Robust spin-orbit torque and spin-galvanic effect at the Fe/GaAs (001) interface at room temperature

Abstract

Interfacial spin-orbit torques (SOTs) enable the manipulation of the magnetization through in-plane charge currents, which has drawn increasing attention for spintronic applications. The search for material systems providing efficient SOTs, has been focused on polycrystalline ferromagnetic metal/non-magnetic metal bilayers. In these systems, currents flowing in the non-magnetic layer generate-due to strong spin-orbit interaction-spin currents via the spin Hall effect and induce a torque at the interface to the ferromagnet. Here we report the observation of robust SOT occuring at a single crystalline Fe/GaAs (001) interface at room temperature. We find that the magnitude of the interfacial SOT, caused by the reduced symmetry at the interface, is comparably strong as in ferromagnetic metal/non-magnetic metal systems. The large spin-orbit fields at the interface also enable spin-to-charge current conversion at the interface, known as spin-galvanic effect. The results suggest that single crystalline Fe/GaAs interfaces may enable efficient electrical magnetization manipulation.

Figure 1. Spin-orbit coupling at the Fe/GaAs interface.

Schematic of Rashba (a) Dresselhaus (b) spin-orbit fields (SOFs) for different crystallographic orientations. Red arrows in a,b denote the direction of spin accumulation induced by a current flow j_x. (c) Atomic structure of the Fe/GaAs (001) spin–orbit interface. Zincblende GaAs exhibits bulk inversion asymmetry (BIA) with D_2d symmetry and adding a single crystalline Fe on top of GaAs further lowers the D_2d symmetry to C_2v. The C_2v symmetry results in the Rashba and Dresselhaus SOFs at the Fe/GaAs (001) interface.

Figure 2. Spin-orbit ferromagnetic resonance measurements.

(a) Depiction of sample structure and experimental set-up. A microwave current passes through the Bias Tee and to the sample to drive the magnetization dynamics in the Fe film. A rectified dc voltage is detected across the Fe stripe. (b) Definition of magnetic-field angle ϕ_H and magnetization angle ϕ_M. (c) Typical spectrum of the dc voltage V obtained at a magnetic-field angle of ϕ_H=45°, microwaves frequency of 12 GHz, and temperature of 300 K, where the offset voltage V_offset is subtracted. (d) Dependence of V_a-sym and V_sym on the magnetization angle ϕ_M for a [100]-orientated device. The solid lines are fits to equation (1).

Figure 3. In-plane spin-orbit fields.

(a) Experimentally determined magnitude and direction of the in-plane spin-orbit fields, which are normalized by a unit current density of 10¹¹ Am⁻². (b) Current density dependence of the magnitudes of μ₀h_R and μ₀h_D obtained from a [010]-orientated device. From the slope of μ₀h_R and μ₀h_D, the ratio of α and β, (α/β)_in-plane, is determined to ∼2.0.

Figure 4. Out-of-plane spin-orbit fields.

(a) Magnetization angle ϕ_M dependences of the out-of-plane spin-orbit field μ₀h^[001] for [100] and [010]-orientated devices; the solid lines are fits by equation (2), from which (α/β)_out-of-plane is obtained (see Supplementary Note 8). The fitting coefficients of sinϕ_M and cosϕ_M are shown in (b). (c) ϕ_M dependences of μ₀h_z for [110] and [10] orientated devices. (d) Fitting coefficients of sinϕ_M and cosϕ_M. The error bar in b,d is the s.d. obtained from the fit. All the fields are normalized by a unit current density of 10¹¹ Am⁻².

Figure 5. Spin-galvanic effect in Fe/GaAs.

(a) Schematic of the device for spin pumping and spin-galvanic effect SGE. The [110]-orientated Fe/GaAs stripe is integrated between the signal (S) and ground (G) lines of the coplanar waveguide (CWG). (b) Magnetic-field μ₀H dependent voltage measured at 300 K for ϕ_M=135° and 315°. This is the ideal configuration for the study of SGE since the parasitic AMR of Fe is absent when H is perpendicular to the stripe. (c) Magnetization angle ϕ_M dependence of V_SGE, the solid line is a fit to cos(ϕ_M+45°). (d) Crystallographic orientation dependence of the magnitude of V_SGE. The error bar is the s.d. obtained by fitting the ϕ_M dependence of V_SGE for different stripe orientations.