Current-driven Domain Wall Motion Research Articles

Doppler effect in a solid medium: Spin wave emission by a precessing domain wall drifting in spin current

The Doppler effect is a fundamental physical phenomenon observed for waves propagating in vacuum or various media, commonly gaseous or liquid. Here, we report on the occurrence of a Doppler effect in a solid medium. Instead of a real object, a topological soliton, i.e., a magnetic domain wall (DW) traveling in a current-carrying ferromagnetic nanowire, plays the role of the moving wave source. The Larmor precession of the DW in an external field stimulates emission of monochromatic spin waves (SWs) during its motion, which show a significant Doppler effect, comparable to the acoustic one of a train whistle. This process involves two prominent spin-transfer-torque effects simultaneously, the current-driven DW motion and the current-induced SW Doppler shift. The latter gives rise to an interesting feature, i.e., the observed SW Doppler effect appears resulting from a stationary source and a moving observer, contrary to the laboratory frame.

Large exchange-dominated domain wall velocities in antiferromagnetically coupled nanowires

Magnetic nanowires supporting field- and current-driven domain wall motion are envisioned for methods of information storage and processing. A major obstacle for their practical use is the domain-wall velocity, which is traditionally limited for low fields and currents due to the Walker breakdown occurring when the driving component reaches a critical threshold value. We show through numerical and analytical modeling that the Walker breakdown limit can be extended or completely eliminated in antiferromagnetically coupled magnetic nanowires. These coupled nanowires allow for large domain-wall velocities driven by field and/or current as compared to conventional nanowires.

Creep turns linear in narrow ferromagnetic nanostrips.

The motion of domain walls in magnetic materials is a typical example of a creep process, usually characterised by a stretched exponential velocity-force relation. By performing large-scale micromagnetic simulations, and analyzing an extended 1D model which takes the effects of finite temperatures and material defects into account, we show that this creep scaling law breaks down in sufficiently narrow ferromagnetic strips. Our analysis of current-driven transverse domain wall motion in disordered Permalloy nanostrips reveals instead a creep regime with a linear dependence of the domain wall velocity on the applied field or current density. This originates from the essentially point-like nature of domain walls moving in narrow, line- like disordered nanostrips. An analogous linear relation is found also by analyzing existing experimental data on field-driven domain wall motion in perpendicularly magnetised media.

Influences of material defects and temperature on current-driven domain wall mobility

Current-induced domain wall motion, which has potential application in the next-generation data storage and logic device, has attracted much interest in recent years. However, how the material defect and its joule heat influence current-driven domain wall motion in magnetic nanostripe is still unclear. This paper is to deal with these issues by using the Landau-Lifshitz-Gilbert spin dynamics. The results show that the material defect can pin domain wall motion and this pinning effect strongly depends on the defect concentration, location and shape. The pinning effect induced by the defect on domain wall motion results in the increase of threshold current, and the domain wall moves steadily and continuously. Specifically, the probability for domain wall motion induced by pinning effect is nonlinearly increasing with the increase of defect concentration. Namely, the increasing of the pinning ability with the increase of the defect concentration becomes fades away. Initially, when the defect is near to domain wall, the pinning ability is obvious. However, the pinning ability is not linearly increasing with the decrease of the initial distance between the defect and the domain wall. The results also show that the single defect is larger, the probability for domain wall motion induced by defect pining is bigger. Moreover, the material defect can suppress the domain wall trending toward breakdown and make domain wall move faster, but the suppressing ability is not obviously increasing with the increase of the defect concentration. On the other hand, the temperature field can remove the pinning phenomenon, which will result in the threshold current decrease. The decrease of the threshold current is of benefit to the working of the data storage and logic device. Also the temperature field can suppress the domain wall trending toward breakdown, but the suppressing ability is less than that of the defect. In addition, the Joule heat around defects can obviously eliminate the pinning effect of the defects, so the pinning effect for a few defects on current-induced domain wall motion can be ignored. Further analysis indicates that these effects are due to the change of the out-of-plane magnetization of the domain wall induced by the material defects and the temperature field, because the velocity of the domain wall motion induced by the applied current greatly depends on the out-of-plane magnetization of the domain wall.

