Effects of altermagnetic order, strain, and doping in RuO2

Abstract

RuO₂, one of the most widely studied transition metal oxides, was recently predicted to host a novel form of collinear magnetic order referred to as altermagnetism. In this study we combine experiment (reflectance, transmittance, ellipsometry and Raman measurements) and first-principles calculations to elucidate the potential role of altermagnetic order, strain and doping on the optical and vibrational properties of RuO₂ grown on TiO₂ (001), (101) and (110) substrates. Our combined experimental and theoretical results point toward a nonmagnetic ground state as the most consistent description of the optical and vibrational properties of bulk RuO₂. RuO₂ strained to TiO₂ (001) remains nonmagnetic in our calculations. Straining to TiO₂ (110) stabilizes ferromagnetic and altermagnetic states that are nearly degenerate and both lower in energy than the nonmagnetic state. The relative energetic ordering of these states is highly sensitive to the level of strain and the choice of exchange–correlation approximation.