Magnetoresistance and microstructure of the sintered Ni doped Fe3O4 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 Fe²⁺ and Fe³⁺ ions indicate that the nonstoichiometric phases of Fe₃O_4+X and NiFe₂O_4-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^-12/ 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 NiFe₂O_4-Y.