Absence of magnetic order in RuO2: insights from μSR spectroscopy and neutron diffraction

Abstract Altermagnets are a novel class of magnetic materials, where magnetic order is staggered both in coordinate and momentum space. The metallic rutile oxide RuO 2 , long believed to be a textbook Pauli paramagnet, recently emerged as a putative workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While some experiments seem consistent with altermagnetism, magnetic order in RuO 2 remains controversial. We show that RuO 2 is nonmagnetic, both in bulk and thin film. Muon spectroscopy complemented by density-functional theory finds at most 1.14 × 10 −4 μ B /Ru in bulk and at most 7.5 × 10 −4 μ B /Ru in 11 nm epitaxial films, at our spectrometers’ detection limit, and dramatically smaller than previously reported neutron results that were used to rationalize altermagnetic behavior. Our own neutron diffraction measurements on RuO 2 single crystals identify multiple scattering as the source for the false signal in earlier studies.