Absence of magnetic order in epitaxial RuO2 revealed by X-ray linear dichroism

Recently, the topic of altermagnetism has attracted tremendous attention and RuO2 have been demonstrated to be one of the most promising altermagnetic candidates. However, disputes still remain on the existence of magnetic order in RuO2. Here in this work, we employ X-ray linear dichroism (XLD), a widely utilized technique for characterizing antiferromagnets, in conjunction with photoemission electron microscopy and multiple scattering calculation to provide clear evidence of the absence of magnetic order in epitaxial RuO2 films. The observed XLD signal is nearly invariant with temperature and independent on cooling field direction, in stark contrast to the substantial magnetic order-related XLD signal predicted by multiple scattering calculation. This finding strongly suggests a nonmagnetic origin for RuO2. Furthermore, we observed significantly distinct XLD signals at the Ru M3 and O K edges in RuO2 films grown on TiO2 substrate with different surface orientations, which can be attributed to the low-symmetry crystal field. These results unequivocally demonstrate the absence of magnetic order in RuO2 and establishes XLD measurement as a robust technique for probing the low-symmetry magnetic materials.

💡 Research Summary

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In this work the authors address the long‑standing controversy over whether epitaxial RuO₂ films possess any form of magnetic order, a question that has become especially urgent after RuO₂ was proposed as a prototypical altermagnet. They combine X‑ray linear dichroism (XLD), photo‑emission electron microscopy (PEEM), and multiple‑scattering calculations to disentangle crystal‑field‑induced anisotropy from a possible antiferromagnetic contribution.

High‑quality RuO₂ thin films were grown on TiO₂ substrates with four different surface orientations