Interface tuned Enhanced and Low Temperature Quenching of Orbital Hall Currents Induce Torque and magnetoresistance in Light Metal/Nickel Bilayers

TL;DR

This paper investigates the orbital Hall effect in light-metal/ferromagnet bilayers, focusing on Ti as the LM and Ni as the FM, with and without a Cu interlayer. Using second-harmonic longitudinal and transverse voltage measurements under an AC current, the authors extract the orbital Hall torque (OHT) efficiency and the unidirectional orbital magnetoresistance (UOMR). They find that inserting Cu enhances both OHT efficiency and UOMR by about fivefold, and that both quantities increase with Ti thickness while decreasing with temperature, with a linear correlation between them that points to a common orbital-current origin in the Ti layer. These results show that orbital currents in light metals can drive torque and magnetoresistance in FM layers and that interface engineering can optimize orbital transport for spintronic functionalities.

Abstract

We investigate orbital current induced effects arising from the orbital Hall effect in light-metal/ferromagnet bilayers. Thin films of Ti in ohmic contact with Ni were studied using second-harmonic longitudinal and transverse voltage measurements under an applied a.c. current. From these signals, we extract the orbital Hall torque (OHT) efficiency and the unidirectional orbital magnetoresistance (UOMR). Insertion of a Cu interlayer between the Ni/Ti interface leads to an enhancement of both OHT efficiency and UOMR compared to both Ni/Ti and Ni/Cu bilayers. Furthermore, systematic variation of Ti thickness reveals that both OHT efficiency and UOMR increase with increasing Ti thickness, indicating that the observed phenomena predominantly originate from the bulk orbital Hall effect rather than purely from interfacial mechanisms and Lowering the temperature leads to a clear reduction in both the orbital Hall torque (OHT) efficiency and the unidirectional orbital magnetoresistance (UOMR). The nearly linear and correlated temperature dependence of both parameters suggests a common underlying mechanism, namely, the orbital Hall effect in the light-metal layer, which governs both the generation of orbital current and its subsequent influence on the ferromagnet through orbital torque and orbital magnetoresistance.

Figures (8)

• Figure 1: (a).Simultaneous measurements of transverse Hall and longitudinal second-harmonic signals were obtained using a lock-in amplifier of FM/LM bilayer device. (b). Field dependence of the second-harmonic transverse resistance ($\frac{R_{xy}^{cos\phi, 2\omega}-R_{\nabla T}}{R_{AHE}}$) plotted as a function of $\frac{1}{B_{ext}+B^k_{eff}}$ for Ni/Ti and Ni/Cu(1 nm)/Ti bilayers. The slope represents the orbital Hall torque efficiency, which shows a significant enhancement upon Cu insertion. (c). Magnetic-field dependence of the unidirectional magnetoresistance $R_{UMR}$ for Ni/Ti and Ni/Cu/Ti samples; the inset shows the angular dependence of the first-harmonic voltage. (d). Comparison of the orbital torque efficiency($\zeta_{SL}$) and normalized UMR amplitude ($R_{UMR}/ER_{xx}^{1\omega}$) for the two structures, highlighting a correlated $\approx 5\times$ enhancement after Cu insertion, indicating their common origin associated with the orbital Hall effect.
• Figure 2: (a). Field dependence of the second-harmonic transverse resistance ($\frac{R_{xy}^{cos\phi, 2\omega}-R_{\nabla T}}{R_{AHE}}$) plotted as a function of $\frac{1}{B_{ext}+B^k_{eff}}$ for the Ni/Cu/Ti bilayer. The slope of the linear fit provides the orbital Hall torque (OHT) efficiency($\zeta_{SL}$). (b).Temperature dependence of $\zeta_{SL}$, showing a monotonic decrease with decreasing temperature. (c). Temperature variation of normalized unidirectional orbital magnetoresistance ($R_{UMR}/ER_{xx}^{1\omega}$), exhibiting a similar trend. (d). Linear correlation between $\zeta_{SL}$ and $R_{UMR}/ER_{xx}^{1\omega}$, indicating that both effects originate from the same underlying mechanism—the orbital Hall effect in the light-metal layer.
• Figure 3: (a).Temperature dependence of the orbital Hall torque efficiency $\zeta_{SL}$ for the Ni/Ti bilayer, showing a gradual decrease with decreasing temperature. (b). Temperature variation of the normalized unidirectional orbital magnetoresistance $R_{UMR}/ER_{xx}^{1\omega}$, exhibiting a similar trend. (c). Linear correlation between $\zeta_{SL}$ and $R_{UMR}/ER_{xx}^{1\omega}$, indicating that both effects share a common origin arising from the orbital Hall effect in the Ti layer.(d)..Thickness dependence of the orbital Hall torque efficiency $\zeta_{SL}$ for the Ni/Ti bilayer, showing a gradual increase with increasing thickness. (e). Thickness variation of the normalised unidirectional orbital magnetoresistance $R_{UMR}/ER_{xx}^{1\omega}$, exhibiting a similar trend. (f). Linear correlation between $\zeta_{SL}$ and $R_{UMR}/ER_{xx}^{1\omega}$, indicating that both effects share a common origin arising from the orbital Hall effect in the Ti layer
• Figure 4: Comparison of (a) the damping-like torque efficiency per unit electric field ($\zeta_{SL}^E$) and (b) the unidirectional magnetoresistance per unit electric field ($R_{UMR}/ER_{xx}^{1\omega}$) for Ni/Ti(10 nm), Ni/Cu(10 nm), and Ni/Cu(10 nm)/Ti(10 nm) bilayers. Both quantities show a significant enhancement in the Ni/Cu/Ti trilayer, indicating efficient orbital-to-spin conversion and a strong correlation between orbital Hall torque and unidirectional orbital magnetoresistance.
• Figure 5: (a) Schematic of the Hall bar device geometry used for simultaneous measurements of longitudinal($V_{xx}$) and transverse ($V_{xy}$) voltages under an applied AC current. The inset shows an optical micrograph of the fabricated Hall bar. The external magnetic field (B) is applied at an in-plane angle ($\phi$) with respect to the current direction. (b) X-ray diffraction (XRD) pattern of the Ti layer showing a distinct (111) peak, indicating preferential crystalline orientation of the Ti film.