Anomalous electrical transport properties of nonstoichiometric nickel ferrite below room temperature

The transport properties of nonstoichiometric nickel ferrite nanoparticles synthesised by the coprecipitation method followed by mechanical milling is reported here. The particle size of ferrite phase in the ball milled samples is found to be ranging from similar to 3.5 nm to similar to 14 nm but in the un-milled sample it becomes similar to 75 nm. A minimum in the conductivity has been observed in dc conductivity versus temperature variation while the activation energies of all the samples show an increasing trend with increasing milling time. The alternating current conductivity has been described by power law sigma'(f,T)alpha f(s)T(n). The frequency exponent 's' shows anomalous behavior, while the magnitude of the temperature exponent 'n' strongly depends on frequency. The dc and ac magnetoresistivities have been observed to be negative. Although the grain boundary contribution is predominated over grain contribution, the magnitude of both grain and grain boundary resistances reduce to lower value under the application of magnetic field. (C) 2011 Elsevier Ltd. All rights reserved.