The crystallographic, magnetic, and electrical properties of Gd3+-substituted Ni–Cu–Zn mixed ferrites

Polycrystalline Ni0.48Cu0.12Zn0.40GdxFe2-xO4 (where x = 0.00, 0.02, 0.04, 0.08, and 0.10) was synthesized by the conventional solid-state reaction technique. The structure and surface morphology were studied by X-ray diffraction (XRD) and high-resolution optical microscopy, respectively. The XRD patterns indicated that the compositions formed a cubic spinel structure for a small amount of Gd3+ substitution. With increasing Gd3+ concentration, a gradual rise in the intensity of the extra peaks was observed, and those peaks were identified as corresponding to GdFeO3 (orthoferrite). Rietveld refinement was performed on the XRD data of Gd3+-substituted Ni0.48Cu0.12Zn0.40 ferrite samples for further verification, and the results were found to be in good agreement with results reported in the literature. Variation of the lattice constant obeyed Vegard's law. Microstructural studies showed that the average grain diameter increases as the Gd3+ content increases up to an optimum concentration and then it decreases. Magnetic properties were characterized by the complex initial permeability (100 Hz–100 MHz). The real part of the complex initial permeability (μi′) increases with Gd3+ concentration up to x = 0.04 and then decreases. Ni0.48Cu0.12Zn0.40Gd0.04Fe1.96O4 shows the highest value of μi′ (at the optimum sintering temperature), which is more than eight times that of the parent sample. Conversely, magnetic loss is reduced remarkably in this composition. The Néel temperature was determined from the temperature-dependent initial complex permeability, which showed a decreasing trend with increase in Gd3+ concentration owing to the weakening of the A-B interaction. DC magnetizations as a function of the applied magnetic field were evaluated with a vibrating-sample magnetometer at room temperature. The magnetization increased with increasing Gd3+ content up to x = 0.04 owing to the variation of cation distribution and then reduced because of the possibility of noncollinear spin arrangement with further increase of Gd3+ content. The number of Bohr magnetons (nB) and the Yaffet-Kittel angle (αY-K) were evaluated. The calculated αY-K values were a minimum for x = 0.04, while the nB variation showed the opposite trend. Because of the reduction of electron exchanges between Fe2+ and Fe3+ at the octahedral site, the room temperature dielectric constant and the dielectric loss were reduced with increasing Gd3+ content. There was an exceptional increase in the dielectric constant for x = 0.02 sample. The AC conductivity spectrum exhibited two regions, and the conductivity was found to decrease with the addition of Gd3+. Impedance spectroscopy studies indicated a non-Debye-type relaxation process. Ferroelectric transition temperatures, TC, were also determined from measurement of the temperature-dependent dielectric constant.