Domain Wall Velocity Research Articles

Engineering of magnetic properties and domain wall dynamics in Fe-Ni-based amorphous microwires by annealing

We present experimental studies related to the influence of annealing on domain wall (DW) dynamics and magnetic hysteresis of Fe-Ni-rich microwires. As-prepared and annealed Fe62Ni15.5Si7.5B15 and Fe49.6Ni27.9Si7.5B15 microwires present perfectly rectangular hysteresis loops and single DW propagation. DW velocity, v, DW mobility, S, and coercivity, Hc, of both studied microwires have been considerably affected after annealing. In Fe62Ni15.5Si7.5B15 microwires, we observed a gradual S increase with annealing time, tann, increasing. However, a non-monotonic S (tann) dependence was observed in Fe49.6Ni27.9Si7.5B15 microwires. We discussed observed dependencies in terms of different magnetoelastic anisotropy, domain walls stabilization, and different Curie temperatures of studied microwires.

Current induced domain wall motion with a Ta/Gd-Fe/Si-N magnetic nanowire for a magneto-optical light modulator

We investigated a magnetic nanowire device for a light modulator which is very important for 3D holography display application. It has a Si-N/Ta/Gd-Fe/Si-N nanowire with 0.5 × 17 μm2 dimension and a Co/Pd multilayer hard magnet with its size of 0.5 × 3 μm2 which is attached underneath the nanowire for initial domain nucleation. Both the nanowire and the hard magnet have perpendicular anisotropy. An initial domain was successfully nucleated by pulse current and found spin-orbit torque from Ta and local magnetic field from the nanomagnet was responsible for the domain nucleation. Current induced domain wall (DW) motion was observed and the DW moved along current direction without magnetic field. DW velocity was significantly enhanced with in-plane magnetic field (Hinp) of 25mT and velocity more than 200 m/s was obtained, which was more than 25 times faster than that without magnetic field. We also show that the fast DW velocity is attributed to chiral Neel domain wall induced by external in-plane magnetic field. This fast DW motion is very useful for large scale light modulator which refreshes large number of pixels, such as 10K × 10K pixels, over a limited time scale of 8 ∼ 16.7 ms which corresponds to TV frame rate of 120 or 60 Hz.

Magnetic properties of MBE grown Mn4N on MgO, SiC, GaN and Al2O3 substrates

Mn4N is a compound magnetic material that can be grown using MBE while exhibiting several desirable magnetic properties such as strong perpendicular magnetic anisotropy, low saturation magnetization, large domain size, and record high domain wall velocities. In addition to its potential for spintronic applications exploiting spin orbit torque with epitaxial topological insulator/ferromagnet bilayers, the possibility of integrating Mn4N seamlessly with the wide bandgap semiconductors GaN and SiC provides a pathway to merge logic, memory and communication components. We report a comparative study of MBE grown Mn4N thin films on four crystalline substrates: cubic MgO, and hexagonal GaN, SiC and sapphire. Under similar growth conditions, the Mn4N film is found to grow single crystalline on MgO and SiC, polycrystalline on GaN, and amorphous on sapphire. The magnetic properties vary on the substrates and correlate to the structural properties. Interestingly, the field dependent anomalous Hall resistance of Mn4N on GaN shows different behavior from other substrates such as a flipped sign of the anomalous Hall resistance.

Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets

Spin–orbit torque can be used to manipulate magnetization in spintronic devices. However, conventional ferromagnetic spin–orbit torque systems have intrinsic limitations in terms of operation speed due to their inherent magnetization dynamics. Antiferromagnets and ferrimagnets with antiparallel exchange coupling exhibit faster spin dynamics and could potentially overcome these limitations. Here, we report ultrafast spin–orbit torque-induced magnetization switching in ferrimagnetic cobalt-gadolinium (CoGd) alloy devices. Using a stroboscopic pump–probe technique to perform time-resolved measurements, we show that the switching time in the ferrimagnets can be reduced to the subnanosecond regime and a domain wall velocity of 5.7 km s‒1 can be achieved, which is in agreement with analytical modelling and atomistic spin simulations. We also find that the switching energy efficiency in the ferrimagnets is one to two orders of magnitude higher than that of ferromagnets. Time-resolved measurements show that current-induced magnetization switching in ferrimagnetic devices is faster and more energy-efficient than in ferromagnet devices.

Fast and efficient switching with ferrimagnets

Domain-wall velocities of 5.7 kilometres per second and switching times of less than a nanosecond have been observed in ferrimagnetic devices.

