Ferrimagnetic to antiferromagnetic transition and complex impedance analysis of Cr-doped magnesium ferrite nanoparticles

The chromium-doped magnesium ferrite (MgCr_xFe_2−xO₄, x = 0.0, 0.4, 0.8, 1.2, 1.6, and 2.0) were prepared using sol–gel technique. X-ray diffraction scans confirmed the cubic spinel structure for all the nanoparticles samples. Reduction in lattice parameter and average crystallite size with increasing Cr³⁺ concentration was observed due to smaller ionic radii of Cr³⁺ (0.67 Å) ions as compared to Fe³⁺ (0.63 Å) ions. In FTIR vibrational studies, the addition of Cr³⁺ shows the splitting of ν₁ band into two sub-bands due to variation of bond strength between same cations present at tetrahedral and octahedral lattice sites. With increasing Cr³⁺ concentration, the ZFC–FC magnetic curves revealed a ferrimagnetic (FiM) to antiferromagnetic (AFM) transition as compound transformed from MgFe₂O₄ to MgCr₂O₄. The M–H loops revealed decrease in saturation magnetization with increase in Cr³⁺ concentration which is attributed to FiM to AFM transition. The coercivity showed a non-monotonic behavior and has maximum value of 750 Oe for x = 1.2 due to overlapping of blocking and Neel transition temperatures. The dielectric properties showed a decreasing trend with increasing Cr³⁺ concentration. The room temperature impedance spectroscopy (Nyquist plots) was simulated using equivalent RC circuit model and an appearance of second semi-circle in the Nyquist plot with increasing Cr³⁺ concentration is observed which can be attributed to the contribution of grains in the hooping mechanism.