Variation in hopping conduction across the magnetic transition in spinel Mn_1.56Co_0.96Ni_0.48O₄ films

The temperature dependent dc resistivity of spinel Mn_1.56Co _0.96Ni_0.48O₄ (MCN) films is measured in the range of 130-304 K. The hopping exponent p of small polaron hopping conduction shows a clear variation from a value of 0.46 in the paramagnetic to 0.91 in the ferromagnetic phase. In order to explain such variation, a model is proposed where Gaussian distributed localized electron states gradually withdraw from the Hubbard band gap below the magnetic transition as a result of increased magnetic order. This correlation between hopping conduction and magnetic order in MCN films may provide a possible approach to fabricate the devices which couple magnetic and electronic properties in one unit.