Probing the N\'eel order in altermagnetic RuO2 films by X-ray magnetic linear dichroism

TL;DR

This study uses X-ray magnetic linear dichroism to confirm antiferromagnetic Neel order in RuO2 films, revealing its orientation and dependence on epitaxial quality, advancing understanding of altermagnetic materials for spintronics.

Contribution

First direct experimental demonstration of Neel order in RuO2 films using XMLD, clarifying its spatial orientation and relation to crystal axes.

Findings

XMLD signals confirm antiferromagnetism in RuO2 films.

Neel order aligns parallel to RuO2 [001] axis.

No in-plane Neel order in non-epitaxial films.

Abstract

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 N\'eel 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 N\'eel 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 N\'eel order in these samples, due to the counterbalanced N\'eel 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 N\'eel 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.