High magnetization and the high-temperature superparamagnetic transition with intercluster interaction in disordered zinc ferrite thin film

Magnetic properties have been investigated for a zinc ferrite thin film deposited on glass at a substrate temperature close to room temperature using a sputtering method. X-ray diffraction analysis reveals that the thin film consists of nanocrystalline ZnFe2O4 with the size of approximately 10 nm. The magnetization at 300 K as a function of the external magnetic field shows ferrimagnetic behaviour, and tends to be saturated to the high value of 32 emu g(-1) when the external magnetic field is higher than 30 kOe. It is considered that the preparation of the ZnFe2O4 thin film by the sputtering method, which involves very rapid cooling of vapour to form the solid-state phase, causes the random distribution of Zn2+ and Fe3+ ions in the spinel structure. In such a situation, Fe3+ ions occupy both octahedral and tetrahedral sites, and the strong superexchange interaction among them gives rise to ferrimagnetic properties accompanied with high magnetization. The static and dynamic magnetic responses, such as the frequency dependence of the linear ac susceptibility, the temperature dependence of the nonlinear ac susceptibility, the discrepancy between the zero-field-cooled (ZFC) and field-cooled dc magnetizations, and the relaxation of the ZFC magnetization, demonstrate that the magnetism of the present thin film is attributable to the superparamagnetism with the interaction among magnetic clusters. Spin freezing occurs at a temperature higher than room temperature (similar to325 K).