  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.
