Current-induced Domain Wall Motion Research Articles

Enhanced perpendicular coercivity of ultrathin perpendicularly magnetized Tb–Fe–Co films on silicon substrates using a thin Pt underlayer

Abstract We have studied the effect of an additional thin Pt underlayer on the magnetic properties of ultrathin perpendicularly magnetized TbFeCo films grown on silicon substrates. We have demonstrated that the presence of a thin Pt underlayer can enhance the perpendicular coercivity of ultrathin TbFeCo films with thickness down to 1 nm while preserving their low saturation magnetization. This can be attributed to the hybridization of Pt and Co orbitals, which induces the strong interfacial perpendicular anisotropy of Co/Pt and/or CoFe/Pt. These characteristics are potentially useful to reduce the critical current density induced magnetization switching and enhance the thermal stability of a magnetic tunnel junction and the current-induced domain wall motion in a nanowire made out of the ultrathin TbFeCo magnetic layer.

Current-induced domain wall motion in magnetic nanowires with various widths down to less than 20 nm

We experimentally and theoretically study the current-induced domain wall motion in magnetic nanowires with various widths, and discuss the issues concerning the domain wall motion in wires with reduced widths down to less than 20 nm. For Co/Ni nanowires, the threshold current density significantly increases as the width decreases below 30 nm and the domain wall motion is not observed within the studied current density range for a number of devices with the wire width of around 20 nm. The relationship between the threshold current density and wire width is reasonably reproduced by a theoretical calculation based on the adiabatic spin-transfer torque model. The micromagnetic simulation suggests that high-anisotropy materials are promising for domain-wall-motion devices with wire widths beyond 20 nm.

Domain wall motion in ultrathin Co70Fe30/Pd multilayer nanowires with perpendicular anisotropy

We report the investigation of spin polarized current induced domain wall (DW) displacement in the perpendicularly magnetized nanowires patterned on ultrathin CoFe/Pd multilayer films by anomalous Hall-effect measurement. We find that DWs can be driven to propagate in the nanowire by the threshold current density (Jth) as low as 5.2 × 1010 A/m2 under a bias field H = 115 Oe. The spin-torque efficiency ε = (1.68 ± 0.09) × 10−14 T·m2/A is derived by measuring the effective field (HJ) generated by the pulsed current as well as through the dependency of Jth on H from the DW depinning field experiment. Our result indicates that the current induced DW motion is essentially dominated by the non-adiabatic spin transfer torque effect and the non-adiabaticity factor β is estimated to be as high as 0.96 ± 0.04.

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.

Influence of temperature on current-induced domain wall motion and its Walker breakdown

The current-driven domain wall propagation along a thin ferromagnetic strip with thermal field is studied by means of micromagnetic simulations. The results show that the velocity of domain wall is almost independent of temperature until Walker breakdown happened. However the thermal field can suppress Walker breakdown and makes domain wall move faster. Further analysis indicates that the thermal field tends to keep the out-of-plane magnetic moment of the domain wall stay in high value, which can promote domain wall motion and suppress the Walker breakdown by breaking the period of domain wall transformation.

Manipulating the magnetization direction of transverse domain walls in Permalloy/Ir strips using nanosecond current pulses

Using magnetic force microscopy and micromagnetic simulations, we studied the effect of Oersted magnetic fields on transverse magnetic domain walls in Fe20Ni80/Ir bilayer nanostrips. Applying nanosecond current pulses with a current density of around 2×1012A/m2, the magnetization direction of a transverse domain wall in the Fe20Ni80-layer could be switched reversibly and reproducibly. This suggests that in bi- and trilayer strips the Oersted field stabilizes the transverse wall magnetization direction during current-induced domain wall motion and prevents domain wall transformations.

Accurate calculation of the transverse anisotropy of a magnetic domain wall in perpendicularly magnetized multilayers

