Anisotropic Spin-Orbit Torque through Crystal-Orientation Engineering in Epitaxial $\mathrm{Pt}$

One of the main objectives of spintronics is to provide power-efficient switching of magnetic layers through electrical means, and in order to achieve this goal, alternate material systems with enhanced spin-orbit torque (SOT) must be engineered. In this work we provide evidence of anisotropy in the SOT and spin Hall effect (SHE) in epitaxial $\mathrm{Pt}$(110) grown on $\mathrm{Mg}\mathrm{O}$(110) single-crystal substrates, and find that the spin Hall angle and the dampinglike torque are 20% larger when current is applied along the [001] crystallographic direction as compared to [$1\overline{1}0$], leading to an equivalent reduction in switching current density along [001]. The anisotropy in SOT is attributed to the bulk contributions of the SHE in the $\mathrm{Pt}$ layer through its anisotropic resistance in this specific orientation. Measurements additionally suggest that the Rashba-Edelstein effect at the $\mathrm{Pt}$/$\mathrm{Ti}$ interface due to the $\mathrm{Pt}$(110) surface has a non-negligible effect on the spin diffusion length and SOT. By providing experimental evidence of the crystal orientation dependence of SOT-induced magnetization switching, this work helps to establish a path for energy-efficient magnetization switching through the alignment of devices with crystallographic directions of enhanced SOT generation.

Figure 1

(a) In plane $\varphi$ scan showing twofold symmetry of epitaxial $\mathrm{Pt}$ (200) and ($2\overline{2}0$) peaks, aligned with corresponding $\mathrm{Mg}\mathrm{O}$ substrate peaks. (b) Extracted $\mathrm{Pt}$ resistivities for the [001] and [$1\overline{1}0$] directions. (c) Out of plane $2\theta$ scan of $\mathrm{Mg}\mathrm{O}$(110)//$\mathrm{Pt}$(10)/$\mathrm{Ti}$(0.8)/$(\mathrm{Co},\mathrm{Fe})\mathrm{B}$(1)/$\mathrm{Mg}\mathrm{O}$(3) sample. (d) Diagram of the ($1\overline{1}0$) plane of $\mathrm{Mg}\mathrm{O}$ and $\mathrm{Pt}$ layers. (e) Diagram of the (001) plane of $\mathrm{Mg}\mathrm{O}$ and $\mathrm{Pt}$ layers.

Figure 2

(a) Example $H_Y$ and $H_Z$ field scans for the [001] and [$1\overline{1}0$] directions for the $t_N=3.5$ nm sample. (b) Schematic of the Hall bar used in this measurement and the axes defined for this work. (c) Example $XY$, $XZ$, and $YZ$ angle scans for the [001] and [$1\overline{1}0$] directions performed at 1.5 T for the $t_N=3.5$ nm sample. (d) SMR as a function of $\mathrm{Pt}$ thickness fit using Eq. (1).

Figure 3

(a) Schematic of the Hall bar used in this measurement and the axes defined for this work. (b) Example first harmonics of the 3 nm sample for the [001] and [$1\overline{1}0$] directions, showing no significant differences. (c) Example second harmonics of the 3 nm sample under a longitudinal field; the inset shows an enlargement of the low field. (d) Example second harmonics of the 3 nm sample under a transverse field; the inset shows an enlargement of the low field. (e) Dampinglike effective fields as a function of Pt thickness for the [001] and [$1\overline{1}0$] directions, fit using Eq. (3). (f) Fieldlike effective fields as a function of Pt thickness for the [001] and [$1\overline{1}0$] directions.

Figure 4

(a) Critical switching current $J_{\mathrm{crit}}$ in the $t_N=4.5$ nm sample as a function of applied field $H_X$ for the [001] and [$1\overline{1}0$] directions. (b) Example loop showing reduced $J_{\mathrm{crit}}$ for [001] at $H_X=-3$ mT.

Figure 5

SMR as a function of thickness with added 99% confidence bands of fitting.

Figure 6

(a) Hysteresis loops with an out-of-plane field $H_Z$ of $t_N=1$, 3, 6, and 10 nm samples as grown, measured in a VSM. (b) Normalized Hall resistances after Hall cross fabrication.