Spontaneous spin and valley polarizations in novel RuO2(MgF)2 and RuO2(ZnF)2 ferrovalley semiconductors with ultrahigh Curie temperatures

Abstract

Two-dimensional room-temperature (RT) ferrovalley semiconductors with spontaneous spin and valley polarizations hold enormous potential in next-generation high-speed and low-power dissipation information technology. By first-principles calculations and Monte Carlo simulations, we discover that the RuO${}_2$(MgF)${}_2$ and RuO${}_2$(ZnF)${}_2$ monolayers with viable fabrication are right such extremely scarce candidate materials. In particular, both RuO${}_2$(MgF)${}_2$ and RuO${}_2$(ZnF)${}_2$ are predicted to be Ising-type ferromagnets with ultrahigh Curie temperatures of 1084 and 972 K, respectively. Due to the interplay between spin–orbital coupling and out-of-plane ferromagnetism, the hexagonal RuO${}_2$(MgF)${}_2$/RuO${}_2$(ZnF)${}_2$ motif spontaneously produces ultraclean valley polarizations as sizable as 195/194 meV in their topmost valence bands, which are further corroborated by a four-band k $\cdot$ p model. More importantly, these two materials remain as above-RT Ising ferromagnets under both hole doping and biaxial strains within experimental reach. The anomalous valley Hall effect can be naturally achieved in both systems through doping moderate holes and infrared light irradiation upon applying an in-plane electric field. Also, we find that the spin and valley polarizations can be inverted via compressive strain and magnetization flipping. These outstanding attributes expose vast potential of both RuO${}_2$(MgF)${}_2$ and RuO${}_2$(ZnF)${}_2$ for a broad range of potential applications in straintronics, spintronics, valleytronics, optoelectronics and their integration.