Driven Magnetization Dynamics Research Articles

Spiking Dynamics in Dual Free Layer Perpendicular Magnetic Tunnel Junctions.

Spintronic devices have recently attracted a lot of attention in the field of unconventional computing due to their non-volatility for short- and long-term memory, nonlinear fast response, and relatively small footprint. Here we demonstrate experimentally how voltage driven magnetization dynamics of dual free layer perpendicular magnetic tunnel junctions can emulate spiking neurons in hardware. The output spiking rate was controlled by varying the dc bias voltage across the device. The field-free operation of this two-terminal device and its robustness against an externally applied magnetic field make it a suitable candidate to mimic the neuron response in a dense neural network. The small energy consumption of the device (4-16 pJ/spike) and its scalability are important benefits for embedded applications. This compact perpendicular magnetic tunnel junction structure could finally bring spiking neural networks to sub-100 nm size elements.

Multitude of exceptional points in van der Waals magnets

Several works have recently addressed the emergence of exceptional points (EPs), i.e., degeneracies of non-Hermitian Hamiltonians, in the long-wavelength dynamics of coupled magnetic systems. Here, by focusing on the driven magnetization dynamics of a van der Waals ferromagnetic bilayer, we show that exceptional points can appear over extended portions of the first Brillouin zone as well. Furthermore, we demonstrate that the effective non-Hermitian magnon Hamiltonian, whose eigenvalues are purely real or come in complex conjugate pairs, respects an unusual wavevector-dependent pseudo-Hermiticity. Finally, for both armchair and zigzag nanoribbon geometries, we discuss both the complex and purely real spectra of the topological edge states and their experimental implications.

Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium

Spin torque is a promising tool for driving magnetization dynamics for computing technologies. These torques can be easily produced by spin-orbit effects, but for most conventional spin source materials, a high degree of crystal symmetry limits the geometry of the spin torques produced. Magnetic ordering is one way to reduce the symmetry of a material and allow exotic torques, and antiferromagnets are particularly promising because they are robust against external fields. We present spin torque ferromagnetic resonance (ST-FMR) measurements and second harmonic Hall measurements characterizing the spin torques in antiferromagnetic iron rhodium alloy. We report extremely large, strongly temperature-dependent exotic spin torques with a geometry apparently defined by the magnetic ordering direction. We find the spin torque efficiency of iron rhodium to be (207 \ifmmode\pm\else\textpm\fi{} 94)% at 170 K and (88 \ifmmode\pm\else\textpm\fi{} 32)% at room temperature. We support our conclusions with theoretical calculations showing how the antiferromagnetic ordering in iron rhodium gives rise to such exotic torques.

Global weak solutions for the Landau–Lifshitz–Gilbert–Vlasov–Maxwell system coupled via emergent electromagnetic fields

Motivated by recent models of current driven magnetization dynamics, we examine the coupling of the Landau–Lifshitz–Gilbert equation and classical electron transport governed by the Vlasov–Maxwell system. The interaction is based on space-time gyro-coupling in the form of emergent electromagnetic fields of quantized helicity that add up to the conventional Maxwell fields. We construct global weak solutions of the coupled system in the framework of frustrated magnets with competing first- and second-order gradient interactions known to host topological solitons such as magnetic skyrmions and hopfions.

Photon spin angular momentum driven magnetization dynamics in ferromagnet/heavy metal bilayers

Thin-film magnetization controlled by optical helicity has been recently reported. Although circularly polarized light has spin angular momentum, helicity-dependent all-optical magnetization switching is mediated by the stochastic thermal process, such as magnetic circular dichroism, and the effect of photon spin angular momentum is considered to be a secondary role. Conversely, the inverse Faraday effect in ferromagnetic thin films and photon spin angular momentum injection into heavy metal thin films have been observed, which can induce torque on metallic thin-film magnets. In this study, we show photon spin angular momentum driven magnetization dynamics in bilayers of Co/(Pt, Au) thin films with various thicknesses. The heavy metal Pt, Au, and ferromagnetic Co layer thickness dependencies of photon spin angular momentum driven torques are discussed in terms of field-like torque owing to the inverse Faraday effect and spin-transfer torque caused by photon spin angular momentum injection into the heavy metal layer with details of optical and magnetic properties. This study provides a better understanding of photon spin angular momentum induced magnetization dynamics in metallic thin-film heterostructures for efficient photon-driven magnetization manipulation.

