Competing ferromagnetic and antiferromagnetic interactions in non-altermagnetic Ru${}_{1-x}$Cr${}_x$O${}_2$

Abstract

We investigate the proposal of hole doping RuO${}_2$ via alloying with Cr ions to induce altermagnetism. Thin film samples of Ru${}_{1-x}$Cr${}_x$O${}_2$ ($0\le x\le 0.28$) were prepared by reactive magnetron co-sputtering on TiO${}_2$ substrates, with epitaxial nature verified by X-ray diffraction and composition depth profiles determined by X-ray photoelectron spectroscopy combined with Ar etching. Neutron diffraction measurements of samples with $x=0$ and $x=0.23$ at low temperatures show no evidence of long-range altermagnetic order with spins oriented along the $c$-axis or within the $ab$-plane. In contrast, temperature dependent susceptibility measurements in samples with $x\ge 0.16$ show a gradual downturn below $T\approx 20$K, suggestive of antiferromagnetic interactions, although this coexists with ferromagnetic hysteresis and remnant magnetization at $4$K for samples with $x\ge 0.23$. Together, our magnetometry and neutron scattering measurements suggest the coexistence of antiferromagnetically coupled ferromagnetic CrO\textsubscript{2} clusters. This indicates that Cr ions do not hole dope Ru bands to induce an altermagnet state, but rather the holes remain localized to the Cr ions.