Emergent Interfacial Magnetism in Epitaxial RuO${}_2$

The magnetic ground state of the altermagnet candidate RuO${}_2$ remains controversial, with magnetic signatures observed mainly in epitaxial films. Here we show, using first-principles calculations, that magnetism in epitaxial RuO${}_2$ can emerge as an interfacial boundary phase at TiO${}_2$/RuO${}_2$ interfaces. While TiO${}_2$-induced epitaxial strain alone does not make (001)-oriented RuO${}_2$ magnetic, explicit TiO${}_2$/RuO${}_2$ 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 TiO${}_2$/RuO${}_2$/TiO${}_2$ heterostructures, the two magnetic interfaces couple through the metallic RuO${}_2$ spacer, producing a thickness-dependent alternation between weak-ferromagnetic and compensated altermagnetic states. Our results identify interface engineering as a practical route to stabilize and control fragile magnetism in RuO${}_2$.