Control of Néel Vector with Spin-Orbit Torques in an Antiferromagnetic Insulator with Tilted Easy Plane

Injecting spin currents into antiferromagnets and realizing efficient spin-orbit-torque switching represents a challenging topic. Because of the diminishing magnetic susceptibility, current-induced antiferromagnetic dynamics remain poorly characterized, complicated by spurious effects. Here, by growing a thin film antiferromagnet, $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, along its nonbasal plane orientation, we realize a configuration where the spin-orbit torque from an injected spin current can unambiguously rotate and switch the Néel vector within the tilted easy plane, with an efficiency comparable to that of classical ferrimagnetic insulators. Our study introduces a new platform for quantitatively characterizing switching and oscillation dynamics in antiferromagnets.

Figure 1

(a) An antiferromagnet with its easy plane parallel with the film surface. Dampinglike torque effective fields ${\mathit{H}}_{\mathrm{DL}}^{A(B)}$ rotate the two magnetic moments ${\mathit{m}}_{A(B)}$ toward out-of-plane directions. (b) An antiferromagnet with the easy plane forming a finite angle with the film surface. ${\mathit{H}}_{\mathrm{DL}}^{A(B)}$ have components for rotating ${\mathit{m}}_{A(B)}$ within the easy plane.

Figure 2

(a) Unit cell of $R$-plane sample. Inset: unit cell orientation for $C$-plane (“$C$”) and $A$-plane (“$A$”) samples. (b) Cross-sectional atomic-resolution high-angle annular dark-field (HAADF) STEM image. Dislocation symbols are marked. (c) XRD symmetric $2\theta \text{−}\omega$ scan. “cps” represents counts per second. Inset: rocking curve. (d) Reciprocal space map. “$x$” represents the expected diffraction peak position if the strain in the film were fully relaxed. (e),(f) $M\text{−}H$ loop for a 30 nm thick film under in-plane (e) and out-of-plane (f) field. Linear background at high fields comes from antiferromagnetic susceptibility. Inset of (e): Photo of the $R$-plane sample with the field direction labeled, which is $\sim 45°$ counterclockwise from the $+b$ axis in the $R$ plane of (a). The scale bar is 3 mm.

Figure 3

(a) SOT geometry, with the current $\mathit{I}\Vert \mathit{x}$ and the injected spins $\mathit{\sigma }\Vert \mathit{y}$. $\mathit{H}$ with an angle of $\beta$ is applied within the $xz$ plane. $\phi$ and $\delta$ define the angles of $\mathit{n}$, and the spanning angle between ${\mathit{m}}_{A(B)}$, respectively. Top-right inset: device illustration. (b),(c) First (b) and second (c) harmonic results of the transverse SMR as a function of $\beta$. The current is 4 mA (root mean square value). ${\tau }_{\mathrm{DL}}$ (red) and ${\tau }_{\mathrm{FL}}$ (blue) contributions to $R_H^{2\omega }$ at $H=20\mathrm{kOe}$ are separately plotted. (d),(e) Current dependence of $R_H^{\omega }$ and $R_H^{2\omega }$, taken at $H=23\mathrm{kOe}$.

Figure 4

(a) $R_H^{\mathrm{SMR}}$ under dc of $I=\pm 6\mathrm{mA}$. The insets show a magnified view of two curves to illustrate the horizontal shift angle $\mathrm{\Delta }\beta$. (b) Summary on $\mathrm{\Delta }\beta (I)$ under different $\mathit{H}$, taking an average across all $\beta$ values. (c),(d) Current-induced $\mathit{n}$ rotation in the $C$ plane for positive (c) and negative (d) field. The darker (lighter) arrows represent the position of $\mathit{n}$ after positive (negative) current pulses. (e) Switching of $R_H^{\mathrm{SMR}}$ under current pulses with different magnitudes. The blue (red) dots show $R_H^{\mathrm{SMR}}$ after positive (negative) pulses. (f) Current-induced switching of $R_H^{\mathrm{SMR}}$ with different $H$. Switching is observed for intermediate fields and the polarity remains the same between $H_z=\pm 5\mathrm{kOe}$.