Current-induced Magnetization Dynamics Research Articles

Terahertz magnetic excitation in a collinear antiferromagnet: Canonical transformation model and atomistic spin simulations

In this study, simple canonical transformation theory is introduced to describe the current-driven magnetization dynamics in collinear antiferromagnets. The theory is confirmed by atomistic level dynamical simulations performed in an antiferromagnetic L10-type MnPt sample. The simulations indicate that when a small spin polarized current is perpendicularly applied to the MnPt sample, a stable initial stationary state remains with a perpendicular antiferromagnetic structure. For a current larger than a threshold, the transition from a stationary state to a stable in-plane precession state is observed without any assistance of magnetic fields, accompanied by a generation of terahertz magnetic signals. The simulations show a good agreement with the analytical theory, from which we could gain a comprehensive understanding of current-induced magnetization dynamics in collinear antiferromagnets at the atomic level.

Control of Néel Vector with Spin-Orbit Torques in an Antiferromagnetic Insulator with Tilted Easy Plane.

Injecting spin currents into antiferromagnets and realizing efficient spin-orbit-torque switching represents a challenging topic. Because of the diminishing magnetic susceptibility, current-induced antiferromagnetic dynamics remain poorly characterized, complicated by spurious effects. Here, by growing a thin film antiferromagnet, α-Fe_{2}O_{3}, along its nonbasal plane orientation, we realize a configuration where the spin-orbit torque from an injected spin current can unambiguously rotate and switch the Néel vector within the tilted easy plane, with an efficiency comparable to that of classical ferrimagnetic insulators. Our study introduces a new platform for quantitatively characterizing switching and oscillation dynamics in antiferromagnets.

Third harmonic characterization of antiferromagnetic heterostructures

Electrical switching of antiferromagnets is an exciting recent development in spintronics, which promises active antiferromagnetic devices with high speed and low energy cost. In this emerging field, there is an active debate about the mechanisms of current-driven switching of antiferromagnets. For heavy-metal/ferromagnet systems, harmonic characterization is a powerful tool to quantify current-induced spin-orbit torques and spin Seebeck effect and elucidate current-induced switching. However, harmonic measurement of spin-orbit torques has never been verified in antiferromagnetic heterostructures. Here, we report harmonic measurements in Pt/α-Fe2O3 bilayers, which are explained by our modeling of higher-order harmonic voltages. As compared with ferromagnetic heterostructures where all current-induced effects appear in the second harmonic signals, the damping-like torque and thermally-induced magnetoelastic effect contributions in Pt/α-Fe2O3 emerge in the third harmonic voltage. Our results provide a new path to probe the current-induced magnetization dynamics in antiferromagnets, promoting the application of antiferromagnetic spintronic devices.

A finite element approximation of a current-induced magnetization dynamics model

Micromagnetics is a continuum theory describing magnetization patterns inside ferromagnetic media. The dynamics of a ferromagnetic material are governed by the Landau-Lifshitz equation. This equation is highly nonlinear and has a non-convex constraint. In this work, a finite element approximation of a current-induced magnetization dynamics model is proposed. The model consists of a modified Landau-Lifshitz-Gilbert (LLG) equation incorporating spin transfer torque. The scheme preserves a non-convex constraint, requires only a linear solver at each time step and is easily applicable to the limiting cases. As the time and space steps tend to zero, a proof of convergence of the numerical solution to a (weak) solution of the modified LLG equation is given. Numerical results are presented to show the effect of the injected current on magnetization switching.

On the Landau-Lifshitz-Bloch equation with spin torque effects

A mathematical model for current-induced magnetization dynamics in ferromagnets at elevated temperatures is considered. The model is represented by a classical Landau-Lifshitz-Bloch equation containing adiabatic and non-adiabatic torques. In the case of a ferromagnet confined to a bounded domain of R3, we prove existence of a weak solution by using Faedo-Galerkin approach and a compactness method. This paper extends the work done by Le et al. (2016) in the absence of adiabatic and non-adiabatic terms. Furthermore, uniqueness is proved in the case of dimension one and two. Finally, the limiting behaviour in large time is studied for the non-adiabatic case.

Real-time Hall-effect detection of current-induced magnetization dynamics in ferrimagnets

Measurements of the transverse Hall resistance are widely used to investigate electron transport, magnetization phenomena, and topological quantum states. Owing to the difficulty of probing transient changes of the transverse resistance, the vast majority of Hall effect experiments are carried out in stationary conditions using either dc or ac. Here we present an approach to perform time-resolved measurements of the transient Hall resistance during current-pulse injection with sub-nanosecond temporal resolution. We apply this technique to investigate in real-time the magnetization reversal caused by spin-orbit torques in ferrimagnetic GdFeCo dots. Single-shot Hall effect measurements show that the current-induced switching of GdFeCo is widely distributed in time and characterized by significant activation delays, which limit the total switching speed despite the high domain-wall velocity typical of ferrimagnets. Our method applies to a broad range of current-induced phenomena and can be combined with non-electrical excitations to perform pump-probe Hall effect measurements.

