Dimensionality-driven in-plane antiferromagnetism in two-dimensional SrRuO3

Abstract

Control of dimensionality is a powerful tool to unlock hidden electronic and magnetic phases. In particular, reduced dimensionality in strongly correlated oxides leads to an intriguing ``dead-layer'' behavior that a transition from (ferromagnetic) metal to (antiferromagnetic) insulator occurs as the thickness approaches the two-dimensional limit. However, the origin of such transition has been a subject of debate that both the intrinsic dimensionality effect and the extrinsic disorder effect are proposed to be the driving force. Here, we reveal a transition from ferromagnetic metal with perpendicular magnetic anisotropy to antiferromagnetic insulator with in-plane magnetic anisotropy in ${\mathrm{SrRuO}}_{3}$ epitaxial films down to the monolayer limit. Experimentally, we demonstrate that ${\mathrm{SrRuO}}_{3}$ films below 3 unit cells become magnetic insulators and the spin easy axis changes to the in-plane ⟨110⟩ directions. First-principles calculations reveal that the interplay of the orbital-selective quantum confinement on Ru $4d$ orbitals and the oxygen octahedral rotation drives the ultrathin films from ferromagnetic metal to antiferromagnetic insulator, reorienting Ru spins from the perpendicular to the ⟨110⟩ directions. Our findings demonstrate how reduced dimensionality can tailor the magnetic state and provide significant advances in one of the debated topics in complex oxide heterostructures that dimensionality effect alone can be the driving force of dead-layer phenomenon in two-dimensional systems.