Probing Magnetic Properties of RuO$_{2}$ Heterostructures Through the Ferromagnetic Layer

TL;DR

This study investigates the magnetic properties of RuO$_{2}$ heterostructures, revealing that RuO$_{2}$ likely lacks intrinsic long-range magnetic order and that observed magnetic effects are interface-related rather than intrinsic.

Contribution

It provides comprehensive magnetometry and magneto-Raman measurements showing RuO$_{2}$ does not have intrinsic magnetic order, clarifying conflicting reports and highlighting interface effects.

Findings

Exchange bias observed below 15 K in heterostructures.

Magno modes only appear with NiFe layers, not in pure RuO$_{2}$.

DFT suggests interface effects like diffusion or spin disorder.

Abstract

RuO$_{2}$ has been proposed as the prototypical altermagnetic material. However, several reports have recently questioned its intrinsic magnetic ordering, leading to conflicting findings, especially in thin film heterostructures pointing to possible interface effects being convoluted with supposed antiferromagnetic/altermagnetic signatures. Here, extensive magnetometry measurements were performed on two independently grown thin film heterostructures of RuO$_{2}$ interfaced with either NiFe or Fe acting as the ferromagnetic layer. Below about 15 K, both samples exhibit exchange bias fields when cooled to approximately 2 K in a $+$1 T field, and a spin transitional feature is observed around 31 K. Magneto-Raman measurements on RuO$_{2}$ thin films only reveal a magnon mode when there is a NiFe layer, suggesting that RuO$_{2}$ does not intrinsically possess long range magnetic ordering.. When in contact with a ferromagnet, RuO$_2$ displays effects that could be ascribed to antiferromagnetism. However, the lack of intrinsic magnon modes points toward possible diffusion between the layers or spin disorder at the interface as seen by density functional theory (DFT) calculations.