Altermagnetism presents intriguing possibilities for spintronic devices due to its unique combination of strong spin-splitting and zero net magnetization. However, realizing its full potential hinges on fabricating single-variant altermagnetic thin films. In this work, we present definitive evidence for formation of single-variant altermagnetic RuO2 (101) thin films with fully epitaxial growth on Al2O3 (1 1 02) r-plane substrates, confirmed through rigorous structural analyses using X-ray diffraction, atomic-resolution transmission electron microscopy and X-ray magnetic linear dichroism. The mutual correspondence of the occupancy of oxygen atoms on the surfaces of RuO2 (101) 010 and Al2O3 (1 1 02) 11 2 0 plays a decisive role in the formation of the single-variant RuO2, which is also supported by our first-principles density functional theory calculations. We further observed spin-splitting magnetoresistance in the single-variant RuO2 (101) /CoFeB bilayers, highlighting the characteristic effect of single variant on spin transport. The demonstration of single-variant RuO2 (101) films marks a significant advancement in the field of altermagnetism and paves the way for exploring their potential applications. Altermagnetism arises from a combination of crystal symmetry and magnetic ordering. For the altermagnetic properties to be clear, and technologically useful, the same crystal variant must be present over the entire sample. Here, He, Wen and coauthors achieve such single variant thin films in RuO2, confirming the altermagnetic properties via XMLD and transport measurements.
He et al. (2025) studied this question.