exchange field, and Gilbert damping factor which varied from 0.4 to 0.9. By using the Micro-magnetic Simulator software, Nano-Barium-Ferrite was simulated as a dot with surface-sized 50 × 50 nm2 and 20 nm of the thickness. The simulation was conducted using Reduced Barrier Writing (RBW) scheme in which the sample was conditioned at room temperature of 298 K while induced by an external magnetic field. The magnetic field intensity linearly enlarged from 0 to 2 Tesla in 2.5 ns. As a result, the magnetization rate rises exponentially to the increase of Gilbert damping values at room temperature. Furthermore, the magnitude of Gilbert damping gives an impact on the propagation direction of the domain wall.