Interfacial spin-orbit torque without bulk spin-orbit coupling

An electric current in the presence of spin-orbit coupling can generate a spin accumulation that exerts torques on a nearby magnetization. We demonstrate that, even in the absence of materials with strong bulk spin-orbit coupling, a torque can arise solely due to interfacial spin-orbit coupling, namely, Rashba-Eldestein effects at metal/insulator interfaces. In magnetically soft NiFe sandwiched between a weak spin-orbit metal (Ti) and insulator (${\mathrm{Al}}_2{\mathrm{O}}_3$), this torque appears as an effective field, which is significantly larger than the Oersted field and qualitatively modified by inserting an additional layer between NiFe and ${\mathrm{Al}}_2{\mathrm{O}}_3$. Our findings point to unconventional routes for tuning spin-orbit torques by engineering interfacial electric dipoles.

Figure 1

(a) Schematic of the second-order PHE measurement. (b) Second-order planar Hall voltage $\mathrm{\Delta }{V}_{\mathrm{PH}}$ curves at different transverse bias fields $H_{\mathrm{y}}$. (c) Current-induced field $H_{\mathrm{I}}$ vs $I_{\mathrm{dc}}$. The dotted line shows $H_{\mathrm{Oe},\mathrm{Ti}}$ based on the estimated fraction of $I_{\mathrm{dc}}$ in Ti. The shaded area is bounded by the maximum possible Oersted field $H_{\mathrm{Oe},\max }$.

Figure 2

(a) Schematic of the ST-FMR setup. (b) ST-FMR spectra at different dc bias currents $I_{\mathrm{dc}}$, with rf current excitation at 5 GHz and +8 dBm and external field $H$ at $\theta ={40}^{\circ }$. Inset: $I_{\mathrm{dc}}$-induced shift of ST-FMR spectra. (c) Shift of resonance field $H_{\mathrm{FMR}}$ due to $I_{\mathrm{dc}}$ at $\theta ={40}^{\circ }$. The error bar is the standard deviation of five measurements. The dotted line shows the estimated Oersted field from Ti, $H_{\mathrm{Oe},\mathrm{Ti}}$. The shaded area is bounded by the maximum possible Oersted field, $H_{\mathrm{Oe},\max }$. (d) Angular dependence of $I_{\mathrm{dc}}$-induced $H_{\mathrm{FMR}}$ shift. The error bar is the error in linear fit of $H_{\mathrm{FMR}}$ vs $I_{\mathrm{dc}}$. The solid curve indicates the fit to $sin\theta$. (e) Transverse current-induced field $H_{\mathrm{I}}=-\mathrm{\Delta }{H}_{\mathrm{FMR}}/sin\theta$ normalized by $H_{\mathrm{Oe},\max }$ at various $\theta$. The solid line indicates the average of the ST-FMR data points. The dotted line indicates estimated $H_{\mathrm{Oe},\mathrm{Ti}}$. The PHE data point at $\theta =0$ is the average of three devices.

Figure 3

(a) NiFe thickness $t_{\mathrm{NiFe}}$ dependence of $H_{\mathrm{I}}$ normalized by $H_{\mathrm{Oe},\max }$. The dotted curve indicates the estimated Oersted field from Ti, $H_{\mathrm{Oe},\mathrm{Ti}}$. Each ST-FMR data point is the mean of results at several frequencies 3.5–7.0 GHz at $\theta ={45}^{\circ }$ and $-{135}^{\circ }$. $H_{\mathrm{I}}/H_{\mathrm{Oe},\max }>0$ is defined as $H_{\mathrm{I}}//+y$ when $I_{\mathrm{dc}}//+x$ [illustrated in Figs. 1 and 2]. (b) Estimated spin-orbit field $H_{\mathrm{SO}}$ per unit current density in NiFe, $J_{\mathrm{NiFe}}$. The solid curve indicates the fit to ${t_{\mathrm{NiFe}}}^{-1}$.

Figure 4

(a)–(f) Structural dependence of $H_{\mathrm{I}}$ (mean of measurements on three PHE devices) normalized by $H_{\mathrm{Oe},\max }$. $H_{\mathrm{Oe},\mathrm{NM}}$ is the Oersted field from current in the nonmagnetic metal layers (Ti, Cu, Pt). The nominal layer thicknesses are as follows: NiFe, 2.3 nm; ${\mathrm{Al}}_2{\mathrm{O}}_3$, 1.5 nm; Ti, 1.2 nm; Cu, 1.0 nm; and Pt, 0.5 nm. (g) Cu thickness $t_{\mathrm{Cu}}$ dependence of $H_{\mathrm{I}}$ normalized by $H_{\mathrm{Oe},\max }$ at NiFe thickness 2.5 nm. The blue dotted curve indicates $H_{\mathrm{Oe},\mathrm{NM}}$. (h) Estimated spin-orbit field $H_{\mathrm{SO}}$ per unit current density in Cu, $J_{\mathrm{Cu}}$.