Orientation dependence of the critical magnetic field for multiferroic BiFeO₃

Multiferroic BiFeO₃ undergoes a transition from a distorted spiral phase to a G-type antiferromagnet above a critical field H_c that depends on the field orientation m. We show that H_c(m) has a maximum when oriented along a cubic diagonal parallel to the electric polarization P and a minimum in the equatorial plane normal to P when two magnetic domains with the highest critical fields are degenerate. The orientational dependence of H_c(m) is more complex than indicated by earlier work, which did not consider the competition between magnetic domains. Some recent measurements might be explained by a mixture of magnetic domains.