Emergence of Unconventional Magnetic Order in Strain-Engineered RuO₂/TiO₂ Superlattices

The spin ordering in RuO₂ remains a highly debated topic, owing to its elusive nature, with reports ranging from a nonmagnetic ground state to signatures of unconventional magnetic order. Here we provide the first unambiguous and direct evidence of unconventional magnetism in epitaxial, fully strained RuO₂/TiO₂ superlattices on a TiO₂ (110) substrate by using hybrid molecular beam epitaxy. Polarized neutron reflectometry reveals a finite magnetic moment localized within the compressively strained RuO₂ layers, consistent with predictions obtained from first-principles calculations. Complementary density functional theory calculations and x-ray photoemission spectroscopy measurements show that epitaxial strain drives the Ru 4d states toward the Fermi level, triggering a Stoner-type instability that stabilizes uncompensated magnetic order. These unique results reveal that RuO₂ exhibits unconventional magnetic states under epitaxial strain, which are not accessible in bulk RuO₂, and establish strain engineering as a powerful route to uncover and control magnetic phases in RuO₂ and related oxides.