Bloch domain walls are the most common type of transition between two out-of-plane magnetized domains (one magnetized upwards, one downwards) in films with perpendicular magnetic anisotropy. The rotation of the spins of such domain walls in the plane of the film requires energy, which is described by an effective anisotropy, the so-called transverse or hard axis anisotropy ${K}_{\ensuremath{\perp}}$. This anisotropy and the related D\"oring mass density of the domain wall are key parameters of the one-dimensional model to describe the motion of magnetic domain walls. In particular, the critical field strength or current density where oscillatory domain wall motion sets in (Walker breakdown) is directly proportional to ${K}_{\ensuremath{\perp}}$. So far, no general framework is available to determine ${K}_{\ensuremath{\perp}}$ from static characterizations such as magnetometry measurements. Here, we derive a universal analytical expression to calculate the transverse anisotropy constant for the important class of perpendicular magnetic multilayers. All the required input parameters of the model, such as the number of repeats, the thickness of a single magnetic layer, and the layer periodicity, as well as the effective perpendicular anisotropy, the saturation magnetization, and the static domain wall width are accessible by static sample characterizations. We apply our model to a widely used multilayer system and find that the effective transverse anisotropy constant is a factor of seven different from that when using the conventional approximations, showing the importance of using our analysis scheme. Our model is also applicable to domain walls in materials with Dzyaloshinskii-Moriya interaction (DMI). The accurate knowledge of ${K}_{\ensuremath{\perp}}$ is needed to determine other unknown parameters from measurements, such as the DMI strength or the spin polarization of the spin current in current-induced domain wall motion experiments.

Determination of magnetic domain-wall types using Dzyaloshinskii–Moriya-interaction-induced domain patterns

The Néel-type domain-wall (DW) configuration caused by the Dzyaloshinskii–Moriya interaction has attracted significant attention because of its crucial role in current-induced DW motion. Here, we propose an experimental technique to determine the DW types (Néel or Bloch) by analyzing the asymmetry in domain expansion patterns. Such asymmetry is caused by the counterbalance between the Dzyaloshinskii–Moriya and Zeeman interactions, which results in the elongation of the domains either longitudinal (for Néel-type DWs) or transverse (for Bloch-type DWs) to the in-plane magnetic field. Therefore, the DW types can be determined simply by examining the elongation axis. The present technique is applicable even to a single image of the domain expansion pattern and provides a rapid determination of the DW configuration for exploring high-efficiency materials for current-induced DW motion in device applications.

Influence of classical electromagnetic effects on current-induced domain wall motion in a perpendicularly magnetized nanowire

The influence of classical electromagnetic effects such as applied current field and eddy current field on current-induced domain wall motion in a perpendicularly magnetized nanowire was investigated using a micromagnetic framework. The method used to calculate these fields was derived from Maxwell equation in order to estimate the effect of an attached non-magnetic (NM) layer. We demonstrate an applied current field and an eddy current field in a perpendicularly magnetized nanowire with and without a non-magnetic layer. Without a NM layer, the eddy current field causes a small correction in the Gilbert damping with a wall angle dependence. Adding a NM layer enhances the effective Gilbert damping, especially, in terms of wall rotation. Furthermore, an applied current field strongly affects the current-induced domain wall motion, and significantly changes the critical current density and the Walker breakdown.

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.

In-plane field-driven crossover in the spin-torque mechanism acting on magnetic domain walls in Co/Ni

We investigate the in-plane field dependence of current-induced magnetic domain wall motion in asymmetric Co/Ni nanowires. The application of an in-plane field changed the direction of domain wall motion. In a systematic study, by varying the magnetic layer thickness, we show that the in-plane field induces a crossover from spin Hall torque to adiabatic spin-transfer torque for the magnetic domain wall motion. Furthermore, the dependence of the threshold current density on the in-plane longitudinal field provides a way for determining the strength of the Dzyaloshinskii-Moriya interaction.

Temperature dependence of current-induced magnetic domain wall motion in an asymmetric Co/Ni nanowire

We have investigated the temperature dependence of current-induced magnetic domain wall (DW) motion in a Co/Ni nanowire, where the spin Hall torque is responsible for the DW motion. The threshold current density (Jth) for DW motion is found to be dependent on the temperature T and the depinning magnetic field Hdep, which is different from the symmetric Co/Ni system where Jth is insensitive to T and Hdep. This result indicates that an extrinsic pinning governs the DW motion when it is driven by the spin Hall torque. Our work therefore suggests that reducing the extrinsic pinning is a key for achieving a low-power-consumption device.

