The Microscopic Model of BiFeO$_3$

Many years and great effort have been spent constructing the microscopic model for the room temperature multiferroic BiFeO3 However, earlier models implicitly assumed that the cycloidal wavevector q was confined to one of the three-fold symmetric axis in the hexagonal plane normal to the electric polarization. Because recent measurements indicate that q can be rotated by a magnetic field, it is essential to properly treat the anisotropy that confines q at low fields. We show that the anisotropy energy $-K_3 S^6 \sin^6 \theta \cos 6 \phi$ confines the wavevectors q to the three-fold axis $\phi =0$ and $+-2 \pi/3$ within the hexagonal plane with $\theta = \pi /2$.