Spin-dependent transport through a Weyl semimetal surface

We experimentally compare two types of interface structures with magnetic and non-magnetic Weyl semimetals. They are the junctions between a gold normal layer and magnetic Weyl semimetal Ti$_2$MnAl, and a ferromagnetic nickel layer and non-magnetic Weyl semimetal WTe$_2$, respectively. Due to the ferromagnetic side of the junction, we investigate spin-polarized transport through the Weyl semimetal surface. For both structures, we demonstrate similar current-voltage characteristics, with hysteresis at low currents and sharp peaks in differential resistance at high ones. Despite this behavior resembles the known current-induced magnetization dynamics in ferromagnetic structures, evolution of the resistance peaks with magnetic field is unusual. We connect the observed effects with current-induced spin dynamics in Weyl topological surface states.

Development of a system for low-temperature ultrafast optical study of three-dimensional magnon and spin orbital torque dynamics.

An ultrafast vector magneto-optical Kerr effect (MOKE) microscope with integrated time-synchronized electrical pulses, two-dimensional magnetic fields, and low-temperature capabilities is reported. The broad range of capabilities of this instrument allows the comprehensive study of spin-orbital interaction-driven magnetization dynamics in a variety of novel magnetic materials or heterostructures: (1) electrical-pump and optical-probe spectroscopy allows the study of current-driven magnetization dynamics in the time domain, (2) two-dimensional magnetic fields along with the vector MOKE microscope allow the thorough study of the spin-orbital-interaction induced magnetization re-orientation in arbitrary directions, and (3) the low-temperature capability allows us to explore novel materials/devices where emergent phenomena appear at low temperature. We discuss the details and challenges of this instrument development and integration and present two datasets that demonstrate and benchmark the capabilities of this instrument: (a) a room-temperature time-domain study of current-induced magnetization dynamics in a ferromagnet/heavy metal bilayer and (b) a low-temperature quasi-static polar MOKE study of the magnetization of a novel compensated ferrimagnet.

Quantitative Study on Current-Induced Effect in an Antiferromagnet Insulator/Pt Bilayer Film.

A quantitative investigation of the current-induced torque in antiferromagnets represents a great challenge due to the lack of an independent method for controlling Néel vectors. By utilizing an antiferromagnetic insulator with the Dzyaloshinskii-Moriya interaction α-Fe_{2}O_{3}, we show that the Néel vector can be controlled with a moderate external field, which is further utilized to calibrate the current-induced magnetic dynamics. We find that the current-induced magnetoresistance change in antiferromagnets can be complicated by resistive switching that does not have a magnetic origin. By excluding nonmagnetic switching and comparing the current-induced dynamics with the field-induced one, we determine the nature and magnitude of current-induced effects in Pt/α-Fe_{2}O_{3} bilayer films.

Deterministic switching of magnetization by chiral buckling

The current-induced magnetization dynamics is an important subject in spintronics research. It contains many interesting physical phenomena and has a high potential for spintronic applications. Most magnetic memory devices are based on the magnetization switching operation. Thus, finding the switching mechanism, as well as the exact understanding of it, is an essential implement for spintronic devices. Here, we show that the current can switch the magnetization via chiral buckling. To destabilize the uniform magnetization, a critical current value is needed. The Dzyaloshinskii-Moriya interaction (DMI) produces a shift in this critical current and the sign of this shift also depends on the domain polarity. Thus, a certain current can select a certain domain polarity. During the switching process, a fluctuation of magnetization is exponentially amplified by the current that forms the magnetization buckling. We explain these phenomena by linear stability analysis and show that two distinct switching operations are possible depending on the DMI.

Current-induced picosecond magnetization dynamics in a Ta/CoFeB/MgO hall bar

Time-resolved Kerr microscopy (TRSKM) has been used to explore the small amplitude picosecond magnetization dynamics induced by spin–orbit torques in a Ta(4 nm)/Co40Fe40B20(1 nm)/MgO(1.6 nm)/Ta(1 nm) Hall bar structure. The time dependent polar magneto optical Kerr effect was recorded following injection of a current pulse of 70 ps duration. Macrospin simulations provide a reasonable description of the precession and a transient background response as the field strength and current polarity are varied, while confirming that the in-plane spin–orbit torque is dominant within this system. Increasing the current density within the simulations leads to coherent magnetization reversal. Inclusion of a modest in-plane bias field is found to reduce both the switching current and the time required for switching. The orientation of the in-plane field relative to the direction of the current determines whether the magnetization can be switched backwards and forwards by current pulses of the same or opposite polarity.

