Angle-Dependent Magnetoresistance Induced by Interface-Generated Spin Current in RuO$_2$/Permalloy Heterostructures

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.

💡 Research Summary

This paper investigates the origin of angle‑dependent magnetoresistance (ADMR) observed in epitaxial RuO₂/Permalloy (Py) heterostructures, with a focus on distinguishing contributions from the recently proposed altermagnetic nature of RuO₂ and interfacial spin‑orbit phenomena. RuO₂, a rutile‑structured oxide, has been theoretically identified as an altermagnet—exhibiting compensated antiparallel magnetic sublattices yet possessing ferromagnet‑like spin‑split bands. However, conflicting reports suggest that bulk RuO₂ may be non‑magnetic, prompting a need for experimental clarification.

The authors grew (100)‑oriented RuO₂ films of 5 nm and 8 nm thickness on TiO₂(100) substrates by pulsed laser deposition (PLD) under ultra‑high vacuum, monitoring crystallinity with in‑situ RHEED. After growth, the samples were transferred without exposure to a sputter chamber where a 4 nm Py layer and a 2 nm Al capping layer were deposited at room temperature. Hall‑bar devices (20 µm × 100 µm) were patterned with the current aligned either along RuO₂