Magnetoresistance and microstructure of the sintered Ni doped Fe/sub 3/O/sub 4/ ferrites

Sintered Ni doped Fe/sub 3/O/sub 4/ ferrites were prepared by mixing Fe/sub 3/O/sub 4/ 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 Fe/sub 3/O/sub 4/ ferrites were investigated. X-ray diffraction and chemical titration analysis of Fe/sup 2+/ and Fe/sup 3+/ ions indicate that the nonstoichiometric phases of Fe/sub 3/O/sub 4+X/ and NiFe/sub 2/O/sub 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/spl deg/C and the optimum sintering time is about 9 h for all samples. After sintering at 1100/spl deg/C for 9 h, the MR value of the undoped Fe/sub 3/O/sub 4/ 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 /spl rho/ versus T/sup -1/2/ curves of all the samples exhibit a linear relationship from the measurement of the electrical resistivity (/spl rho/) 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 Fe/sub 2/O/sub 3/, NiFe/sub 2/O/sub 4/ or NiFe/sub 2/O/sub 4-Y/.