Orbital ordering as the determinant for ferromagnetism in biferroic BiMnO₃

Abstract

The ferromagnetic structure of BiMnO₃, T_c=105 K, has been determined from powder neutron-diffraction data collected at 20 K on a sample synthesized at high pressures using a cubic anvil press. BiMnO₃ is a distorted perovskite that crystallizes in the monoclinic space group C2 with unit-cell parameters a=9.5317(7) Å, b=5.6047(4) Å, c=9.8492(7) Å, and β=110.60(1)° (Rp=6.78%, wRp=8.53%, reduced χ²=1.107). Data analysis reveals a collinear ferromagnetic structure with the spin direction along [010] and a magnetic moment of 3.2μB. There is no crystallographic phase transition on cooling the polar room-temperature structure to 20 K, lending support to the belief that ferromagnetism and ferroelectricity coexist in BiMnO₃. Careful examination of the six unique Mn-O-Mn superexchange pathways between the three crystallographically independent Mn³⁺ sites shows that four are ferromagnetic and two are antiferromagnetic, thereby confirming that the ferromagnetism of BiMnO₃ stems directly from orbital ordering.