Probing the Néel Order in Altermagnetic RuO₂ Films via 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 that are closely related to the magnetic easy axis. Ru M₃-edge X-ray magnetic linear dichroism (XMLD) measurements were performed to probe the Néel order in RuO₂. For epitaxial RuO₂ films, characteristic XMLD signals were observed for either RuO₂(100) or RuO₂(110) at normal incidence or RuO₂(001) at oblique incidence. The signals disappeared when the test temperature exceeded the Néel temperature. For non-epitaxial 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 the Néel order in these samples because of the counterbalanced Néel order of the twin crystals, as evidenced by X-ray diffraction phi-scan measurements. Our experimental results unambiguously demonstrate antiferromagnetism in RuO₂ films and reveal the spatial relation of the Néel order parallel to the RuO₂ 001 crystalline axis. These findings deepen the understanding of RuO₂ and other attractive altermagnetic materials used in the field of spintronics.