Optical signatures of the coupled spin-mechanics of a levitated magnetic microparticle

We propose a platform that combines the fields of cavity optomagnonics and levitated optomechanics to control and probe the coupled spin-mechanics of magnetic dielectric particles. We theoretically study the dynamics of a levitated Faraday-active dielectric microsphere serving as an optomagnonic cavity, placed in an external magnetic field and driven by an external laser. We find that the optically driven magnetization dynamics induces angular oscillations of the particle with low associated damping. Further, we show that the magnetization and angular motion dynamics can be probed via the power spectrum of the outgoing light. Namely, the characteristic frequencies attributed to the angular oscillations and the spin dynamics are imprinted in the light spectrum by two main resonance peaks. Additionally, we demonstrate that a ferromagnetic resonance setup with an oscillatory perpendicular magnetic field can enhance the resonance peak corresponding to the spin oscillations and induce fast rotations of the particle around its anisotropy axis.

Laser-Induced Magnetization Precession in Individual Magnetoelastic Domains of a Multiferroic Co40Fe40B20/BaTiO3 Composite

Using a magneto-optical pump-probe technique with micrometer spatial resolution we show that magnetization precession can be launched in individual magnetic domains imprinted in a Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) layer by elastic coupling to ferroelectric domains in a BaTiO$_{3}$ substrate. The dependence of the precession parameters on external magnetic field strength and orientation reveal that by laser-induced ultrafast partial quenching of the magnetoelastic coupling parameter of CoFeB by $\approx$27% along with 10% ultrafast demagnetization trigger the magnetization precession. The relation between the laser-induced reduction of the magnetoelastic coupling and the demagnetization is approximated by the $n(n+1)/2$-law with n$\approx$2. This correspondence confirms the thermal origin of the laser-induced anisotropy change. Based on the analysis and modeling of the excited precession we find signatures of laser-induced precessional switching, which occurs when the magnetic field is applied along the hard magnetization axis and its value is close to the effective magnetoelastic anisotropy field. The precession excitation process in an individual magnetoelastic domain is found to be unaffected by neighboring domains. This makes laser-induced changes of magnetoelastic anisotropy a promising tool for driving magnetization dynamics and switching in composite multiferroics with spatial selectivity.

The role of ultrafast magnon generation in the magnetization dynamics of rare-earth metals.

Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin-lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.

Single-shot dynamics of spin-orbit torque and spin transfer torque switching in three-terminal magnetic tunnel junctions.

Current-induced spin-transfer torques (STT) and spin-orbit torques (SOT) enable the electrical switching of magnetic tunnel junctions (MTJs) in non-volatile magnetic random access memories. To develop faster memory devices, an improvement in the timescales that underlie the current-driven magnetization dynamics is required. Here we report all-electrical time-resolved measurements of magnetization reversal driven by SOT in a three-terminal MTJ device. Single-shot measurements of the MTJ resistance during current injection reveal that SOT switching involves a stochastic two-step process that consists of a domain nucleation time and propagation time, which have different genesis, timescales and statistical distributions compared to STT switching. We further show that the combination of SOT, STT and the voltage control of magnetic anisotropy leads to reproducible subnanosecond switching with the spread of the cumulative switching time smaller than 0.2 ns. Our measurements unravel the combined impact of SOT, STT and the voltage control of magnetic anisotropy in determining the switching speed and efficiency of MTJ devices.

Spin–Orbit Torque and Spin Hall Effect-Based Cellular Level Therapeutic Spintronic Neuromodulator: A Simulation Study

Artificial modulation of a neuronal subset through ion channels activation can initiate firing patterns of an entire neural circuit in vivo. As nanovalves in the cell membrane, voltage-gated ion channels can be artificially controlled by the electric field gradient that caused by externally applied time varying magnetic fields. Herein, we theoretically investigate the feasibility of modulating neural activities by using magnetic spintronic nanostructures. An antiferromagnet/ferromagnet (AFM/FM) structure is explored as neuromodulator. For FM layer with perpendicular magnetization, stable bidirectional magnetization switching can be achieved by applying in-plane currents through AFM layer to induce the spin-orbit torque (SOT) due to the spin Hall effect (SHE). This Spin-orbit Torque Neurostimulator (SOTNS) utilizes in-plane charge current pulses to switch the magnetization in FM layer. The time changing magnetic stray field induces electric field that modulates the surrounding neurons. The Object Oriented Micromagnetic Framework (OOMMF) is used to calculate space and time dependent magnetic dynamics of SOTNS structure. The current driven magnetization dynamics in SOTNS has no mechanically moving parts. Furthermore, the size of SOTNS can be down to tens of nanometers, thus, arrays of SOTNSs could be fabricated, integrated together and patterned on a flexible substrate, which gives us much more flexible control of the neuromodulation with cellular resolution.

