Enhanced Spin-Orbit Torque via Modulation of Spin Current Absorption

The magnitude of spin-orbit torque (SOT), exerted to a ferromagnet (FM) from an adjacent heavy metal (HM), strongly depends on the amount of spin current absorbed in the FM. We exploit the large spin absorption at the Ru interface to manipulate the SOTs in $\mathrm{HM}/\mathrm{FM}/\mathrm{Ru}$ multilayers. While the FM thickness is smaller than its spin dephasing length of 1.2 nm, the top Ru layer largely boosts the absorption of spin currents into the FM layer and substantially enhances the strength of SOT acting on the FM. Spin-pumping experiments induced by ferromagnetic resonance support our conclusions that the observed increase in the SOT efficiency can be attributed to an enhancement of the spin-current absorption. A theoretical model that considers both reflected and transmitted mixing conductances at the two interfaces of FM is developed to explain the results.

Figure 1

(a) Film structure of the multilayers (thicknesses are in nm). (b) $R_H$ (offset for clarity) as a function of in-plane pulsed currents with different Ru thicknesses ($t_{\mathrm{Ru}}$). $t_{\mathrm{Ru}}$ is indicated for each curve. (c) Switching current density, $J_S$ (defined as the average absolute value of switching current densities between the high to low $R_H$ and vice versa) as a function of $t_{\mathrm{Ru}}$. (d) The SOT switching efficiency ($\eta$) as a function of $t_{\mathrm{Ru}}$. (e) Longitudinal ($H_L$) and transverse ($H_T$) effective fields versus $t_{\mathrm{Ru}}$.

Figure 2

$R_H$ (offset for clarity) as a function of pulsed currents with different FM thicknesses ($t_{\mathrm{FM}}$) with a 0.6 nm Ru (a), and 2 nm MgO (b) capping layer. $t_{\mathrm{FM}}$ is indicated for each curve. (c) The SOT switching efficiency ($\eta$). (d) Longitudinal ($H_L$) effective field. (e) Transverse ($H_T$) effective field as a function of $t_{\mathrm{FM}}$. The solid (open) symbols in (c)–(e) represent the data with Ru (MgO) capping layer.

Figure 3

(a) In a $\mathrm{Pt}/\mathrm{FM}/\mathrm{MgO}$ structure, the polarized spins from the Pt layer are partially reflected at the $\mathrm{FM}/\mathrm{MgO}$ interfaces and compensate the torque at the $\mathrm{Pt}/\mathrm{FM}$ interface. (b) In a $\mathrm{Pt}/\mathrm{FM}/\mathrm{Ru}$ structure, with strong spin relaxation near the $\mathrm{FM}/\mathrm{Ru}$ interface, the absorption of spin currents from the Pt layer into FM is greatly enhanced. The white arrows denote the spin currents.

Figure 4

(a) Schematic of the field modulated FMR setup. (b) FMR spectra with $t_{\mathrm{Ru}}=0$ and 0.6 nm. (c) Full width at half maximum (FWHM) at different frequencies to extract the damping constant ($\alpha$) for the samples with $t_{\mathrm{Ru}}=0$, 0.6, and 1.4 nm. The inset shows the data from the reference sample. (d) Enhanced damping ($\mathrm{\Delta }\alpha$) versus $t_{\mathrm{Ru}}$.