Orbital ordering as the determinant for ferromagnetism in biferroicBiMnO3

Abstract

The ferromagnetic structure of ${\mathrm{BiMnO}}_3,$ $T_c=105\mathrm{}\hspace{0.5em}\mathrm{K},$ 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 $C2\mathrm{}$ with unit-cell parameters $a=9.5317\mathrm{}\left(7\right)\mathrm{}\hspace{0.5em}\AA ,$ $b=5.6047\mathrm{}\left(4\right)\mathrm{}\hspace{0.5em}\AA ,$ $c=9.8492\mathrm{}\left(7\right)\mathrm{}\hspace{0.5em}\AA ,$ and $ß=110.60\mathrm{}\left(1\right)\mathrm{}°$ $(Rp=6.78\%,$ $wRp=8.53\%,$ reduced ${\chi }^2=1.107).\mathrm{}$ Data analysis reveals a collinear ferromagnetic structure with the spin direction along [010] and a magnetic moment of $3.2\mu B.\mathrm{}$ 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.