Interface-Generated Spin Current Induced Magnetoresistance in RuO2/Py Heterostructures

TL;DR

This work investigates magnetotransport in RuO$_2$/Py heterostructures to disentangle interfacial spin-transport effects from potential altermagnetic contributions. Structural characterization confirms epitaxial RuO$_2$ growth, while transport measurements reveal a pronounced negative ADMR in the β-plane that is largely independent of crystallography. Through a drift-diffusion framework incorporating interface-generated spin currents (IGSC) and the inverse spin Hall effect (iSHE), the authors show that interfacial mechanisms dominate the observed magnetoresistance, with $Q_{zy}$ depending on whether $M$ is along $y$ or $z$ and with thickness controls predicting crossovers between negative and positive ADMR. The study argues that IGSC, rather than bulk altermagnetic effects, governs the signal, highlighting the critical role of interfacial spin transport in RuO$_2$-based spintronic devices and outlining pathways to isolate altermagnetic contributions in future work.

Abstract

Altermagnets, a recently discovered class of magnetic materials exhibiting ferromagnetic-like spin-split bands and antiferromagnetic-like compensated magnetic order, have attracted significant interest for next-generation spintronic applications. Ruthenium dioxide (RuO2) has emerged as a promising altermagnetic candidate due to its compensated antiparallel magnetic order and strong spin-split electronic bands. However, recent experimental and theoretical reports also suggest that RuO2 may be non-magnetic in its ground state, underscoring the need for deeper investigations into its magnetic character. Specifically, the (100)-oriented RuO2 films are expected to generate spin currents with transverse spin polarization parallel to the Néel vector. Here, we investigate magnetotransport in epitaxial RuO2/Permalloy (Py) heterostructures to examine spin Hall magnetoresistance and interfacial effects generated in such systems. Our measurements reveal a pronounced negative angular-dependent magnetoresistance for variation of magnetic field direction perpendicular to the charge current direction. Detailed temperature-, magnetic field-, and crystallographic orientation-dependent measurements indicate that interface-generated spin current (IGSC) at the RuO2/Py interface predominantly governs the observed magnetoresistance. This shows that strong interface effects dominate over possible altermagnetic contributions from RuO2. Our results show that the role of interface-generated spin currents is crucial and should not be overlooked in studies of altermagnetic systems. A critical step in this direction is disentangling interfacial from altermagnetic contributions. The insight into interfacial contributions from altermagnetic influences is essential for the advancement of RuO2 based spintronic memory and sensing applications.

Figures (6)

• Figure 1: Structural and surface characterization of TiO$_2$(100)//RuO$_2$(100)/Py films. (a) XRD patterns measured with the scattering vector normal to the (100)-oriented rutile substrate. (b) RHEED pattern for TiO$_2$(100)//RuO$_2$(100), with the electron beam aligned along TiO$_2$[001].
• Figure 2: Longitudinal resistance variations for magnetic field direction in the $\alpha$-, $\beta$-, and $\gamma$-planes. (a) Measurement scheme for the $\alpha$-, $\beta$-, and $\gamma$-planes (b) RuO$_2$(100)/Py heterostructure, and (c) Py thin film measured at 20K, for $J_C$ being parallel to the c-axis under a 0.95T magnetic field.
• Figure 3: Longitudinal resistance variations in the $\beta$-plane at different temperatures for (a) $J_C \perp c$ and (b) $J_C \parallel c$ configurations under a 0.95T magnetic field. (c) Temperature dependence of the peak-to-peak magnetoresistance amplitude in the $\beta$-plane. (d) ADMR measured at 2T and 5T in the $\beta$-plane at 20K. (e, f) Schematic representation of IGSC-induced negative ADMR. When a charge current $J_C$ (red arrow) is injected, a spin current $Q_{zy}$ (yellow arrow) is generated perpendicular to the interface. The magnitude of the spin current $Q_{zy}$ is maximized and minimized when the magnetization $M$ (black arrow) is (e) parallel and (f) perpendicular to the spin index on the y-axis, respectively. Consequently, the inverse process of SHE converts the interfacial spin current back into an induced charge current $J_C^{\text{ind}}$ (orange arrow), resulting in a resistance change.
• Figure 4: Plots of $Q_{zy}$ versus $z$ for the (a) interfacial and (b) spin Hall contribution to the longitudinal magnetoresistance. Each case considers a different source of $Q_{zy}$ (i.e. interfacial versus bulk), but in both cases, the iSHE in the NM layer is assumed to convert $Q_{zy}$ to an in-plane charge current $j_x$, which opposes the original charge current from the applied electric field. Panel (a) shows that $Q_{zy}$ is greatest for $m \parallel y$, indicating a negative ADMR from the interfacial contribution. Panel (b) shows that $Q_{zy}$ is greatest for $m \parallel z$, indicating a positive ADMR from the bulk spin Hall contribution.
• Figure 5: Thickness dependencies of the average, dimensionless spin current in the NM layer, obtained by integrating $Q_{zy}$ from $z = -t_\text{NM}$ to $z = 0$ and dividing by $t_\text{NM}$ and the source spin current strength. Panels (a) and (c) show that the interfacial contribution to the MR ratio can switch from negative to positive in certain regimes.