Room-temperature bipolar ferrovalley semiconductors and anomalous valley Hall effect in Janus CeClI and CeBrI

Ferrovalley materials with perpendicular magnetic anisotropy (PMA), high Curie temperature (T_C) and large valley polarization are very crucial to spintronic and valleytronic applications. We herein propose two 2D f-electron Janus CeClI and CeBrI with strong spin-orbit coupling, and use first-principles calculations, Monte Carlo simulations and Wannier functions to investigate the magnetic anisotropy, magnetic transition temperature, valley characteristic and anomalous valley Hall effect (AVHE). Both CeClI and CeBrI monolayers are found to be above-room-temperature ferromagnetic semiconductors with T_Cs of 468 and 418 K, respectively, which are robust to biaxial strain. Monolayer CeClI possesses the PMA and the spontaneous bipolar valley polarization of 54.5 (75.4) meV at the valence (conduction) band. Monolayer CeBrI exhibits the in-plane magnetic anisotropy (IMA), but a very slight compressive biaxial strain (less than −1 %) can induce the IMA-to-PMA transition with the bipolar valley polarization of 61.8 (80.2 meV) at the valence (conduction) band under −1 % compressive strain. The compressive strain increases the PMA mainly contributed by Ce-d and I-p electrons, and decreases the valley polarization due to the weakened spin-orbit coupling, but the valley polarization is still considerable. Interestingly, under only −1 % (−2 %) compressive strain, the valley locates at the valence band maximum, leading to the AVHE for CeBrI (CeClI). Our present work highlights the robust high T_C, large bipolar valley polarization and AVHE in 2D CeClI and CeBrI, which will stimulate extensive studies on f-electron Janus ferrovalley systems for spintronic and valleytronic applications.