|
内容記述 |
Ruthenium dioxide (RuO2) has recently emerged as a prototypical altermagnetic material, characterized by long-range antiferromagnetic order accompanied by large momentum-dependent spin splitting in its electronic structure. Despite the absence of net magnetization, altermagnets retain spin-polarized electronic states, offering advantages such as robustness against stray magnetic fields and ultrafast spin dynamics.1) These properties make RuO2 a promising candidate for antiferromagnetic spintronics, particularly when high-quality epitaxial films with controlled domain structures can be realized. Due to the threefold symmetry of the c-Al2O3 (0001) surface and the rutile structure of RuO2, a triple-domain epitaxial structurehas been expected; however, direct experimental confirmation has not been reported so far.In this study, we investigate the epitaxial growth and magnetic properties of RuO2 thin films on c-plane sapphire substrates. RuO2 films with a thickness of 50 nm were grown on c-Al2O3 (0001) substrates by reactive sputtering at 300 °C. Using plane-view scanning transmission electron microscopy (STEM) combined with inverse fast Fourier transform (iFFT) analysis, we successfully visualized the triple-domain structure. The iFFT images clearly resolve three rotational domains and their spatial distribution, providing direct evidence of the predicted triple-domain epitaxial relationship between RuO₂ and the c-Al2O3 (0001) substrate. The antiferromagnetic order was further investigated by X-ray magnetic linear dichroism (XMLD) measurements. XMLD experiments were performed at the Ru M3,2 edge at KEK (PF, BL16) and at the Ru L3,2 edge at Nanoterasu (BL 13U). No XMLD signal was observed under normal-incidence geometry. This result is attributed to the averaging effect caused by the X-ray beam size being significantly larger than the individual domain size, leading to mutual cancellation of the Néel vectors from different domains. In addition, first-principles calculations that explicitly account for spin-polarized core-level transitions were performed to simulate the XMLD spectra. These results provide insight into the interplay between epitaxial domain structures and the magnetic response of RuO2, contributing to a deeper understanding of altermagnetic films.Acknowledgments: This work was partly supported by the KAKENHI (Nos. 23H03803 and 24K01346).H.N. was partly supported by Nagoya University Program for Research Enhancement. XMLD measurements were performed under proposal No. 24G640.Reference 1) N.T. V. Anh, H. Naganuma et al., Adv. Sci., 12, 2413165 (2025). |
