Microstructure, magnetic and magneto-transport properties of polycrystalline Fe3O4 films

The structure, magnetic and magnetoresistance properties of polycrystalline Fe3O4 films have been examined. It has been found that Fe3O4 grains are surrounded by disordered atoms formed by lattice mismatch of adjacent grains. The exchange bias has been observed because of the pinning effect at the grain boundaries. The exchange bias field becomes lower when the number of spins frozen along the cooling field direction is reduced. The Verwey transition has been observed at ∼103 K, being lower than that of the bulk Fe3O4 at 125 K due to the structure defects. This phase transition is also confirmed by magnetoresistance measurements as the MR{=[R(H)-R(0)]/R(0)}MR{=[R(H)-R(0)]/R(0)} reaches the maximum at ∼100 K. The transport mechanism is tunneling between Fe3O4 grains, satisfying the logρ∼T-1/2logρ∼T-1/2 relation. The alignment of the moments at grain boundaries affects the MR significantly, which is responsible for the weak saturation of the high-field magnetoresistance.