Role of B diffusion in the interfacial Dzyaloshinskii-Moriya interaction in $\mathrm{Ta}/{\mathrm{Co}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}/\mathrm{MgO}$ nanowires

We report on current-induced domain wall motion in $\mathrm{Ta}/\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}/\mathrm{MgO}$ 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 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 $D=+0.06\mathrm{mJ}/{\mathrm{m}}^2$. The positive DMI coefficient is interpreted to be a consequence of B diffusion into the Ta buffer layer during annealing, which was observed by chemical depth profiling measurements. The experimental results are compared to one-dimensional model simulations including the effects of pinning. This modeling allows us to reproduce the experimental outcomes and reliably extract a spin-Hall angle ${\theta }_{\mathrm{SH}}=\text{–}0.11$ for Ta in the nanowires, showing the importance of an analysis that goes beyond the model for perfect nanowires.

Figure 1

(a) Schematic of the experimental setup for current pulse injection, including an SEM micrograph of the sample used during the experiment. The inset shows the shape of one of the pulses applied to the device, measured with the oscilloscope (across the 50-Ω internal resistance). (b) Differential Kerr microscopy image of nucleated magnetic domains by Oersted field in initialized nanowires. The magnetization in the reversed domains is pointing upwards ($+z$, black areas). (c) Differential Kerr microscopy image of the same wires in (b), after domain walls motion due to injection of a burst of 50 current pulses ($\Delta t=25\mathrm{ns},j_a=\text{–}2.75\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2,j_a>0$ in the $+x$ direction) through the nanowires. The direction of the applied conventional current $j_a$ and of the electron current $j_e$ is indicated by the red and blue arrow, respectively.

Figure 2

(a) Average velocity of the DW as a function of the current density injected in the magnetic wires, for different durations of the current pulse. The DW velocity increases with the pulse duration, due to the fact that the 5-ns rise and fall times of the injected pulses have less influence on the measured domain wall velocity during longer pulses. The DW moves with the conventional current $j_a$ (against the electron flow $j_e$). The average velocities and the error bars (standard deviations) are calculated from 30 different DW motions, at each current density. (b) Average velocity of the DW as a function of $j_a$, free of the rise- and fall-time influence.

Figure 3

Effect of a longitudinal magnetic field on the current-induced DW motion. (a) Differential Kerr microscopy image of nucleated magnetic domains in presaturated nanowires. The magnetization in the reversed domains points in the $+z$ direction (black areas). The green lines indicate the position of the DWs. The red arrows describe the DWs magnetization configuration. (b) Differential Kerr microscopy image of the domain walls moved due to current pulse injection $\left(j_a=+3.6\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2\right)$, when a longitudinal field is applied $\left({\mu }_0{H}_x=\text{–}35\mathrm{mT}\right)$. The dashed green lines indicate the starting position of the DWs, while the solid orange lines indicate their final position. The blue arrows show the DW motion. Down-up (DU, ↓↑) and up-down (UD, ↑↓) DWs move in opposite directions. (c) Average velocity of ↓↑- and (d) ↑↓-DWs as a function of the longitudinal field $\left({\mu }_0{H}_x\right)$, for two different current densities. Solid symbols refer to $j_a=3.6\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$, while empty symbols refer to $j_a=2.8\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$. Squares refer to positive $j_a$, while triangles refer to negative $j_a$. The solid (dashed) lines are the 1D-model fitting curves for $j_a=\pm 3.6\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2 \left(j_a=\pm 2.8\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2\right)$ (see text for details). (e) Average velocity of ↓↑ (empty symbols) and ↑↓ (solid symbols) -domain walls as a function of ${\mu }_0{H}_x$, for a current density of $j_a=+3.6\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$ (squares), and $j_a=\text{–}3.6\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$ (triangles). Lines represent the 1D-model fitting curves.

Figure 4

(a) TEM cross-section image of the $\mathrm{Ta}(5)/\mathrm{C}\mathrm{o}\mathrm{F}\mathrm{e}\mathrm{B}(1)/\mathrm{MgO}(2)/\mathrm{Ta}(5)$ stack showing that $\mathrm{MgO}$ and $\mathrm{CoFeB}$ crystallize in the cubic phase after annealing. Marks indicating the different layers are superimposed as a guide to the eye. Layer thicknesses are reported in nanometers. (b) SIMS depth profiles of as-deposited (ad) and annealed (ann) ($300{}^{\circ }\mathrm{C}$, 2h) structures. Signals related to B (dots), MgO (squares), Fe (up-triangles) and Co (down-triangles) are shown. Following the B profile, B diffusion from the CoFeB layer towards the Ta layer (and partially the MgO layer) is evident. For the sake of clarity profiles are aligned at the CoFeB/Ta interface. Secondary ions are collected in negative mode, and the measurement parameters are as reported in [38].

Figure 5

XMCD-PEEM images of current-induced DW motion in $\mathrm{Pt}/\mathrm{C}{\mathrm{o}}_{68}\mathrm{F}{\mathrm{e}}_{22}{\mathrm{B}}_{10}/\mathrm{MgO}$ nanowires. (a) Image of the full device consisting of a magnetic nanowire with a magnetic pad at one side, connected to two gold pads at its ends (yellow areas) used for current pulse injection. The black line indicates the position of the DW in the nanowire. The contrast in the wire is not completely homogeneous due to some residual resist from the patterning process. In (b–d) the current-induced DW motion is shown. The dashed (solid) line indicates the initial (final) position of the DW. The DW moves with the electron flow, as indicated by the arrows.