  Identifying intrinsic low-dimensional ferromagnets with high transition
temperature and electrically tunable magnetism is crucial for the development
of miniaturized spintronics and magnetoelectrics. Recently long-range 2D
ferromagnetism was observed in van der Waals crystals CrI$_3$ and
Cr$_2$Ge$_2$Te$_6$, however their Curie temperature is significantly lowered
when reducing down to monolayer/few layers. Herein, using renormalized
spin-wave theory and first-principles electronic structure theory, we present a
theoretical study of electrically tunable 2D ferromagnetism in van der Waals
layered CrSBr and CrSeBr semiconductors with high Curie temperature of ~150K
and sizable band gap. High transition temperature is attributed to strong
anion-mediated superexchange interaction and a sizable spin-wave excitation gap
due to large exchange and single-ion anisotropy. Remarkably, hole and electron
doping can switch magnetization easy axis from in-plane to out-of-plane
direction. These unique characteristics establish monolayer CrSBr and CrSeBr as
promising platform for realizing 2D spintronics and magnetoelectrics such as 2D
spin field effect transistor.
