Novel Critical Temperature Resistor of Sintered Ni–Fe–O Nanosized Powders

Nanosized powders of Ni–Fe–O were synthesized by a pulsed wire discharge method and sintered at 600 °C for 1 h in air. Abrupt electrical resistivity changes were observed in the temperature dependence of resistivity for the sintered Ni–Fe–O powders above 203 °C. Similar resistivity curves, which had been observed in V–O samples and had been used for the critical temperature resistors, had never been reported in Ni–Fe–O samples. Possible mechanisms to explain the resistivity change in NiFe₂O₄, including order-disorder transition, semiconductor-metal transition, and surface spin pinning, are discussed.