Large fieldlike torque in amorphous ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ originated from the intrinsic spin Hall effect

We investigated temperature dependent current driven spin-orbit torques in magnetron sputtered ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ (4 and 10 nm)/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ (5 nm) layered structures with in-plane magnetic anisotropy. The room temperature dampinglike and fieldlike spin torque efficiencies of the amorphous ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ films were measured to be $0.14\pm 0.008\left(0.07\pm 0.012\right)$ and $–0.03\pm 0.006\left(–0.20\pm 0.009\right)$, for the four (10 nm) films, respectively, by utilizing the second-harmonic Hall technique. The large fieldlike torque in the relatively thicker ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ (10 nm) thin film is unique compared to the traditional spin Hall materials interfaced with thick magnetic layers with in-plane magnetic anisotropy which typically have dominant dampinglike and negligible fieldlike torques. Additionally, the observed room temperature fieldlike torque efficiency in ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ (10 nm)/CoFeB (5 nm) is up to three times larger than the dampinglike torque ($–0.20\pm 0.009$ and $0.07\pm 0.012$, respectively) and 30 times larger at 50 K ($–0.29\pm 0.014$ and $0.009\pm 0.017$, respectively). The temperature dependence of the fieldlike torques shows dominant contributions from the intrinsic spin Hall effect while the dampinglike torques show dominate contributions from the extrinsic spin Hall effects, skew scattering, and side jump. Through macrospin calculations, we found that including fieldlike torques on the order of or larger than the dampinglike torque can reduce the switching critical current and decrease magnetization procession for a perpendicular ferromagnetic layer.

Figure 1

(a) Bright-field transmission and (b) high-angle annular dark-field electron microscope image of the RS10 sample. (c) Selected area diffraction pattern of the RS10 sample. The diffuse ring seen suggests an amorphous film with no long-range order.

Figure 2

(a) Diagram of the harmonic Hall measurement: a Hall bar with a length of 85 μm and a width of 10 μm is rotated in the $xy$ plane from 0 ° to 360 °, while the first- and second-harmonic Hall voltages are measured via two lock-in amplifiers. (b) The first-harmonic and (c) second-harmonic Hall voltages for the RS10 sample rotated in a 1500 Oe external field at 300 K fitted to Eqs. (1) and (2), respectively.

Figure 3

(a) The contribution from the DL and (b) FL torques on the second-harmonic Hall voltage for the RS4 sample.

Figure 4

(a) Temperature dependence of the SOT efficiency from the DL torque and (b) FL torque. (c), (d) Relation between magnitude ${\zeta }_{\mathrm{DL},\mathrm{FL}}$ and the resistivity of the RS4 and RS10 samples, respectively. The dashed lines show the fit to Eq. (4).