Anomalous electrical transport properties of nonstoichiometric nickel ferrite below room temperature

The transport properties of nonstoichiometric nickel ferrite nanoparticles synthesised by the co-precipitation 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 ∼3.5 nm to ∼14 nm but in the un-milled sample it becomes ∼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 σ′(f,T) ∝ fsTn. 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.

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► Ball milling introduces deformations and results in lattice expansion.
► Conductivity minimum was observed at Tmin, which decreases with milling.
► The values of activation energy increase from 63 to 131 meV with milling.
► Magnitude of the temperature exponent ‘n’ of σ′(f,T) strongly depends on frequency.
► The total resistance due to grain and grain boundary decreases with magnetic field.