We argue that the centrosymmetric C2/c symmetry in BiMnO3 is spontaneously broken by antiferromagnetic (AFM) interactions existing in the system. The true symmetry is expected to be Cc, which is compatible with the noncollinear magnetic ground state, where the ferromagnetic order along one crystallographic axis coexists with the the hidden AFM order and related to it ferroelectric polarization along two other axes. The C2/c symmetry can be restored by the magnetic field B∼35 Tesla, which switches off the ferroelectric polarization. Our analysis is based on the solution of the low-energy model constructed for the 3d-bands of BiMnO3, where all the parameters have been derived from the first-principles calculations. Test calculations for isostructural BiCrO3 reveal an excellent agreement with experimental data.
@article{arxiv.0904.0348,
title = {Magnetic ground state and multiferroicity in BiMnO$_3$},
author = {I. V. Solovyev and Z. V. Pchelkina},
journal= {arXiv preprint arXiv:0904.0348},
year = {2009}
}