Probing the Néel order in altermagnetic RuO₂ films by X-ray magnetic linear dichroism

The emerging altermagnetic RuO₂ with both compensated magnetic moments and broken time-reversal symmetry possesses nontrivial magneto-electronic responses and nonrelativistic spin currents, which are closely related to magnetic easy axis. To probe the Néel order in RuO₂, we conducted Ru M₃-edge X-ray magnetic linear dichroism (XMLD) measurement. For epitaxial RuO₂ films, characteristic XMLD signals can be observed in either RuO₂(100) and RuO₂(110) at normal incidence or RuO₂(001) at oblique incidence, and the signals disappear when test temperature exceeds Néel temperature. For nonepitaxial RuO₂ films, the flat lines in the XMLD patterns of RuO₂(100) and RuO₂(110) demonstrate that there is no in-plane uniaxial alignment of Néel order in these samples, due to the counterbalanced Néel order of the twin crystals evidenced by X-ray diffraction phi-scan measurements. Our experimental results unambiguously demonstrate the antiferromagnetism in RuO₂ films and reveal the spatial relation of Néel order to be parallel with RuO₂ 001 crystalline axis. These research findings would deepen our understanding of RuO₂ and other attractive altermagnetic materials applied in the field of spintronics.