The magnetic ground state of ${\mathrm{RuO}}_2$ has been under intense debate. Using first-principles calculations, we show that compressive strain along the [001] direction stabilizes an itinerant magnetism in ${\mathrm{RuO}}_2$ thin films grown on (100) and (110) ${\mathrm{TiO}}_2$ 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) ${\mathrm{RuO}}_2$ hosts an ideal altermagnetic order, whereas broken symmetry in (110) films leads to a ferrimagnetic state. Finally, we discuss the effects of Hubbard $U$ parameters and evaluate the realistic tunneling magnetoresistance of (100) ${\mathrm{RuO}}_2$.

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• Altermagnetism
• Electronic structure
• Tunneling magnetoresistance
• Density functional calculations
• Strain engineering
• X-ray diffraction
• X-ray photoelectron spectroscopy