Confinement-induced altermagnetism in RuO₂ ultrathin films

The magnetic properties of bulk RuO₂remain a subject of active debate, despite its pivotal role in the emergence of altermagnetism. The latter is a novel paradigm in magnetic phases, characterized by the absence of net magnetization due to anti-parallel alignment of magnetic moments, yet displaying finite spin-splitting in the electronic band structure. This unique behavior unlocks opportunities for advanced applications in information technology devices. Recent experimental and theoretical investigations suggest that bulk RuO₂, contrary to prior assumptions, is non-magnetic. In this work, we propose the fabrication of RuO₂ultrathin films to robustly stabilize the altermagnetic phase. Unlike their bulk counterparts, ultrathin films up to about 2 nm thickness experience substantial strain relaxation, leading to a dramatic impact on the electronic structure that triggers a transition towards an altermagnetic behavior, which mimics the impact of an artificially applied Hubbard-<i>U</i>correction to account for electronic correlations. Our results imply that the surface sensitiveness of the probing experimental techniques is essential to sense the altermagnetic behavior emerging at RuO₂surfaces. Our findings promote the use and exploration of ultrathin films for the realization of spintronic devices based on altermagnets.