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.