Orbital ordering as the determinant for ferromagnetism in biferroic BiMnO₃

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 X² = 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.