Magnetoresistance and microstructure of the sintered Ni doped Fe ₃O₄ ferrites

Sintered Ni doped Fe3O4 ferrites were prepared by mixing Fe3O4 powder with NiO powder and compressing into pellet, then sintering in argon atmosphere. The contents of Ni in the sintered samples were between 0 at.% and 5.08 at.%. The effects of the Ni content, sintering temperature and sintering time on the magnetoresistance (MR) and microstructure of sintered Fe3O4 ferrites were investigated. X-ray diffraction and chemical titration analysis of Fe 2+ and Fe3+ ions indicate that the nonstoichiometric phases of Fe3O4+X and NiFe2O4-Y were co-existed in the Ni doped sample. The optimum sintering temperature, at which the maximum MR value could be obtained, is around 1100°C and the optimum sintering time is about 9 h for all samples. After sintering at 1100°C for 9 h, the MR value of the undoped Fe3O4 ferrite is about 6% at room temperature. Maximum MR value is about 7.5% when 1.12 at.% Ni was doped. It was found that the log ρ versus T-1/2 curves of all the samples exhibit a linear relationship from the measurement of the electrical resistivity (ρ) of the sintered samples between 80 K and room temperature. This implies that the MR effect is mainly spin-dependent tunneling where the electrons flow through insulating barriers of Fe2O3, NiFe2O4 or NiFe2O4-Y.