Pinning Effect on Current-Induced Domain Wall Motion in Nanostrip

AbstractPinning effect on current-induced magnetic transverse domain wall dynamics in nanostrip is studied for its potential application to new magnetic memory devices. In this study, we carry out a series of calculations by solving generalized Landau-Lifshitz equation involving a current spin transfer torque in one and two dimensional models. The critical current for the transverse wall depinning in nanostrip depends on the size of artificial rectangular defects on the edges of nanostrip. We show that there is intrinsic pinning potential for a defect such that the transverse wall oscillates damply around the pinning site with an intrinsic frequency if the applied current is below critical value. The amplification of the transverse wall oscillation for both displacement and maximum value of m3 is significant by applying AC current and current pulses with appropriate frequency. We show that for given pinning potential, the oscillation amplitude as a function of the frequency of the AC current behaves like a Gaussian distribution in our numerical study, which is helpful to reduce strength of current to drive the transverse wall motion.

Current-induced damping of nanosized quantum moments in the presence of spin-orbit interaction

Motivated by the need to understand current-induced magnetization dynamics at the nanoscale, we have developed a formalism, within the framework of Keldysh Green function approach, to study the current-induced dynamics of a ferromagnetic (FM) nanoisland overlayer on a spin-orbit-coupling (SOC) Rashba plane. In contrast to the commonly employed classical micromagnetic LLG simulations the magnetic moments of the FM are treated {\it quantum mechanically}. We obtain the density matrix of the whole system consisting of conduction electrons entangled with the local magnetic moments and calculate the effective damping rate of the FM. We investigate two opposite limiting regimes of FM dynamics: (1) The precessional regime where the magnetic anisotropy energy (MAE) and precessional frequency are smaller than the exchange interactions, and (2) The local spin-flip regime where the MAE and precessional frequency are comparable to the exchange interactions. In the former case, we show that due to the finite size of the FM domain, the \textquotedblleft Gilbert damping\textquotedblright does not diverge in the ballistic electron transport regime, in sharp contrast to Kambersky's breathing Fermi surface theory for damping in metallic FMs. In the latter case, we show that above a critical bias the excited conduction electrons can switch the local spin moments resulting in demagnetization and reversal of the magnetization. Furthermore, our calculations show that the bias-induced antidamping efficiency in the local spin-flip regime is much higher than that in the rotational excitation regime.

Phase-Field Simulation of Strain-Assisted Current-Induced Magnetization Dynamics in a Magnetic Tunnel Junction

We investigated the effect of substrate misfit strain theoretically by using a phase-field microelasticity model in a simple sandwich structure of a magnetic tunnel junction with perpendicular anisotropy. The critical current for magnetization flipping decreases with increasing tensile strain except in the region above a threshold strain, which results from magneto-elastic coupling interaction between perpendicular magnetic anisotropy and substrate strain. Magnetization reversal speeds with different substrate strains and switching currents are also presented, which suggest ways to improve the magnetization switching efficiency and decrease the power consumption of magnetic devices.

Current induced magnetization dynamics and magnetization switching in superconducting ferromagnetic hybrid (F|S|F) structures

We investigate the current induced magnetization dynamics and magnetization switching in an unconventional p-wave superconductor sandwiched between two misaligned ferromagnetic layers by numerically solving the Landau-Lifshitz-Gilbert equation modified with current induced Slonczewski's spin torque term. A modified form of the Ginzburg-Landau free energy functional has been used for this purpose. We demonstrated the possibility of current induced magnetization switching in the spin-triplet ferromagnetic superconducting hybrid structures with a strong easy axis anisotropy and the condition for magnetization reversal. The switching time for such arrangement is calculated and is found to be highly dependent on the magnetic configuration along with the biasing current. This study would be useful in designing the practical superconducting-spintronic devices.

Current-driven antivortex core resonance measured by the rectifying effect

We demonstrate the current-driven resonance of a single antivortex core confined in a cross-shaped Ni81Fe19 wire. The antivortex core dynamics can be excited purely by spin transfer torque; therefore, it is significant to understand the current-induced magnetization dynamics. The antivortex core resonance can be measured from the frequency dependence of a rectified voltage generated by an alternating current application. We found that the resonance frequency and peak amplitude greatly depend on the external magnetic field. This result is in good agreement with micromagnetic simulation.

