Emergent Interfacial Magnetism in Epitaxial RuO₂

The magnetic ground state of the altermagnet candidate RuO₂ remains controversial, with magnetic signatures observed mainly in epitaxial films. Here we show, using first-principles calculations, that magnetism in epitaxial RuO₂ can emerge as an interfacial boundary phase at TiO₂/RuO₂ interfaces. While TiO₂-induced epitaxial strain alone does not make (001)-oriented RuO₂ magnetic, explicit TiO₂/RuO₂ interfaces stabilize sizable Ru moments confined to the first few Ru layers. Charge-density and orbital-resolved analyses reveal interfacial electronic reconstruction, and substrate doping provides a route to tune the induced moments. In symmetric TiO₂/RuO₂/TiO₂ heterostructures, the two magnetic interfaces couple through the metallic RuO₂ spacer, producing a thickness-dependent alternation between weak-ferromagnetic and compensated altermagnetic states for even RuO₂ thicknesses. Our results identify interface engineering as a practical route to stabilize and control fragile magnetism in RuO₂.