Intrinsic magnetism in two-dimensional Cr₂X₂Se (X = I, Br, Cl) monolayers

A novel family of two-dimensional Cr2X2Se (X = I, Br, Cl) monolayers with intrinsic magnetism has been predicted using first-principles calculations. The monolayer with the five atomic sublayers has two structures: structure I with ABCAB-type atomic sublayers and structure II with ABCBA-type atomic sublayers. It is found that the lowest energy states are ferromagnetic (FM) half metal for the structure I, and antiferromagnetic (AFM) semiconductor for the structure II, respectively. Phonon spectrum calculations and molecular dynamics simulations show that these lowest energy magnetic states are stable. Monte Carlo simulations based on the Ising model show that the Curie temperatures are about 94, 156 and 93 K for Cr2I2Se, Cr2Br2Se and Cr2Cl2Se monolayers in the structure I, respectively, which are above 77 K of the liquid-nitrogen transition temperature. The large magnetocrystalline anisotropy energies of about 1879, 68, 28 μeV per Cr atom for Cr2X2Se (X = I, Br, Cl) monolayers in the structure I show their importance in magnetic memory and storage. The magnetization direction is out of plane in Cr2I2Se and Cr2Br2Se, but is in plane in Cr2Cl2Se. Our study implies a new route in 2D electronics and spintronics.