While ferromagnetism and antiferromagnetism are well-established classes of magnetic order, a third class of collinear magnetic order, termed altermagnetism, has recently attracted scientific interest. We measured magnetic circular dichroism (MCD) in core-level photoemission (XPS) at the Ru $2p_{3/2}$ and $2p_{1/2}$ core levels in epitaxial ${\mathrm{RuO}}_2\left(110\right)/{\mathrm{TiO}}_2\left(110\right)$ films using circularly polarized x rays at 6 keV, as well as x-ray magnetic circular dichroism (XMCD) in resonant x-ray absorption at the Ru ${\mathrm{M}}_{3,2}$ ($3p_{3/2}$ and $3p_{1/2}$) edges. Charge transfer multiplet calculations show that the MCD-XPS and the XMCD can be explained by an altermagnetic locking of Ru magnetic moments and a distorted crystal field orientation. The distortion is caused by the epitaxial strain. The collinear magnetic moments in ${\mathrm{RuO}}_2$ occupy sublattice sites with distorted octahedral crystal fields that are rotated by ${90}^{\circ }$ with respect to each other. A change in the sign of the MCD-XPS at different sample positions indicates the presence of altermagnetic domains with the size of around hundreds of micrometers.

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• Altermagnets
• Thin films
• X-ray magnetic circular dichroism
• X-ray photoelectron spectroscopy