Temperature-driven enhancement and sign reversal of field-like torque in Py/FePS$_3$ bilayers

Abstract

Electrical manipulation of magnetization via current-induced spin orbit torques offers a promising route toward nonvolatile and energy efficient spintronic devices. In this work, we present a comprehensive investigation of SOTs in Py/FePS$_3$ bilayer devices, where Py/FePS$_3$ is a layered van der Waals antiferromagnetic insulator. Using low frequency harmonic Hall measurements, we quantify both field like and damping like torque components and examine their dependence on temperature. We find that interfacing Py with Py/FePS$_3$ leads to a pronounced enhancement of the field-like torque efficiency compared to Py reference devices, while the damping-like torque remains largely unaffected. Strikingly, the field like torque efficiency exhibits a strong temperature dependence, including a clear sign reversal upon cooling. This behavior occurs despite negligible charge current flow through the Py/FePS$_3$ layer, indicating that the observed torque modulation arises from interfacial effects rather than bulk transport. The close correlation between the temperature evolution of the field like torque and the antiferromagnetic ordering of Py/FePS$_3$ highlights the active role of antiferromagnetic insulators in controlling spin orbit torque symmetry and efficiency, and suggests new pathways for torque engineering in magnetic heterostructures.

Figures (4)

• Figure 1: (a) Schematic illustration of the FePS$_3$/Py bilayer device geometry and measurement configuration. An in-plane charge current $J_c$flowing through the Py layer generates spin–orbit torques, giving rise to field-like ($B_{FL}$) and damping-like ($B_{DL}$) effective fields acting on the magnetization M. (b) Optical microscope image of a representative FePS$_3$/Py Hall bar device used for harmonic Hall measurements. (c) Angular dependence of the second-harmonic Hall resistance $R_{xy}^{1\omega}$measured as a function of the in-plane magnetic field angle $\phi$for Py and Py/FePS$_3$ devices. The solid lines represent fits using a $sin(2\phi)$ dependence. (d) First-harmonic Hall resistance $R_{xy}^{1\omega}$ as a function of $\phi$, showing comparable magnetic response for Py and Py/FePS$_3$, indicating that the static magnetic properties of Py remain largely unaffected by the FePS$_3$ layer.
• Figure 2: (a,b) Angular dependence of the second-harmonic Hall resistance $R_{xy}^{2\omega}$ measured at different in-plane magnetic fields (0.3 T and 0.01 T) at (a) T=290 K and (b) T=50 K for the FePS$_3$/Py bilayer device. The solid lines represent fits based on the standard harmonic Hall model. (c) Extracted quantity $R^{2\omega}_{xy,cos2\phi cos\phi}/R_{PHE}$ plotted as a function of inverse magnetic field 1/B at various temperatures, where the linear dependence is used to separate the field-like torque contribution. (d) Temperature dependence of the effective field-like effective field ($B_{FL}$) , showing a pronounced evolution and sign reversal upon decreasing temperature, while the damping-like torque remains negligible within experimental uncertainty shown in fig. \ref{['4']}.
• Figure 3: Normalized second-harmonic transverse resistance $\frac{R^{2\omega}_{xy,cos\phi}}{R_{AHE}}$ plotted as a function of $\frac{1}{B_{ext}+B^k_{eff}}$ for the Py/FePS$_3$ bilayer at T=290 K. Symbols represent the experimental data and the solid line is a linear fit. The nearly vanishing slope indicates a negligible SL like effective field contribution within the experimental uncertainty. Inset: Corresponding measurement at T=20 K. The absence of any appreciable linear dependence confirms that the SL like torque contribution remains negligible over the investigated temperature range.
• Figure 4: (a) Temperature dependence of the electric field across the device calculated for an applied current of 5 mA, showing a monotonic decrease with decreasing temperature, consistent with metallic transport dominated by the Py layer. Temperature-dependent magnetization of FePS$_3$ measured using a SQUID magnetometer under an applied magnetic field of 1 T, with the anomaly near T=118 identifying the antiferromagnetic Néel temperature of FePS$_3$.(c) Temperature dependence of the field-like torque efficiency $\zeta_{FL}^E$ for Py/FePS$_3$ (red circles) and Py reference devices (blue squares). The pronounced enhancement and sign reversal of $\zeta_{FL}^E$ in Py/FePS$_3$ correlates strongly with the antiferromagnetic ordering of FePS$_3$, while the Py-only device shows a weak and monotonic temperature dependence.