Frequency dependence of the complex impedances and dielectric behaviour of some Mg-Zn ferrites

Complex impedances of some magnesium-zinc ferrites, MgxZn1-xFe2O4 (x = 0.4, 0.5, 0.6 and 0.7), in the frequency range of 1 mHz-10 MHz were measured at 300 K. The complex-plane impedance spectrum from each sample indicates that the capacitive and the resistive properties of the materials are mainly attributed to the processes that associated with the bulk (grain) and the grain boundary. A low-frequency dispersion (LFD) and a negative capacitance regions are also observed at low frequencies. A small series resistance of about the same value for all samples is also observed. The bulk and the grain boundary components are analysed by assuming a two-layer leaky capacitor. The simulated complex impedances using the Cole-Cole expression are in agreement with the measured values. The dielectric properties of the materials are attributed to the interfacial polarization and LFD at low frequencies and the orientational polarization at high frequencies. These properties result in the dielectric loss which composed of a Debye-like loss peak at high frequency end and a strong LFD below 1 Hz. Both regions exhibit universality with regard to the power-law relation between the dielectric loss (epsilon(r) '' or chi '') and frequency as given by epsilon(r) '' proportional to omega(n-1), where n = 0.14 and n = 0.02, respectively. The exponent is marginally negative, n = -0.03, in the region of negative capacitance below 3-4 mHz. The variations of the above properties and the characteristic parameters for different compositions of the Mg-Zn ferrites are discussed.