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 orientation m of the field. 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 measured critical field along a cubic axis is about 19 T but H_c is predicted to vary by as much as 2.5 T above and below this value. The orientational dependence of H_c(m) is more complex than indicated by earlier work, which did not consider the competition between magnetic domains.
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