  We present the microscopic theory of improper multiferroicity in BiMnO3,
which can be summarized as follows: (1) the ferroelectric polarization is
driven by the hidden antiferromagnetic order in the otherwise centrosymmetric
C2/c structure; (2) the relativistic spin-orbit interaction is responsible for
the canted spin ferromagnetism. Our analysis is supported by numerical
calculations of electronic polarization using Berry's phase formalism, which
was applied to the low-energy model of BiMnO3 derived from the first-principles
calculations. We explicitly show how the electric polarization can be
controlled by the magnetic field and argue that BiMnO3 is a rare and
potentially interesting material where ferroelectricity can indeed coexist and
interplay with the ferromagnetism.