Study of the velocity plateau of Dzyaloshinskii domain walls

We study field-driven domain wall (DW) velocities in asymmetric multilayer stacks with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction (DMI), both experimentally and by micromagnetic simulations. Using magneto-optical Kerr microscopy under intense and nanoseconds-long fields, we show that DWs in these films propagate at velocities up to hundreds of m/s and that, instead of the expected decrease of velocity after the Walker field, a long plateau with constant velocity is observed, before breakdown. Both the maximum speed and the field extent of the velocity plateau strongly depend on the values of the spontaneous magnetization and the DMI strength, as well as on the magnetic anisotropy. Micromagnetic simulations reproduce these features in sufficiently wide strips, even for perfect samples. A physical model explaining the microscopic origin of the velocity plateau is proposed.

Interaction of Magnetization Centers of Different Signs as the Cause of the Nonmonotonic Field Dependence of the Domain Wall Velocity in Synthetic Pt/Co/Ir/Co/Pt Ferrimagnets

The purpose of the work is to find the dependence of growth rate of magnetization centers of various types on their surrounding by other nucleation centers in a synthetic Pt/Co/Ir/Co/Pt ferrimagnet with perpendicular magnetic anisotropy. The following four types of nucleation centers exist in a sample with two ferromagnetic layers of different thicknesses: P+ centers correspond to the regions where the magnetizations of the thick and thin Co layers are directed along an applied field (↑↑); P– centers are the regions where the magnetizations of the layers are opposite to an applied field (↓↓); and AP+ and AP– centers correspond to the regions where the magnetizations of the thick and thin Co layers are opposite to each other and the total magnetic moment is along (↑↓) or opposite to (↓↑) an applied field, respectively. P– nucleation centers are found to be surrounded by AP+ regions in any field and exhibit a monotonic field dependence of their boundary. The field dependence of the growth rate of AP– nucleation centers is nonmonotonic since, as the field increases, they are surrounded by AP+ nucleation centers, AP+ and P– regions, and only P– nucleation centers in strong fields.

Study of Domain Wall Propagation in FePt and FePd Nanowires Driven by Sub-nanosecond Pulse of Magnetic Field by Micromagnetic Simulation

In this study, we investigated the domain wall propagation behavior in FePt and FePd nanowires by micromagnetic simulation based on LLG equation. A rectangular shape nanowire model was generated with a fixed length of 2000 nm and varied thicknesses of 2.5 nm and 5 nm. The wire widths were varied of 50 nm, 100 nm, and 150 nm. The 500 ps length of magnetic-pulse was applied in-plane to a transverse type domain wall. We observed the Walker Breakdown field (HWB), which means the maximum domain wall velocity before it was abruptly decreased. The transverse DW structure was maintained below HWB while vortex/antivortex wall structure was formed in the higher magnetic field. Interestingly, the HWB value of both FePt and FePd were decreased as the width and thickness of the nanowires increased. We have also analyzed the changing of the demagnetization and exchange energy for the domain wall structure transition from transverse wall to vortex/anti-vortex wall.

Antiferromagnetic domain wall control via surface spin flop in fully tunable synthetic antiferromagnets with perpendicular magnetic anisotropy

Antiferromagnetic (AF) domain walls have recently attracted revived attention, not only in the emerging field of AF spintronics, but also more specifically for offering fast domain wall velocities and dynamic excitations up to the terahertz frequency regime. Here we introduce an approach to nucleate and stabilize an AF domain wall in a synthetic antiferromagnet (SAF). We present experimental and micromagnetic studies of the magnetization reversal in [(Co/Pt)$_{X-1}$/Co/Ir]$_{N-1}$(Co/Pt)$_X$ SAFs, where interface induced perpendicular magnetic anisotropy (PMA) and AF interlayer exchange coupling (IEC) are completely controlled via the individual layer thicknesses within the multilayer stack. By combining strong PMA with even stronger AF IEC, the SAF reveals a collective response to an external magnetic field applied normal to the surface, and we stabilize the characteristic surface spin flop (SSF) state for an even number N of AF-coupled (Co/Pt)$_{X-1}$/Co multilayer blocks. In the SSF state our system provides a well-controlled and fully tunable vertical AF domain wall, easy to integrate as no single crystal substrates are required and with uniform 2D-magnetization in the film plane for further functionalization options, such as for example lateral patterning via lithography.

