Strain-explorable valley-polarized topological phase transition and perpendicular magnetocrystalline anisotropy in hexagonal M ClBr ( M = Ru, Os) monolayers

The coexistence of ferrovalley and band topology has garnered significant attention, due to its valley-polarized quantum anomalous Hall effect in condensed matter physics for two-dimensional (2D) ferromagnetic materials, in which the spontaneous out-of-plane magnetization and strong spin-orbit coupling effect are both satisfied. Here, using first-principles calculations and a tight-binding model, we reported the valley-polarized band topological phase transition and perpendicular magnetocrystalline anisotropy in a series of 2D Janus magnetic transition metal dihalides $H\ensuremath{-}M\mathrm{ClBr}$ ($M$ = Ru, Os) via strain manipulation. Due to both the inversion and time-reversal asymmetry, valley polarization could be spontaneously achieved in hexagonal RuClBr and hexagonal OsClBr monolayers, with considerable values of 247 and 150 meV, respectively, which exceed those of most reported 2D ferrovalley materials. By applying an external biaxial strain, it is noteworthy that the easy magnetization axis of $H$-RuClBr can be switched to the out-of-plane direction due to distinct spin channel interactions between the Ru $d$ orbitals. Moreover, transitions from the anomalous valley Hall effect to the nontrivial topological state in both $H$-RuClBr and $H$-OsClBr are observed. Due to the spontaneous valley polarization, band topology phase transition, and perpendicular magnetocrystalline anisotropy, it is expected that 2D Janus magnetic transition metal dihalides expand the application for the interplay among valleytronics, spintronics, and band topology.