Magnetic circular dichroism in core level x ray photoelectron spectroscopy of altermagnetic RuO2 films

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 2 amp; 8290; amp; 119901;3 2 and 2 amp; 8290; amp; 119901;1 2 core levels in epitaxial RuO2 amp; 8290; 110 TiO2 amp; 8290; 110 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 M3,2 3 amp; 8290; amp; 119901;3 2 and 3 amp; 8290; amp; 119901;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 RuO2 occupy sublattice sites with distorted octahedral crystal fields that are rotated by 90 amp; 8728; 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