Confinement-induced altermangetism in RuO2 ultrathin films

The magnetic properties of bulk RuO2 remain a subject of active debate, despite its pivotal role in the emergence of altermagnetism. The latter is a novel paradigm in magnetic phases, charac- terized 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 RuO2 ultrathin films to robustly stabilize the altermag- netic phase. Unlike their bulk counterparts, ultrathin films up to about 2nm 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-U 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 RuO2 surfaces. Our findings promote the use and exploration of ultrathin films for the realization of spintronic devices based on altermagnets.