First-principles study of ferroelectricity induced by p-d hybridization   in ferrimagnetic NiFe2O4
  We investigate the ferrimagnetism and ferroelectricity of bulk NiFe$_2$O$_4$ with tetragonal $P4_122$ ~symmetry by means of density functional calculations using generalized gradient approximation + Hubbard $U$ approach. Special attention is paid to finding the most energetically favorable configuration on magnetic ordering and further calculating the reliable spontaneous electric polarization. With the fully optimized crystalline structure of the most stable configuration, the spontaneous polarization is obtained to be 23 $\mu$C/cm$^2$ along the z direction, which originates from the hybridization between the 3d states of the Fe$^{3+}$ cation and the 2p states of oxygen induced by Jahn-Teller effect. 
Un-Gi Jong, Chol-Jun Yu, Yong-Su Park, Chong-Suk Ri
cond-mat.mtrl-sci