Dielectric loss, conductivity relaxation process and magnetic properties of Mg substituted Ni–Cu ferrites

Abstract

The dielectric properties, dc and ac electrical resistivities of Mg substituted Ni–Cu ferrites with general formula Ni_0.5Cu_0.5−xMg_xFe₂O₄ (0.0≤x≤0.5) have been investigated as a function of frequency, temperature and composition. ac resistivity of all the samples decreases with increase in the frequency exhibiting normal ferrimagnetic behavior. The frequency dependence of dielectric loss tangent showed a maximum in between 10 Hz and 1 kHz in all the ferrites. The conductivity relaxation of the charge carriers was examined using the electrical modulus formulism, and the results indicate the presence of the non-Debye type of relaxation in the prepared ferrites. Similar values of activation energies for dc conduction and for conductivity relaxation reveal that the mechanism of electrical conduction and dielectric polarization is the same in these ferrites. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates that the distribution of relaxation time is temperature independent. The saturation magnetization and coercivity as calculated from the hysteresis loop measurement show striking dependence on composition.

Introduction

Spinel ferrites with general formulae AB₂O₄ are the most important materials for fundamental as well as for applied research point of view due to their high electrical resistivity and sufficient low dielectric losses [1], [2], [3]. Polycrystalline ferrites are supposed to be very good dielectric materials with low conductivity and have a wide field of technological applications in the range from microwave to radio wave frequencies [4], [5], [6], [7]. The distribution of cations over tetrahedral and octahedral sites in ferrites has a remarkable effect on the electrical and magnetic behavior [8], [9], [10], [11]. Spinel ferrites have been shown to exhibit interesting dielectric and magnetic properties in the nanocrystalline form compared with those of bulk form, because the cation distribution is distinctly different in ferrite nanoparticles compared with the bulk ferrites [12], [13]. The physical properties of ferrites are also dependent on synthesis technique, chemical composition, the quantity and type of additives [14], [15], [16]. Interesting results have been reported on the electrical and magnetic behavior of Ni–Cu ferrites substituted with different metal ions such as Zn, Al and Co [9], [17], [18]. All these factors play a major role in deciding the overall electrical properties of the ferrites. The sol–gel auto-combustion synthesis [19] provides an easy alternative for the preparation of nano-size particles that require low annealing temperature. Therefore, in the present work Mg substituted Ni–Cu ferrites of different compositions prepared by the sol–gel technique for the first time have been reported. Further, the electrical and magnetic properties of Ni–Cu ferrites have been studied as a function of Mg²⁺ ions concentration for widening its range of applications. A detailed investigation on the transport properties of the ferrites belonging to a series of type Ni_0.5Cu_0.5−xMg_xFe₂O₄ can throw light on the mechanism of electric conduction studied over a wide range of frequencies.