Theory of magnetic-field-induced critical end point in BiMn₂O₅

A recent experiment on the multiferroic BiMn₂O₅ compound under a strong applied magnetic field revealed a rich phase diagram driven by the coupling of magnetic and charge (dipolar) degrees of freedom. Based on the exchange-striction mechanism, we propose here a theoretical model with the intent to capture the interplay of the spin and dipolar moments in the presence of a magnetic field in BiMn₂O₅. Experimentally observed behavior of the dielectric constants, magnetic susceptibility, and the polarization is, for the most part, reproduced by our model. The critical behavior observed near the polarization reversal (P=0) point in the phase diagram is interpreted as arising from the proximity to the critical end point.