Current-Induced Magnetization Dynamics in 1-D Bicomponent Magnonic Crystal

Nonlinear localized magnetic excitations in 1-D bicomponent magnonic crystal are investigated under periodic magnetic field of spatially varying strength with spin current. The governing modified Landau–Lifshitz equation with spin current is transformed into the variable coefficient nonlinear Schrodinger (VCNLS) equation using a stereographic projection. In general, the VCNLS equation is nonintegrable and by using similarity transformation technique, the VCNLS equation is reduced into the standard NLS equation with certain integrability conditions. A systematic connection between the solution of VCNLS and NLS equations is established. The solution of VCNLS equation is obtained using the solution of the NLS equation. The excitation of magnetization is in the form of solitons on the oscillatory background with structure similar to the form of spin Bloch waves, and in addition, the spin current controls the soliton velocity. The velocity of soliton is directly proportional to the strength of current density and inversely proportional to the average saturation magnetization value, which is the value of saturation magnetization of the composite material at the interface of the two ferromagnetic materials, which are used to form the bicomponent magnonic crystal.

Stray Field Dependence of Threshold Current in a Spin-Torque Oscillator With a Perpendicular Spin Polarizer

We theoretically investigate the effect of stray field on the current-induced magnetization dynamics in a nanopillar with a perpendicular spin polarizer. The threshold current to excite out-of-plane (OP) magnetization procession is analytically calculated based on a simple Newton-like dynamic equation. The numerical calculations of this analytical expression are confirmed by macrospin simulations, revealing that the critical current linearly depends on the stray field. The stray field versus current phase diagram of magnetization oscillation states in such a device is presented, showing three regions of magnetization dynamics including parallel, antiparallel, and OP precession states. The physical insight of the phase transitions between the three states can be explained by calculating the energy profiles as well as the phase portraits of the free layer.

Charging effect reduction in electron beam lithography and observation of single nanopillars on highly insulating substrates

Electron beam writing and imaging of nanoscale structures on highly insulating substrates severely suffer from charging effects, which cause reduction in pattern resolution, positioning precision, and imaging quality. Conductive layers deposited above or below the resist layer can effectively reduce charge accumulation, but often give rise to contamination impairing the physical and chemical properties of functional nanostructures. Here we deal with top and bottom contacted, sub-micron-sized nanopillars made from multilayer stacks comprising ferromagnetic and non-magnetic materials for the study of current-induced magnetization dynamics. We show how the charging effects in a previously established fabrication process for single-crystalline nanopillars by H. Dassow et al. (2006) [1] can be significantly reduced by using the bottom electrode layer as charge dissipater and only isolating and disconnecting the bottom electrodes from ground after the fabrication of the delicate nanopillar structure by electron beam lithography. The modified process is successfully applied to Co2MnSi/Ag/Co2MnSi(001) multilayer stacks grown on highly insulating MgO substrates. Ellipsoidal nanopillars with a cross-section of 75×120nm2 reveal 2% giant magnetoresistance and angular dependent magnetization behavior due to the magnetic anisotropy of the elliptical nanomagnets.

Role of spin diffusion in current-induced domain wall motion for disordered ferromagnets

Current-induced spin transfer torque and magnetization dynamics in the presence of spin diffusion in disordered magnetic textures is studied theoretically. We demonstrate using tight-binding calculations that weak, spin-conserving impurity scattering dramatically enhances the nonadiabaticity. To further explore this mechanism, a phenomenological drift-diffusion model for incoherent spin transport is investigated. We show that incoherent spin diffusion indeed produces an additional spatially dependent torque of the form $\ensuremath{\sim}{\ensuremath{\nabla}}^{2}[\mathbf{m}\ifmmode\times\else\texttimes\fi{}(\mathbf{u}\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\nabla})\mathbf{m}]+\ensuremath{\xi}{\ensuremath{\nabla}}^{2}[(\mathbf{u}\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\nabla})\mathbf{m}]$, where $\mathbf{m}$ is the local magnetization direction, $\mathbf{u}$ is the direction of injected current, and $\ensuremath{\xi}$ is a parameter characterizing the spin dynamics (precession, dephasing, and spin-flip). This torque, which scales as the inverse square of the domain wall width, only weakly enhances the longitudinal velocity of a transverse domain wall but significantly enhances the transverse velocity of vortex walls. The spatial-dependent spin transfer torque uncovered in this study is expected to have significant impact on the current-driven motion of abrupt two-dimensional textures such as vortices, skyrmions, and merons.