Spin-valley coupling enhanced high-TC ferromagnetism in a non-van der Waals monolayer Cr2Se3 on graphene

Abstract

Spin-valley magnetic ordering is restricted to layered van der Waals type transition-metal dichalcogenides with ordering temperatures below 55 K. Recent theoretical studies on non-van der Waals structures have predicted spin-valley polarization induced semiconducting ferromagnetic ground states, but experimental validation is missing. We report high-Curie temperature (TC ~ 225 K) metallic ferromagnetism with spontaneous spin-valley polarization in monolayer Cr2Se3 on graphene. Angle-resolved photoemission spectroscopy (ARPES) reveals systematic temperature-dependent energy shifts and splitting of localized Cr 3 d↑-t2g bands, accompanied by occupancy of the itinerant Cr 3d-eg valleys. The t2g-eg spin-valley coupling at the K/K’ points of hexagonal Brillouin zone leads to ferromagnetic ordering. Circular dichroism in ARPES shows clear evidence of spin-valley polarized states. Comparison with bilayer and trilayer Cr2Se3 reveals the crucial role of valley carrier density in enhancing TC and provides a guiding principle to realize 2D ferromagnetism at higher temperatures in non-van der Waals materials.