Orbital ordering as the determinant for ferromagnetism in biferroicBiMnO3

Abstract

The ferromagnetic structure of ${\mathrm{BiMnO}}_{3},$ ${T}_{c}=105 \mathrm{K},$ has been determined from powder neutron-diffraction data collected at 20 K on a sample synthesized at high pressures using a cubic anvil press. ${\mathrm{BiMnO}}_{3}$ is a distorted perovskite that crystallizes in the monoclinic space group $C2$ with unit-cell parameters $a=9.5317(7) \AA{},$ $b=5.6047(4) \AA{},$ $c=9.8492(7) \AA{},$ and $\ss{}=110.60(1)\ifmmode^\circ\else\textdegree\fi{}$ $(Rp=6.78%,$ $wRp=8.53%,$ reduced ${\ensuremath{\chi}}^{2}=1.107).$ Data analysis reveals a collinear ferromagnetic structure with the spin direction along [010] and a magnetic moment of $3.2\ensuremath{\mu}B.$ 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 ${\mathrm{BiMnO}}_{3}.$ Careful examination of the six unique Mn-O-Mn superexchange pathways between the three crystallographically independent ${\mathrm{Mn}}^{3+}$ sites shows that four are ferromagnetic and two are antiferromagnetic, thereby confirming that the ferromagnetism of ${\mathrm{BiMnO}}_{3}$ stems directly from orbital ordering.