Current-Driven Domain Wall Motion: Velocity, Current and Phase Transition

The relation between domain wall motion and intensity of driven current is examined in a phenomenological theory where the kinetic energy is expanded as a series of polynomial function of current density just as the Landau phase transition theory. The dependency of velocity on current density is square root which degenerates into linear if the current is much higher than the critical value. The theory result is consistent with several previous experiments and also can explain the change of critical current in the presence of temperature. The role of temperature playing in the dynamics of domain wall motion is also discussed. The phase transition theory in terms of current density is employed to explain the critical behavior of domain wall motion.

Detection of ferromagnetic domain wall pinning and depinning with a semiconductor device

We demonstrate the detection of a ferromagnetic domain wall using a nanoscale Hall cross. A narrow permalloy wire is defined lithographically on top of a Hall cross fabricated from an InAs quantum well. The width of the Hall cross (500 nm–1 μm) is similar to the width of the ferromagnetic wire (200–500 nm), and a geometric pinning site is fabricated in the ferromagnetic wire to trap a domain wall within the area of the Hall cross. The devices provide a signal that is often the same order of magnitude as the offset Hall voltage when a domain wall is located above the Hall cross, and may be useful for memory applications. Different geometries for the Hall cross and ferromagnetic wire are tested, and radiofrequency pulses are sent into the wire to demonstrate current driven domain wall motion. Further changes to the Hall bar geometry with respect to the wire geometry are investigated by numerical computation. A large gain in signal is seen for Hall bars only slightly wider than the ferromagnetic wires as compared to those twice as wide, as well as a larger sensitivity to the exact position of the domain wall with respect to the center of the Hall cross.

Detection of current-driven magnetic domains in [Co/Pd] nanowire by tunneling magnetoresistive sensor

Current-driven magnetic domain walls in magnetic nanowires have attracted a great deal of interest in terms of both physical studies and engineering applications. The anomalous Hall effect measurement is widely used for detecting the magnetization direction of current-driven magnetic domains in a magnetic nanowire. However, the problem with this measurement is that the detection point for current-driven domain wall motion is fixed at only the installed sensing wire across the specimen nanowire. A potential solution is the magnetic domain scope method, whereby the distribution of the magnetic flux leaking from the specimen can be analyzed directly by contact-scanning a tunneling magnetoresistive field sensor on a sample. In this study, we fabricated specimen nanowires consisting of [Co (0.3)/Pd (1.2)]21/Ta(3) films (units in nm) with perpendicular magnetic anisotropy on Si substrates. A tunneling magnetoresistive sensor was placed on the nanowire surface and a predetermined current pulse was applied. Real-time detection of the current-driven magnetic domain motion was successful in that the resistance of the tunneling magnetoresistive sensor was changed with the magnetization direction beneath the sensor. This demonstrates that magnetic domain detection using a tunneling magnetoresistive sensor is effective for the direct analysis of micro magnetic domain motion.

Current-driven vortex domain wall motion in wire-tube nanostructures

We have investigated the current-driven domain wall motion in nanostructures comprised of a pair of nanotube and nanowire segments. Under certain values of external magnetic fields, it is possible to pin a vortex domain wall in the transition zone between the wire and tube segments. We explored the behavior of this domain wall under the action of an electron flow applied in the opposite direction to the magnetic field. Thus, for a fixed magnetic field, it is possible to release a domain wall pinned simply by increasing the intensity of the current density, or conversely, for a fixed current density, it is possible to release the domain wall simply decreasing the magnetic external field. When the domain wall remains pinned due to the competition between the current density and the magnetic external field, it exhibits a oscillation frequency close to 8 GHz. The amplitude of the oscillations increases with the current density and decreases over time. On the other hand, when the domain wall is released and propagated through the tube segment, this shows the standard separation between a steady and a precessional regime. The ability to pin and release a domain wall by varying the geometric parameters, the current density, or the magnetic field transforms these wire-tube nanostructures in an interesting alternative as an on/off switch nano-transistor.

