  The recent demonstration of MoSi2N4 and its exceptional stability to air,
water, acid, and heat has generated intense interest in this family of
two-dimensional (2D) materials. Among these materials, NbSi2N4, VSi2N4, and
VSi2P4 are semiconducting, easy-plane ferromagnets with negligible in-plane
magnetic anisotropy. They thus satisfy a necessary condition for exhibiting a
dissipationless spin superfluid mode. The Curie temperatures of monolayer
VSi2P4 and VSi2N4 are determined to be above room temperature based on Monte
Carlo and density functional theory calculations. The magnetic moments of
VSi2N4 can be switched from in-plane to out-of-plane by applying tensile
biaxial strain or electron doping.
