Exploring altermagnetism in RuO2: from conflicting experiments to emerging consensus

Altermagnetism has recently emerged as a new class of magnetic order that combines the advantages of both ferromagnets and antiferromagnets. The compensated antiparallel spin structure, in combination with crystallographic rotational symmetry, gives rise to distinct magnetic properties, opening new opportunities for next-generation spintronic applications. In this review, we introduce a variety of experimental approaches-including electronic, optical, and particle-based spectroscopies-used to probe theoretically suggested altermagnetism. In particular, we review recent studies on the altermagnetic candidate RuO₂, whose magnetic ground state remains under debate with conflicting experimental results, organizing the discussion according to the experimental techniques. Furthermore, we highlight recent findings on fully strained RuO₂ thin films that emphasize the critical role of strain in the emergence of altermagnetism. We believe that this review will provide not only practical guidelines for investigating altermagnetic systems but also valuable insights toward reaching consensus on the ongoing controversies surrounding RuO₂'s altermagnetism.