Questioning Altermagnetism in RuO₂: A Study of Spin-to-Charge Conversion Mechanisms by Time-Domain Terahertz Spectroscopy

Altermagnets are a recently discovered class of magnetic material with great potential for applications in the field of spintronics, owing to their non-relativistic spin-splitting and simultaneous antiferromagnetic order. One of the most studied altermagnetic candidate materials is rutile structured RuO₂, however it has recently come under significant scrutiny as evidence emerges for its lack of any ordered magnetism at all. In this work we examine the spin-to-charge conversion in epitaxial RuO₂ thin films in bilayer with ferromagnetic permalloy (Fe₈₁Ni₁₉) by time-domain terahertz spectroscopy, as a probe for the inverse altermagnetic spin-splitting effect (IASSE) signature of altermagnetism. Our results do not indicate presence of IASSE for either as-deposited or field annealed samples, but rather are consistent with spin-to-charge conversion by only the relativistic inverse spin-Hall effect, as well as the non-relativistic and non-magnetic electrical anisotropic conductance, casting further doubt on the existence of altermagnetism in RuO₂.