Fabrication of L-shaped Fe4N ferromagnetic narrow wires and position control of magnetic domain wall with magnetic field

We grow a 15-nm-thick ferromagnetic Fe4N epitaxial film on a SrTiO3(001) substrate by molecular beam epitaxy, and process it into approximately 0.5-µm-wide and 24-µm-long L-shaped ferromagnetic narrow wires by electron-beam lithography and Cl2 reactive ion etching. Their longitudinal directions are set in parallel to the magnetic easy axes, Fe4N[100] and [010]. With applying external magnetic field in the direction parallel to Fe4N[100] or [010], the position of domain wall is controlled either on the upper side or lower side of the corner. This experiment is the preliminary step toward current-driven domain wall motion in Fe4N having a negative spin polarization.

Field and Current Driven Magnetic Domain Wall Motion in Disordered A1-FePt Nanowires

Arc-shaped magnetic FePt nanowires (10nm thick, 375nm wide) displaying in-plane anisotropy were fabricated. A vortex domain wall was formed in the middle of the arc after in-plane saturation in a field perpendicular to the wire. Translational domain wall propagation was observed at a field of 40 Oe applied parallel to the wire, while complete propagation to the end of the arc occurred at 100 Oe. Domain wall displacement at low velocity was achieved by current pulses of 2.5 1011 A m2 at zero applied magnetic field, comparable to the zero-field threshold current density of Permalloy. A field-assisted spin transfer torque drove the domain wall 1.8m to the end of the arc.

Current-driven domain wall motion enhanced by the microwave field

The magnetic domain wall (DW) motion driven by a spin-polarized current opens a new concept for memory and logic devices. However, the critical current density required to overcome the intrinsic and/or extrinsic pinning of DW remains too large for practical applications. Here, we show, by using micromagnetic simulations and analytical approaches, that the application of a microwave field offers an effective solution to this problem. When a transverse microwave field is applied, the adiabatic spin-transfer torque (STT) alone can sustain a steady-state DW motion without the sign of Walker breakdown, meaning that the intrinsic pinning disappears. The extrinsic pinning can also be effectively reduced. Moreover, the DW velocity is increased greatly for the microwave-assisted DW motion. This provides a new way to manipulate the DW motion at low current densities.

Current-driven domain wall mobility in polycrystalline Permalloy nanowires: A numerical study

A complete understanding of domain wall motion in magnetic nanowires is required to enable future nanowire based spintronics devices to work reliably. The production process dictates that the samples are polycrystalline. In this contribution, we present a method to investigate the effects of material grains on domain wall motion using the GPU-based micromagnetic software package MuMax3. We use this method to study current-driven vortex domain wall motion in polycrystalline Permalloy nanowires and find that the influence of material grains is fourfold: an extrinsic pinning at low current densities, an increasing effective damping with disorder strength, shifts in the Walker breakdown current density, and the possibility of the vortex core to switch polarity at grain boundaries.

Nonreciprocal spin-wave channeling along textures driven by the Dzyaloshinskii-Moriya interaction

Ultrathin metallic ferromagnets on substrates with strong spin-orbit coupling can exhibit induced chiral interactions of the Dzyaloshinskii-Moriya (DM) form. For systems with perpendicular anisotropy, the presence of DM interactions has important consequences for current-driven domain-wall motion and underpins possible spintronic applications involving skyrmions. We show theoretically how spin textures driven by the DM interaction allow nonreciprocal channeling of spin waves, leading to measurable features in magnetic wires, dots, and domain walls. Our results provide methods for detecting induced DM interactions in metallic multilayers and controlling spin wave propagation in ultrathin nanostructures.

Magnetic-texture-controlled transverse spin injection

We propose an effect whereby an electric current along the interface between a ferromagnetic and normal metal leads to injection of pure spin current into the normal metal, if the magnetization direction in the ferromagnet varies spatially along the direction of current. For the specific example of a ferromagnetic spin spiral, we compute the inverse spin Hall effect voltage this spin current gives rise to when injected into a Pt layer. Furthermore, we show that this pure spin current leads to modification of the parameters that govern spin transfer and current-driven domain-wall motion, which can be used to tune the latter in layered magnetic systems. This effect in principle enables in situ control over the location, magnitude, and polarization of spin-current injection in devices.

