  Based on a simple tight-binding model, we propose a general theory of
light-induced magnetic phase transition (MPT) in antiferromagnets based on the
general conclusion that the bandgap of antiferromagnetic (AFM) phase is usually
larger than that of ferromagnetic (FM) one in a given system. Light-induced
electronic excitation prefers to stabilize the FM state over the AFM one, and
once the critical photocarrier concentration ({\alpha}_c) is reached, an MPT
from AFM phase to FM phase takes place. This theory has been confirmed by
performing first-principles calculations on a series of two-dimensional (2D)
van der Waals (vdW) antiferromagnets and a linear relationship between
{\alpha}_c and the intrinsic material parameters is obtained. Importantly, our
conclusion is still valid even considering the strong exciton effects during
photoexcitation. Our general theory provides new ideas to realize reversible
read-write operations for future memory devices.