Limiting Velocity and Dispersion Law of Domain Walls in Ferrimagnets Close to the Spin Compensation Point

The motion of domain walls in GdFeCo-type ferrimagnets near the point of compensation of sublattice spins s1 and s2, when the effects of the exchange increase in the limiting wall velocity take place, is theoretically studied. The energy E and momentum P of the wall are determined and the dispersion relation E = E(P) is derived. It is found that some walls become unstable at a sufficiently small decompensation value s1 - s2 νc(s1 + s2), this dispersion relation, as well as that for ferromagnets, is periodic.

Voltage-induced high-speed DW motion in a synthetic antiferromagnet

Voltage-induced motion of a magnetic domain wall (DW) has potential in developing novel devices with ultralow dissipation. However, the speed for the voltage-induced DW motion (VIDWM) in a single ferromagnetic layer is usually very low. In this work, we proposed VIDWM with high speed in a synthetic antiferromaget (SAF). In the medium with a small damping coefficient, the velocity for the coupled DWs in the SAF is significantly higher than its counterpart in a single ferromagnetic layer. Strong interlayer antiferromagnetic exchange coupling plays a critical role for the high DW velocity since it inhibits the tilting of DW plane with Dzyaloshinskii–Moriya interaction. On the other hand, in the material with a large damping coefficient, the Walker breakdown of DW motion is also inhibited due to the stabilization of moment orientation under a strong interlayer antiferromagnetic coupling. In theory, the voltage-induced gradient of magnetic anisotropy is proved to be equal to an effective magnetic field that drives DW.

Enhancing domain wall velocity through interface intermixing in W-CoFeB-MgO films with perpendicular anisotropy

We study the influence of He+ irradiation induced interface intermixing on magnetic domain wall (DW) dynamics in W-CoFeB (0.6 nm)-MgO ultrathin films, which exhibit high perpendicular magnetic anisotropy and large Dzyaloshinskii-Moriya interaction (DMI) values. Whereas the pristine films exhibit strong DW pinning, we observe a large increase in the DW velocity in the creep regime upon He+ irradiation, which is attributed to the reduction of pinning centers induced by interface intermixing. Asymmetric in-plane field-driven domain expansion experiments show that the DMI value is slightly reduced upon irradiation, and a direct relationship between DMI and interface anisotropy is demonstrated. Our findings provide insights into the material design and interface control for DW dynamics, as well as for DMI, enabling the development of high-performance spintronic devices based on ultrathin magnetic layers.

Analogy between growth of crystals and ferroelectric domains. Application of Wulff construction

In this article we applied a concept of kinetic Wulff plot (v-plot), which was proposed originally for understanding and control of the crystal growth, for characterization of the domain wall motion anisotropy in uniaxial ferroelectrics. Only the minima of v-plot can be obtained by the observation of the convex growth of isolated domains, whereas we can get insight of the v-plot maxima considering the concave growth during shape transformation after domain coalescence. Within this approach we reconstructed experimentally the compete v-plot for lithium niobate and potassium titanyl phosphate crystals. The giant effect of the domain wall velocity dependence on its orientation has been revealed. The effect results in fast restoration of the convex faceted domain shape with the slowest walls after merging. The proposed approach can be used for understanding and control of domain growth during electric field poling by patterned electrodes.

Creep of chiral domain walls

Recent experimental studies of magnetic domain expansion under easy-axis drive fields in materials with a perpendicular magnetic anisotropy have shown that the domain wall velocity is asymmetric as a function of an external in plane magnetic field. This is understood as a consequence of the inversion asymmetry of the system, yielding a finite chiral Dzyaloshinskii-Moriya interaction. Numerous attempts have been made to explain these observations using creep theory, but, in doing so, these have not included all contributions to the domain wall energy or have introduced additional free parameters. In this article we present a theory for creep motion of chiral domain walls in the creep regime that includes the most important contributions to the domain-wall energy and does not introduce new free parameters beyond the usual parameters that are included in the micromagnetic energy. Furthermore, we present experimental measurements of domain wall velocities as a function of in-plane field that are well decribed by our model, and from which material properties such as the strength of the Dzyaloshinskii-Moriya interaction and the demagnetization field are extracted.

Engineering of Magnetic Softness and Domain Wall Dynamics of Fe-rich Amorphous Microwires by Stress- induced Magnetic Anisotropy

We observed a remarkable improvement of domain wall (DW) mobility, DW velocity, giant magnetoimpedance (GMI) effect and magnetic softening at appropriate stress-annealing conditions. Beneficial effect of stress-annealing on GMI effect and DW dynamics is associated with the induced transverse magnetic anisotropy. An improvement of the circumferential permeability in the nearly surface area of metallic nucleus is evidenced from observed magnetic softening and remarkable GMI effect rising. We assumed that the outer domain shell with transverse magnetic anisotropy associated to stress-annealing induced transverse magnetic anisotropy affects the travelling DW in a similar way as application of transversal bias magnetic field allowing enhancement the DW velocity. Observed decreasing of the half-width of the EMF peak in stress-annealed microwires can be associated to the decreasing of the characteristic DW width. Consequently, stress annealing enabled us to design the magnetic anisotropy distribution beneficial for optimization of either GMI effect or DW dynamics.

