Spin-valley coupling enhanced high-TC ferromagnetism in a non-van der Waals monolayer Cr2Se3 on graphene
C. W. Chuang, Tappei Kawakami, K. Sugawara, K. Nakayama, S. Souma, M. Kitamura, Kenta Amemiya, Koji Horiba, Hiroshi Kumigashira, Geoffroy Kremer, Y. Fagot‐Révurat, D. Malterre, Chiara Bigi, F. Bertran, F. H. Chang, H. J. Lin, Chien‐Te Chen, T. Takahashi, A. Chainani, T. Sato
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. In most van der Waals ferromagnets, reducing the number of layers reduces the Curie temperature. Here, Chuang et al., find that Cr2Se3 has an increased Curie temperature for thinner samples, and through angle-resolved photoemission spectroscopy they attribute this to differences in the valley carrier density in different thickness samples.