Altermagnetism refers to a wide class of magnetic orders featuring magnetic sublattices with opposite spins related by rotational symmetries, resulting in non-trivial spin splitting and magnetic multipoles. However, the direct observation of the altermagnetic order parameter remains elusive. Here, by combining theoretical analysis, electrical transport, X-ray and optical spectroscopies, we establish a phase diagram in hybrid molecular beam epitaxy-grown RuO2/TiO2 (110) films, mapping symmetries along with altermagnetic/electronic/structural phase transitions as functions of film thickness and temperature. This features a novel altermagnetic metallic polar phase in epitaxially-strained 2 nm films, extending the concept of multiferroicity to altermagnets. Such a clear signature of a magnetic phase transition at ~500 K is observed exclusively in ultrathin strained films, unlike in bulk RuO2 single crystals. These results demonstrate the potential of epitaxial heterostructure design to induce altermagnetism, paving the way for emergent novel phases with multifunctional properties.
@article{arxiv.2405.05838,
title = {Altermagnetic Polar Metallic phase in Ultra-Thin Epitaxially-Strained RuO2 Films},
author = {Seung Gyo Jeong and In Hyeok Choi and Sreejith Nair and Luca Buiarelli and Bita Pourbahari and Jin Young Oh and Nabil Bassim and Daigorou Hirai and Ambrose Seo and Woo Seok Choi and Rafael M. Fernandes and Turan Birol and Liuyan Zhao and Jong Seok Lee and Bharat Jalan},
journal= {arXiv preprint arXiv:2405.05838},
year = {2025}
}