Impact of Stress Annealing on the Magnetization Process of Amorphous and Nanocrystalline Co-Based Microwires.

The domain wall (DW) dynamics of amorphous and nanocrystalline Co-based glass-coated microwires are explored under the influence of stress annealing. Different annealing profiles have enabled remarkable changes in coercivity and magnetostriction values of Co-based amorphous microwires with initially negative magnitude, allowing induced magnetic bistability in stress-annealed samples and, consequently, high DW velocity has been observed. Similarly, Co-based nanocrystalline microwires with positive magnetostriction and spontaneous bistability have featured high DW velocity. Different values of tensile stresses applied during annealing have resulted in a redistribution of magnetoelastic anisotropy showing a decreasing trend in both DW velocities and coercivity of nanocrystalline samples. Observed results are discussed in terms of the stress dependence on magnetostriction and microstructural relaxation.

Dynamics of noncollinear antiferromagnetic textures driven by spin current injection

We present a theoretical formalism to address the dynamics of textured, noncolliear antiferromagnets subject to spin current injection. We derive sine-Gordon type equations of motion for the antiferromagnets, which are applicable to technologically important antiferromagnets such as Mn3Ir and Mn3Sn, and enables an analytical approach to domain wall dynamics in those materials. We obtain the expression for domain wall velocity, which is estimated to reach around 1 km/s in Mn3Ir by exploiting spin Hall effect with electric current density around 10^11A/m^2.

In situ monitoring of electric field effect on domain wall motion in Co ultrathin films in direct contact with an electrolyte

We present experimental data on the electric field effect on the magnetic domain wall dynamics in Co ultrathin films in direct contact with an aqueous electrolyte and in the absence of any oxide layer. We use a three electrode electrochemical setup to apply a large and uniform electric field and to precisely separate chemical effects induced by hydrogen from pure electric field effects. We show that in the case of the pure electric field effect, the domain wall velocity varies exponentially with the electric field and that these variations are larger than those observed previously on similar systems due to a larger magnetoelectric coefficient in our case.

Current-driven domain wall dynamics in ferrimagnets: Micromagnetic approach and collective coordinates model

Theoretical studies dealing with current-driven domain wall dynamics in ferrimagnetic alloys and, by extension, other antiferromagnetically coupled systems as some multilayers, are here presented. The analysis has been made by means of micromagnetic simulations that consider these systems as constituted by two subsystems coupled in terms of an additional exchange interlacing them. Both subsystems differ in their respective gyromagnetic ratios and temperature dependence. Other interactions, as for example anisotropic exchange or spin-orbit torques, can be accounted for differently within each subsystem according to the physical structure. Micromagnetic simulations are also endorsed by means of a collective coordinates model which, in contrast with some previous approaches to these antiferromagnetically coupled systems, based on effective parameters, also considers them as formed by two coupled subsystems with experimentally definite parameters. Both simulations and the collective model reinforce the angular moment compensation argument as accountable for the linear increase with current of domain wall velocities in these alloys at a certain temperature or composition. Importantly, the proposed approach by means of two coupled subsystems permits to infer relevant results in the development of future experimental setups that are unattainable by means of effective models.

Atomic antiferromagnetic domain wall propagation beyond the relativistic limit

We theoretically investigate the dynamics of atomic domain walls (DWs) in antiferromagnets driven by a spin-orbit field. For a DW with the width of a few lattice constants, we identify a Peierls-like pinning effect, with the depinning field exponentially decaying with the DW width, so that a spin-orbit field moderately larger than the threshold can drive the propagation of an atomic DW in a step-wise manner. For a broad DW, the Peierls pinning is negligibly small. However, the external spin-orbit field can induce a fast DW propagation, accompanied by a significant shrinking of its width down to atomic scales. Before stepping into the pinning region, noticeable spin waves are emitted at the tail of the DW. The spin-wave emission event not only broadens the effective width of the DW, but also pushes the DW velocity over the magnonic barrier, which is generally believed to be the relativistic limit of the DW speed. While the existing dynamic theory based on the continuum approximation fails in the atomic scale, we develop an energy conversion theory to interpret the DW dynamics beyond the relativistic limit.