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Enhanced Magnetization and Conductivity in NiFe2O4

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.