Role of B diffusion in the interfacial Dzyaloshinskii-Moriya interaction inTa/Co20Fe60B20/MgOnanowires

We report current-induced domain wall motion (CIDWM) in Ta\Co20Fe60B20\MgO nanowires. Domain walls are observed to move against the electron flow when no magnetic field is applied, while a field along the nanowires strongly affects the domain wall motion direction and velocity. A symmetric effect is observed for up-down and down-up domain walls. This indicates the presence of right-handed domain walls, due to a Dzyaloshinskii-Moriya interaction (DMI) with a DMI coefficient D=+0.06 mJ/m2. The positive DMI coefficient is interpreted to be a consequence of boron diffusion into the tantalum buffer layer during annealing. In a Pt\Co68Fe22B10\MgO nanowire CIDWM along the electron flow was observed, corroborating this interpretation. The experimental results are compared to 1D-model simulations including the effects of pinning. This advanced modelling allows us to reproduce the experiment outcomes and reliably extract a spin-Hall angle {\theta}SH=-0.11 for Ta in the nanowires, showing the importance of an analysis that goes beyond the currently used model for perfect nanowires.

Reversal-mechanism of perpendicular switching induced by an in-plane current

We propose a magnetization reversal model to explain the perpendicular switching of a single ferromagnetic layer induced by an in-plane current. Contrary to previously proposed reversal mechanisms that such magnetic switching is directly from the Rashba or spin Hall effects, we suggest that this type of switching arises from the current-induced chirality dependent domain wall motion. By measuring the field dependent switching behaviors, we show that such switching can also be achieved between any two multidomain states, and all of these switching behaviors can be well explained by this model. This model indicates that the spin Hall angle in such structures may be overestimated and also predicts similar switching behaviors in other ferromagnetic structures with chiral domain walls or skyrmions.

Spin Hall torque magnetometry of Dzyaloshinskii domain walls

Current-induced domain wall motion in the presence of the Dzyaloshinskii-Moriya interaction (DMI) is experimentally and theoretically investigated in heavy-metal/ferromagnet bilayers. The angular dependence of the current-induced torque and the magnetization structure of Dzyaloshinskii domain walls are described and quantified simultaneously in the presence of in-plane fields. We show that the DMI strength depends strongly on the heavy metal, varying by a factor of 20 between Ta and Pa, and that strong DMI leads to wall distortions not seen in conventional materials. These findings provide essential insights for understanding and exploiting chiral magnetism for emerging spintronics applications.

Field- and Current-Induced Domain Wall Motion in Tb/Co Multilayers in the Presence of Spin-Orbit Coupling-Induced Torques

We have studied the field- and current-induced domain wall (DW) motion in perpendicularly magnetized Tb/Co multilayer wires with asymmetric interface structures. We found that both up/down and down/up walls of a domain move at the same velocity. In the current-induced case, the DWs were moved along the current flow, not electron flow. This can be attributed to spin-orbit torques induced by structural inversion asymmetry (SIA). In addition, the DW velocity was changed by inserting ultrathin Pt interlayers into the wire stack. These results suggest that the SIA of the wire is tunable by changing layered structures.

Magnetic Properties of Anti-Perovskite Type Transition Metal Ferromagnetic Nitrides

