Probing the Néel Order in Altermagnetic RuO₂ Films via X-Ray Magnetic Linear Dichroism

The emerging altermagnetic RuO_2 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 M_3-edge X-ray magnetic linear dichroism (XMLD) measurements were performed to probe the Néel order in RuO_2. For epitaxial RuO_2 films, characteristic XMLD signals were observed for either RuO_2(100) or RuO_2(110) at normal incidence or RuO_2(001) at oblique incidence. The signals disappeared when the test temperature exceeded the Néel temperature. For non-epitaxial RuO_2 films, the flat lines in the XMLD patterns of RuO_2(100) and RuO_2(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 RuO_2 films and reveal the spatial relation of the Néel order parallel to the RuO_2 001 crystalline axis. These findings deepen the understanding of RuO_2 and other attractive altermagnetic materials used in the field of spintronics.