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This article has been retracted. A statement of retraction is published in Mod. Phys. Lett. B Volume 28, Issue 26 (2014) http://dx.doi.org/10.1142/S0217984914930014

The influence of nickel doping on the electrical properties and dielectric relaxation in Zn${}_{1-x}$Ni${}_x$Fe₂O₄ (ZNFO, $0.2\le x\le 0.5$) ceramics has been investigated via the dielectric and complex impedance spectra measurements. According to the modified Curie–Weiss law, the diffusivity factor of the ZNFO ceramics from 1.69 to 2.02 with $x$ increasing from 0.2 to 0.5, respectively. Two relaxation peaks are observed in the nickel doped samples, by employing the modified Arrhenius equation, two activation energy values of different sintering temperatures were calculated and analyzed in combination with oxygen vacancy. The Cole–Cole plots showed that the semicircular arcs which are nonideal Debye type, and the grain boundaries resistance increases with increasing Ni concentration.

In this work, Sr-substituted samples of single-phase spinel monoferrites with chemical formula ${\mathrm{\text{Ba}}}_{1-x}{\mathrm{\text{Sr}}}_x{\mathrm{\text{Fe}}}_2{\mathrm{\text{O}}}_4$ (x = 0.00, 0.33, 0.67, 1.00) were synthesized using sol–gel auto-combustion method. In order to confirm the single-phase formation of these samples, a sample (x = 0.00) was chosen for heat treatment at different temperatures (100, 300, 400, 600 and $700^{\circ }\mathrm{\text{C}}$) for 4 h. The heat treated sample was then investigated by X-ray diffraction (XRD) analysis and results showed that a single-phase sample can be successfully synthesized at a temperature of $600^{\circ }\mathrm{\text{C}}$, which is much lower than that reported in earlier literature for synthesis of same structured samples. All the synthesized samples were then sintered at $700^{\circ }\mathrm{\text{C}}$ for 4 h to achieve better crystallinity. From XRD patterns, lattice parameters, cell volume and XRD density as a function of Sr-substitution were calculated. Scanning electron microscopy (SEM) results showed that the grain size increased as the temperature was increased. Fourier transform infrared spectroscopy (FTIR) results confirmed the single-phase spinel monoferrites at $700^{\circ }\mathrm{\text{C}}$. From M–H loops (x = 0.0, 0.33, 0.67 and 1.00), different magnetic parameters such as saturation magnetization $(M_s)$, remanance $(M_r)$, coercivity $(H_c)$ and magnetic moment $(n_B)$ were calculated. Magnetocrystalline anisotropy constant and Y–K angles of Sr-doped Ba monoferrites were also calculated. In addition, the variation of different dielectric parameters (real permittivity, imaginary permittivity, real permeability, imaginary permeability, ac conductivity and loss tangent) as a function of frequency (1–6 GHz) has been discussed in this work. The results suggest that the synthesized materials have many advantages over previously reported single-phase spinel monoferrites.