Alternate current conductivity and dielectric properties of nonstoichiometric nanocrystalline Mg–Zn ferrite below room temperature

Nanocrystalline Mg–Zn ferrite is prepared by ball milling the stoichiometric powder mixture of MgO, ZnO and α-Fe2O3. The ternary Mg–Zn ferrite phase may not be obtained by heat treatment of the stoichiometric powder mixture of the oxides. The particle size of ferrite phase in the ball milled samples remains almost unaltered (∼3 nm) and that of in annealed samples increases up to ∼330 nm. Alternate current conductivity and dielectric properties of nanocrystalline Mg–Zn ferrite in the temperature range 77≤T≤300 K, applying magnetic field up to 1 T in the frequency range 20 Hz–1 MHz, are being reported. The frequency dependence of conductivity has been described by power law σ′(f,T)∝fs and the frequency exponent ‘s’ is found to be anomalous temperature dependent. A detailed analysis of the temperature dependence of the universal dielectric response parameter ‘s’ revealed that the correlated barrier hopping is the dominating charge transport mechanism. The real part of the dielectric permittivity at a fixed frequency was found to follow the power law ε′(f,T)∝Tn and the magnitude of the temperature exponent ‘n’ strongly decreases with increasing frequency. The dielectric permittivity shows a large degree of dispersion at low frequency, but rapid polarization at high frequencies, which can be interpreted by the Maxwell-Wagner capacitor model. The temperature dependence of resistance due to grain and grain boundary contribution exhibits two activation regions. The observed positive ac conductivity may be due to the change of grain and grain boundary resistances caused by the magnetic field.