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

Abstract The emerging altermagnetic RuO 2 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 RuO 2 , we conducted Ru M 3 -edge X-ray magnetic linear dichroism (XMLD) measurement. For epitaxial RuO 2 films, characteristic XMLD signals can be observed in either RuO 2 (100) and RuO 2 (110) at normal incidence or RuO 2 (001) at oblique incidence, and the signals disappear when test temperature exceeds Néel temperature. For nonepitaxial 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 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 RuO 2 films and reveal the spatial relation of Néel order to be parallel with RuO 2 [001] crystalline axis. These research findings would deepen our understanding of RuO 2 and other attractive altermagnetic materials applied in the field of spintronics.