Ferromagnetic anti-perovskite type nitrides (Fig.1) have special features suitable for application to spintronics devices. We have succeeded in growing Co x Fe4 - x N and Co x Mn4 - x N films epitaxially on SrTiO3(001) substrates using molecular beam epitaxy for the first time, and characterized their magnetic properties.1-3 Co3FeN is theoretically predicted to have the large negative spin-polarization of density of states at the Fermi level (P D),4 when Fe atoms are located at the corner (I) sites and Co atoms at the face center (II) sites of the cubic unit cell. We performed x-ray magnetic circular dichroism (XMCD) measurements for the Co3FeN epitaxial films, and evaluated the orbital and spin magnetic moments (m spin) of Co and Fe atoms. Clear XMCD spectra were observed at Fe and Co L 2,3 absorption edges for the Co3FeN films. The m spin per Fe and Co atoms in the Co3FeN film was evaluated to be 2.08 and 1.52 μ B, respectively.3 In the ordered large spin-polarized Co3FeN phase, the m spin per Fe and Co atoms is theoretically predicted to have 3.09 and 1.41 μ B.3 The experimentally obtained m spin of Fe atoms, 2.08 μ B, is close to 2.29 μ B, the site averaged value in Fe4N.5 This result shows that the Fe atoms in the Co3FeN film are located at both I and II sites. Mössbauer measurements were also performed in the Co3FeN film and indicated the consistent results. Growth conditions should be optimized so that Fe and Co atoms are located only at I and II sites, respectively, in order to achieve large spin-polarized Co3FeN. Perpendicular magnetic anisotropy (PMA) was reported in the Mn4N films.6 In addition, large positive P D is predicted by the first principle calculation.7 Co x Mn4 - x N is predicted to have PMA and small saturation magnetization (M S), which is good for application to current-induced magnetic domain wall motion devices. PMA was observed in the Co x Mn4 - x N(x≤0.8) epitaxial films from the magnetization curves measured by a superconductive quantum interface device magnetometer. The M S of the Mn4N and Co0.8Mn3.2N films was 145 and 15 emu/cc at 300 K, respectively. The effective PMA energy of the Mn4N films was about 2.2 Merg/cc.2 The c/a ratio, where c and a are lattice constants along perpendicular and in-plane direction of Mn4N films, respectively, was approximately 0.99.2 We think that PMA in the Mn4N films is induced by the in-plane tensile strain. Distinct XMCD spectra at Mn and Co L 2,3 absorption edges were observed for the Co0.8Mn3.2N film. Shape of the XMCD signal at Mn L 2,3 edges was broader than that of Mn4N. In contrast, XMCD signal at Co L 2,3 edges was sharp. This indicates that the Mn atoms are more likely to locate at II sites, and the Co atoms at I sites instead. It is predicted that the Mn atoms at II sites have the itinerant feature due to band hybridization with neighboring N atoms, while the Co atoms at I sites the localized feature. The element specific magnetization curves measured at the Mn and Co L 3 edges, under the applied magnetic field perpendicular to the films, showed the clear open hysteresis, indicating that PMA surely appeared in the Mn4N and Co0.8Mn3.2N films. 1 T. Sanai, K. Ito, T. Suemasu et al., J. Cryst. Growth 357, 53 (2011). 2 Y. Yasutomi, K. Ito, T. Suemasu et al., J. Appl. Phys. 115, 17A935 (2014). 3 K. Ito, T. Suemasu et al., Appl. Phys. Lett. 103, 232403 (2013). 4 Y. Takahashi et al., J. Magn. Magn. Mater. 323, 2941 (2011). 5 A. Sakuma, J. Magn. Magn. Mater. 102, 127 (1991). 6M. Tsunoda and K. Kabara, presented at ICAUMS2012, 2pPS-47. 7 M. S. Miao et al., Phys. Rev. B 72, 033204 (2005).Fig.1 Lattice structure of anti-perovskite type nitrides.

Modeling on current-induced multiple domain-wall motion in permalloy nanowires

We performed micromagnetic modeling on moving multiple data bits (4 to 64 bits) in permalloy nanowires and investigated the important factors affecting domain-wall movement. It was observed that current-driven multiple transverse domain walls motion (MTDWM) in cylindrical nanowires with a diameter below 20 nm is characteristic of an exceptional massless mobility, which is independent of the number of bits. Besides, two phenomena are noteworthy: First, the linear velocity and the domain-wall width are closely related to the diameter of a wire. Second, the domain walls collapse when the wire diameter is more than 30 nm.

Domain wall motions in perpendicularly magnetized CoFe/Pd multilayer nanowire

Current-induced domain wall (DW) motion is investigated in a 600 nm wide nanowire using multilayer film with a structure of Ta(5 nm)/Pd(5 nm)/[CoFe(0.4 nm)/Pd(1.2 nm)]15/Ta(5 nm) in terms of anomalous Hall effect measurements. It is found that motion of DWs can be driven by a current density as low as 1.44 × 1011 Am−2. The effect of the Oersted field (HOe) and spin transfer torque field (HST), which are considered as effective fields for DW motion, is quantitatively separated from the dependence of depinning fields on the current. The results show that the motion of the walls is essentially dominated by the non-adiabaticity with a high non-adiabatic factor β of up to 0.4.

Current-induced domain wall motion in nanostrip–nanobars system

Transverse domain wall motion in strips with a series of nanobars placed perpendicularly on either side has been investigated systematically by micromagnetic simulation. Domain wall motion in strips is driven by electric current owing to the spin transfer torque effect. In a strip in contact with nanobars, a domain wall can withstand a high current density (above A/cm2) to maintain structural stability, but the domain wall velocity cannot be increased markedly. Domain wall motion in strips not in contact with nanobars can reach a higher speed under a relatively low current density. To examine the effects of nanobars on domain wall motion, the DW depinning energy and the Walker limit uw at different parameters are discussed systematically.