Evolution of structure, magnetic and transport properties of sputtered films from Fe to Fe3O4

Polycrystalline iron oxide films were fabricated using the reactive sputtering method without substrate heating. Structure characterization indicates that the dominant phases in the films evolve from α-Fe to pure Fe3O4 with the increasing O2 flow rate. In polycrystalline Fe3O4 films, disordered atoms exist at the grain boundaries. Magnetic properties analyses reveal that the room-temperature magnetization first decreases and later increases due to the variation of the volume fraction of the paramagnetic FeO phase with a Neel temperature of 198 K. The magnetoresistance MR (= [R(H) − R(0)]/R(0)) of the films increases from 0.1% for pure Fe film to 10.6% for the Fe3O4 film at 80 K under a 90 kOe field. The transport mechanism of FeO–Fe3O4 and Fe3O4 films is suggested to be the tunnelling process, which satisfies the log ρ ~ T−1/2 relation. The Hall resistivity of the Fe3O4 film decreases with increasing temperature. The ordinary and extraordinary Hall coefficients of the Fe3O4 film at 300 K are about 100 and 420 times larger than those of bulk Fe.