Density functional study of weak ferromagnetism in a thick BiCrO ₃ film

Very thick films of BiCrO ₃ have been grown on a SrTiO ₃ substrate, maintaining a tetragonal lattice up to thicknesses of 14,000 Å. Assuming that we can treat films of this thickness as bulk crystals, we first calculated the experimentally undetermined atomic positions within the unit cell with the measured lattice constant of the film, and then relaxed the lattice constants along with the atomic positions. All of the calculations result in an antiferroelectric crystal with the Pnma space group with noncollinear Bi displacements. We find G-type antiferromagnetism with a spin-orbit induced weak ferromagnetic component; however, the weak ferromagnetic component can be canceled if the antiferromagnetic spins are oriented along a particular one of the three twofold rotation axes.