Enhanced spin-dependent tunneling magnetoresistance in magnetite films coated by polystyrene

Hematite films were deposited by magnetron sputtering. A phase transformation from hematite to magnetite occurred when polystyrene PS coated hematite films were annealed above 200 °C in hydrogen flow. Giant negative magnetoresistance MR was observed with the best MR ratio of over 8% at room temperature and in a field of 5.5 T found in samples annealed at 230 °C. The temperature dependence of the resistivity is characteristic of intergranular tunneling. After the PS layer was removed and the films annealed again at 230 °C in hydrogen flow, the resistivity increased by about one order of magnitude and the MR ratio decreased to 4.3%. These data show that PS coating layer can protect magnetite films from oxidation and enhance interganular spin-dependent tunneling magnetoresistance.