  Different configurations of magnetic orders and cation distributions for
NiFe2O4 are studied by the density functional based methods with the possible
inclusion of the on-site Coulomb interaction U. The lowest energy state is an
inverse spinel structure of tetragonal space group P4322 with Ni locating at
the octahedral site (B site) while the Fe distributing equally at the
tetrahedral (A site) and octahedral site and with antiparallel moments between
cations at A and B sites. However, a quadruple enhanced magnetization is
obtained for a structure with the same cation distribution as that of the
lowest energy state, i.e. still an inverse spinel structure, but with
antiparallel moments between Ni and Fe cations. Its energy is higher than that
of the ground state by about 0.4eV/fomula. Both phases are insulating. Another
phase with enhanced magnetization is a normal spinel structure with Ni and Fe
locating at A and B site respectively, i.e. corresponding to a structure of
cation inversion. Its electronic structure displays a conductive and nearly
half-metallic behavior. These two phases could be the possible candidates for
the experimentally observed NiFe2O4 thin films possessing considerably enhanced
magnetization but different electric properties, i.e. one insulating and the
other conductive with spin-polarized current.