Another view on Gilbert damping in two-dimensional ferromagnets

A keen interest towards technological implications of spin-orbit driven magnetization dynamics requests a proper theoretical description, especially in the context of a microscopic framework, to be developed. Indeed, magnetization dynamics is so far approached within Landau-Lifshitz-Gilbert equation which characterizes torques on magnetization on purely phenomenological grounds. Particularly, spin-orbit coupling does not respect spin conservation, leading thus to angular momentum transfer to lattice and damping as a result. This mechanism is accounted by the Gilbert damping torque which describes relaxation of the magnetization to equilibrium. In this study we work out a microscopic Kubo-Středa formula for the components of the Gilbert damping tensor and apply the elaborated formalism to a two-dimensional Rashba ferromagnet in the weak disorder limit. We show that an exact analytical expression corresponding to the Gilbert damping parameter manifests linear dependence on the scattering rate and retains the constant value up to room temperature when no vibrational degrees of freedom are present in the system. We argue that the methodology developed in this paper can be safely applied to bilayers made of non- and ferromagnetic metals, e.g., CoPt.

Scaling Study of Spin-Hall-Assisted Spin Transfer Torque Driven Magnetization Switching in the Presence of Dzyaloshinskii–Moriya Interaction

Spin-hall-assisted spin transfer torque (SHA-STT) can achieve high-speed, magnetic-field-free, and high-reliable magnetization switching in a three-terminal device consisting of magnetic tunnel junctions (MTJ) above a heavy-metal. Nowadays, the development of perpendicular magnetic anisotropy drives the continuous scaling of the MTJ. In addition, an asymmetric exchange interaction called Dzyaloshinskii–Moriya interaction (DMI) inevitably exists at the heavy metal/ferromagnet interface and has a considerable influence on the magnetization dynamics. Considering these factors, in this work, we study the scaling performance of the SHA-STT driven magnetization dynamics in the presence of DMI. Simulation results demonstrate that, for nonzero DMI, the magnetization switching is activated by domain nucleation, whose mechanism is strongly dependent on the MTJ size and DMI magnitude. The critical SHE current density for magnetization switching decreases with the enlarged MTJ or enhanced DMI. In the presence of DMI, the switching time decreases with the scaling of the MTJ. Moreover, compared with the case of zero DMI, the switching speed is improved or deteriorated for the weak or strong DMI, respectively. Our work demonstrates that the MTJ size and DMI magnitude should be optimized in order to achieve a good tradeoff among a set of performance metrics of the SHA-STT devices.

Micromagnetic simulations of spin-torque driven magnetization dynamics with spatially resolved spin transport and magnetization texture

We present a simple and fast method to simulate spin-torque driven magnetisation dynamics in nano-pillar spin-valve structures. The approach is based on the coupling between a spin transport code based on random matrix theory and a micromagnetics finite-elements software. In this way the spatial dependence of both spin transport and magnetisation dynamics is properly taken into account. Our results are compared with experiments. The excitation of the spin-wave modes, in- cluding the threshold current for steady state magnetisation precession and the nonlinear frequency shift of the modes are reproduced correctly. The giant magneto resistance effect and the magnetisa- tion switching also agree with experiment. The similarities with recently described spin-caloritronics devices are also discussed.

Unidirectional spin-torque driven magnetization dynamics

The rich physics associated with magnetism often centers around directional effects. Here we demonstrate how spin-transfer torques in general result in unidirectional ferromagnetic resonance dynamics upon field reversal. The unidirectionality is a direct consequence of both field-like and damping-like dynamic torques simultaneously driving the motion. This directional effect arises from the field-like torque being odd and the damping-like torque being even under field reversal. The directional effect is observed when the magnetization has both an in-plane and out-of-plane component, since then the linear combination of the torques rotates with a different handedness around the magnetization as the magnetization is tipped out-of-plane. The effect is experimentally investigated via spin-torque ferromagnetic resonance measurements with the field applied at arbitrary directions away from the interface normal. The measured asymmetry of the voltage spectra are well explained within a phenomenological torque model.

Thickness dependence study of current-driven ferromagnetic resonance in Y3Fe5O12/heavy metal bilayers

We use ferromagnetic resonance to study the current-induced torques in YIG/heavy metal bilayers. YIG samples with thickness varying from 14.8 nm to 80 nm, with the Pt or Ta thin film on top, are measured by applying a microwave current into the heavy metals and measuring the longitudinal DC voltage generated by both spin rectification and spin pumping. From a symmetry analysis of the FMR lineshape and its dependence on YIG thickness, we deduce that the Oersted field dominates over spin-transfer torque in driving magnetization dynamics.

