Growth, strain, and spin-orbit torques in epitaxial Ni-Mn-Sb films sputtered on GaAs

We report current-induced spin torques in epitaxial NiMnSb films on a commercially available epiready GaAs substrate. The NiMnSb was grown by cosputtering from three targets using optimized parameters. The films were processed into microscale bars to perform current-induced spin-torque measurements. Magnetic dynamics were excited by microwave currents, and electric voltages along the bars were measured to analyze the symmetry of the current-induced torques. We found that the extracted symmetry of the spin torques matches those expected from spin-orbit interaction in a tetragonally distorted half-Heusler crystal. Both fieldlike and dampinglike torques are observed in all the samples characterized, and the efficiency of the current-induced torques is comparable to that of ferromagnetic metal/heavy-metal bilayers.

Figure 1

(a) and (b) XRD profiles for a GaAs(001) substrate/NiMnSb (NMS) 17 nm/MgO 3 nm film. (a) Out-of-plane and (b) in-plane profiles along various scattering vectors $q$ of GaAs. The inset of (b) shows the NMS222 $\varphi$ scan profile (pole figure). (c) In-plane $M-H$ curves along GaAs$\left[1\overline{1}0\right]$, [010], [110], and [100]. (d) An AFM image of the 17 nm thick NiMnSb film used in the present study.

Figure 2

(a) Schematic of the measurement setup using this study with circuit configuration. (b) Typical FMR voltages measured at a microwave excitation frequency of 7 GHz with the external magnetic field applied along $\theta$ = ${60}^{\circ }$ in the [110] device. Dots are experimental data points, and curves are produced by best-fit parameters using Eq. (2) and voltage offsets that are constant with field and linear to field.

Figure 3

(a) Two-dimensional color plot of FMR voltages $V_{\text{dc}}$ measured from an in-plane rotational scan of external magnetic field as a function of applied magnetic field and angle. We use the microwave excitation frequency of 7 GHz. (b) FMR field $H_{\text{res}}$ as a function of the in-plane crystallographic angle. We plot results from fitting FMR scans.

Figure 4

(a) and (b) Angle dependence of the symmetric (open red circles) and antisymmetric (open black circles) components of the resonance measured for bars patterned along the (a) $\left[1\overline{1}0\right]$ and (b) [110] crystal directions. The current density in the bar is ${10}^{10}\mathrm{A}$/${\text{m}}^2$ and measured at a microwave excitation frequency of 7 GHz. (c) and (d) Plot of the magnitude and direction of the effective fields for (c) Dresselhaus and Rashba types of symmetry and (d) the sum of the two components. (e) $\theta$ dependence of the effective $h_z$ field. (f) Plot of magnitude of the voltage for the symmetric and antisymmetric components as a function of injected microwave powers.