Emergence of unconventional magnetic order in strain-engineered RuO2/TiO2 superlattices

TL;DR

This study demonstrates that epitaxial strain in RuO2/TiO2 superlattices induces unconventional magnetic order, providing direct experimental evidence and theoretical insights into strain-driven magnetic phase emergence in these oxides.

Contribution

First direct evidence of strain-induced unconventional magnetism in RuO2/TiO2 superlattices, combining experimental and theoretical approaches.

Findings

Polarized neutron reflectometry confirms magnetic moments in strained RuO2 layers.

Strain shifts Ru 4d states toward the Fermi level, causing a Stoner instability.

Unconventional magnetic states are stabilized under epitaxial strain, not present in bulk.

Abstract

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 TiO2 (110) substrate grown by 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 and X-ray photoemission spectroscopy show that epitaxial strain drives the Ru 4d states toward the Fermi level, triggering a Stoner-type instability that stabilizes non-compensated magnetic order. These unique results reveal that RuO2 exhibits unconventional magnetic states under epitaxial strain, which are not accessible in bulk and establish strain engineering as a powerful route to uncover and control magnetic phases in RuO2 and related oxides.