Intrinsic valley polarization in 2D magnetic MXenes: surface engineering induced spin-valley coupling

Abstract

Generating valley polarization for valleytronics applications requires breaking the inversion symmetry of two-dimensional (2D) hexagonal crystals. As such, 2D MXenes naturally lose their inversion symmetry upon surface functionalization, thereby opening up new design opportunities for controlling their physical and chemical properties. However, no spin-valley couplings have been proposed for magnetic MXenes thus far. Herein, we demonstrate that surface engineering not only breaks the inversion symmetry of 2D MXenes but also induces valley polarizations with diverse magnetic orders, including ferromagnetism, ferrimagnetism and antiferromagnetism. Using Cr₂C-based MXenes as prototypes, our theoretical calculations showed that Janus MXenes Cr₂COX (X = F, Cl and OH) are excellent candidates for ferrovalley materials, especially Cr₂COF, which has a strong valley polarization of 334 meV and a high Curie temperature of 1146 K. Concurrently, Cr₂C-based MXenes with mixed functionalizations (Cr₂CO_0.75F_1.25 and Cr₂CO_1.25F_0.75) displayed properties of ferrivalley and ferrovalley semiconductors, with 11 and 15 meV valley splitting, respectively. In other magnetic MXenes, surface engineering also induced valley properties, as shown by the new bipolar antiferrovalley identified in Cr₂TiC₂FCl MXene. Therefore, our study is the first proposal of an experimentally viable approach (i.e., surface engineering) for generating valley polarization in 2D MXenes by breaking their inversion symmetry while simultaneously providing a computational paradigm for probing other 2D nanomaterials with potential applications in valleytronics.