Magnetic and electrical properties of epitaxial NiFe₂O₄ (001) films fabricated by reactive sputtering

Thin films of NiFe₂O₂ were fabricated on MgO (001) and MgAl₂O₂ (MAO) (001) substrates by reactive radio frequency magnetron sputtering and were evaluated with regards to their electrical and magnetic properties. A saturation magnetization of 285 emu/cm3 was obtained with a 30 nm thick film grown on an MAO substrate at an oxygen flow rate, Q, of 9 sccm. This enhanced magnetization was found to be due to a normal spinel arrangement at the interface. From the thickness dependence of the magnetization, the intrinsic magnetization was determined as 241 emu/cm3 with the enhancement region estimated to be around 3/4 of the lattice constant, thus providing 8 μB/f.u. The magnetization of those films grown on an MgO substrate exhibited remarkably smaller amplitudes to those of the MAO substrates. It is suggested that this substrate-dependent magnetization can be attributed to the number of antiphase boundaries. The resistivity was found to increase with Q, with a resistivity state in the same order as that of the bulk achieved with films prepared in which Q was greater than 10 sccm.