Time-resolved scanning electron microscopy with polarization analysis

We demonstrate the feasibility of investigating periodically driven magnetization dynamics in a scanning electron microscope with polarization analysis based on spin-polarized low-energy electron diffraction. With the present setup, analyzing the time structure of the scattering events, we obtain a temporal resolution of 700 ps, which is demonstrated by means of imaging the field-driven 100 MHz gyration of the vortex in a soft-magnetic FeCoSiB square. Owing to the efficient intrinsic timing scheme, high-quality movies, giving two components of the magnetization simultaneously, can be recorded on the time scale of hours.

Terahertz-driven magnetism dynamics in the orthoferrite DyFeO3

Terahertz (THz) driven magnetization dynamics are explored in the orthoferrite DyFeO3. A high-field, single cycle THz pulse is used to excite magnon modes in the crystal together with other resonances. Both quasi-ferromagnetic and quasi-antiferromagnetic magnon modes are excited and appear in time-resolved measurements of the Faraday rotation. Other modes are also observed in the measurements of the time-resolved linear birefringence. Analysis of the excitation process reveals that despite larger than expected electro-optical susceptibility, it is mainly the THz magnetic field that couples to the quasi-ferromagnetic and quasi-antiferromagnetic magnon branches.

Current driven magnetization dynamics of a self-polarised synthetic ferrimagnet

Spin torque driven excitations in spin valves and tunnel junctions are often investigated for a two magnetic layer system for which a polarizer (fixed magnetization) and a free layer can be distinguished. In the search for improved microwave properties and to understand the role of different coupling mechanisms between the magnetic layers, here, the excitation spectrum of an exchange coupled two layer synthetic ferrimagnet (SyF) system is investigated numerically with spin momentum transfer acting on both layers simultaneously. This self-polarised two layer system does not contain an external polarizer, and excitation of coupled modes arises due to the mutual spin transfer torque and the Ruderman-Kittel-Kasuya-Yosida interlayer exchange coupling. The current-field state diagrams of static and dynamic states are reported as a function of the interlayer exchange coupling strength. The numerically determined critical boundaries are well reproduced by an analytical stability analysis. The dynamic steady states reveal an optic-like mode at low magnetic fields, which becomes progressively acoustic-like for increased magnetic fields and currents. The frequency of these modes can be tuned by the film thickness and the strength of the interlayer exchange interaction. The results presented here will provide an important guide for designing spin torque oscillators that exploit the dynamic coupling between layers and, furthermore, they will provide a basis to test analytical models of spin torque driven coupled excitations.

Micromagnetic simulations of spin-wave normal modes and the spin-transfer-torque driven magnetization dynamics of a ferromagnetic cross

We studied spin-transfer-torque (STT) switching of a cross-shaped magnetic tunnel junction in a recent report [Roy et al., J. Appl. Phys. 113, 223904 (2013)]. In that structure, the free layer is designed to have four stable energy states using the shape anisotropy of a cross. STT switching showed different regions with increasing current density. Here, we employ the micromagnetic spectral mapping technique in an attempt to understand how the asymmetry of cross dimensions and spin polarization direction of the injected current affect the magnetization dynamics. We compute spatially averaged frequency-domain spectrum of the time-domain magnetization dynamics in the presence of the current-induced STT term. At low currents, the asymmetry of polarization direction and that of the arms are observed to cause a splitting of the excited frequency modes. Higher harmonics are also observed, presumably due to spin-wave wells caused by the regions of spatially non-uniform effective magnetic field. The results could be used towards designing a multi-bit-per-cell STT-based random access memory with an improved storage density.

Experimental observation of the optical spin transfer torque

The spin transfer torque is a phenomenon in which angular momentum of a spin polarized electrical current entering a ferromagnet is transferred to the magnetization. The effect has opened a new research field of electrically driven magnetization dynamics in magnetic nanostructures and plays an important role in the development of a new generation of memory devices and tunable oscillators. Optical excitations of magnetic systems by laser pulses have been a separate research field whose aim is to explore magnetization dynamics at short time scales and enable ultrafast spintronic devices. We report the experimental observation of the optical spin transfer torque, predicted theoretically several years ago building the bridge between these two fields of spintronics research. In a pump-and-probe optical experiment we measure coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by circularly polarized laser pulses. During the pump pulse, the spin angular momentum of photo-carriers generated by the absorbed light is transferred to the collective magnetization of the ferromagnet. We interpret the observed optical spin transfer torque and the magnetization precession it triggers on a quantitative microscopic level. Bringing the spin transfer physics into optics introduces a fundamentally distinct mechanism from the previously reported thermal and non-thermal laser excitations of magnets. Bringing optics into the field of spin transfer torques decreases by several orders of magnitude the timescales at which these phenomena are explored and utilized.