Current-driven domain wall motion along high perpendicular anisotropy multilayers: The role of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction

The current-induced domain wall motion along a thin cobalt ferromagnetic strip sandwiched in a multilayer (Pt/Co/AlO) is theoretically studied with emphasis on the roles of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction. The results point out that these ingredients, originated from the spin-orbit coupling, are consistent with recent experimental observations in three different scenarios. With the aim of clarifying which is the most plausible the influence of in-plane longitudinal and transversal fields is evaluated.

Observation of current-driven oscillatory domain wall motion in Ni80Fe20/Co bilayer nanowire

Direct observation of current-driven oscillatory domain wall motion above the Walker breakdown by x-ray magnetic circular dichroism in photoemission electron microscopy is reported in Ni80Fe20/Co nanowire, showing micrometer-scale displacement at ∼13 MHz. We identify two key factors that enhance the oscillatory motion: (i) increase of the hard-axis magnetic anisotropy field value |H⊥| and (ii) increase of the ratio between non-adiabatic spin-transfer parameter to the Gilbert damping coefficient, β/α, which is required to be larger than 1. These findings point to an important route to tune the long-scale oscillatory domain wall motion using appropriate geometry and materials.

Domain Wall Dynamics in Asymmetric Stacks: The Roles of Rashba Field and the Spin Hall Effect

The influence of spin-orbit torques, either Rashba and/or spin Hall effects, on the current-driven domain wall motion along high perpendicular magnetocrystalline anisotropy stacks, is analyzed by means of micromagnetic simulations and the one-dimensional model. The study indicates that the domain wall motion can be either in the direction of the current or opposing to it depending on the strength of the spin Hall effect in the presence of the Rashba spin-orbit torque, and therefore, it points out that both spin-orbit torques could be essential ingredients to describe recent experiments.

The influence of the spin-orbit torques on the current-driven domain wall motion

The current-induced domain wall motion along a ferromagnetic strip with high perpendicular magnetocristalline anisotropy sandwiched in an multilayer stack is theoretically studied, by means of micromagnetic simulations and the one-dimensional model, with emphasis on the roles of the Rashba spin-orbit coupling and the spin Hall effect. The results point out that in the presence of a strong Rashba field the domain wall motion can be either in the direction of the current or opposing to it depending on the amplitude of the spin Hall effect. The predictions are in agreement with the experiments only in a reduced range of positive spin Hall angles under a strong Rashba torque.

Assessment of Rashba Field Effects in Ultrathin Pt/Co/GdOx Submicrometer Strips

The influence of injected current on magnetization dynamics is investigated in submicrometer-wide strips of out-of-plane magnetized Pt/Co/GdOx. This ultrathin film structure is similar to Pt/Co/AlOx exhibiting highly efficient current-driven domain wall (DW) motion, which has recently been attributed in part to the Rashba effect. In Pt/Co/GdOx, the reduction of the out-of-plane magnetization with increasing injected current is largely due to Joule heating, and the effective transverse Rashba field that tilts the magnetization is estimated to be at most ~0.3 T/(1012 A/m2). Measurements of high-speed DW motion reveal no evidence of a strong Rashba field to suppress DW precession. These findings indicate that the Rashba effect plays a limited or negligible role in current-driven DW dynamics, and alternative torques are required to explain the high efficiency of DW motion in ultrathin heavy-metal/ferromagnet/oxide systems.

Chiral spin torque at magnetic domain walls

Spin-polarized currents provide a powerful means of manipulating the magnetization of nanodevices, and give rise to spin transfer torques that can drive magnetic domain walls along nanowires. In ultrathin magnetic wires, domain walls are found to move in the opposite direction to that expected from bulk spin transfer torques, and also at much higher speeds. Here we show that this is due to two intertwined phenomena, both derived from spin-orbit interactions. By measuring the influence of magnetic fields on current-driven domain-wall motion in perpendicularly magnetized Co/Ni/Co trilayers, we find an internal effective magnetic field acting on each domain wall, the direction of which alternates between successive domain walls. This chiral effective field arises from a Dzyaloshinskii-Moriya interaction at the Co/Pt interfaces and, in concert with spin Hall currents, drives the domain walls in lock-step along the nanowire. Elucidating the mechanism for the manipulation of domain walls in ultrathin magnetic films will enable the development of new families of spintronic devices.