Structure and magnetic properties of polycrystalline Fe3O4 films deposited by reactive sputtering at room temperature

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature, which is much lower than the reported temperature of above 500 K to deposit high quality Fe3O4 films. Transmission electron microscopy image shows that uniform Fe3O4 grains are well separated by grain boundaries and their size increases with film thickness. The magnetization of the Fe3O4 films measured at 300 K under 50 kOe magnetic field is much lower than the reported value of 471 emu/cc, which can be interpreted as the existence of strong antiferromagnetic coupling within the grain boundaries. The coupling between frozen surface spins and core spins and the switch of surface spin configurations cause hysteresis loop shift below 200 K and high field irreversibility, respectively. The observed decrease of magnetization with film thickness can be explained as the reduction in magnetization caused by the increased volume fraction of surface layer and the grain boundaries.