Strain Engineering of Altermagnetism in RuO₂ thin films

The magnetic ground state of RuO₂ has been under intense debate. While early experiments revealed signatures of an altermagnetic order, more recent measurements reported a non-magnetic ground state in bulk RuO₂ and relatively thick films. Here, we investigate the role played by the epitaxial strain resulting from the lattice mistmatch with a substrate. Using first-principles calculations, we show that compressive strain along [001] direction stabilizes an altermagnetic phase in RuO₂ thin films grown on (100) and (110) TiO₂ substrates. We further identify that compressive strain enhances the density of states near the Fermi level, resulting in a Fermi surface instability and the emergence of altermagnetism. The magnitude of strain and the associated increase in the density of states can be tuned by varying the film thickness, as systematically confirmed by x-ray diffraction and photoemission spectroscopy measurements. Symmetry analysis further reveals that (100) RuO₂ hosts an ideal altermagnetic order, whereas broken symmetry in (110) films leads to an uncompensated ferrimagnetic state.