Effect of chromium in magnetic and dielectric properties of inverse spinel FeMn2O4

We prepared single-phase polycrystalline Fe(Mn 1− x Cr x ) 2 O 4 ( x = 0.0–0.5) samples, and they are found to be in tetragonal structure up to x = 0.05 and beyond that in cubic form. A decrease in particle size with Cr concentration has been observed by FESEM and TEM analyses. The FTIR characterization shows the stretching, as well as the bending of C–H and O–H functional groups and the presence of the strong absorption bands of Fe–O, Mn–O and Cr–O which correspond to the lattice vibrations in octahedral and tetrahedral coordination of atoms. Ferrimagnetic (FIM) transition above room temperature ( T C > 350 K) followed by low-temperature spin reorientation transition has been observed from magnetization data. The FIM T C is in the range of 378–365 K for chromium substitution x = 0.0–0.5. Suppression of spin reorientation as well as magnetic irreversibility is observed, and it is attributed to the reduction in effective magneto-crystalline anisotropy by an order of magnitude due to Cr substitution. Room temperature dielectric measurement shows a large dielectric constant of the order of 10 3 –10 5 at 100 Hz. However, the frequency-dependent curves suggest the presence of considerable concentration of free charge carriers. From the high-temperature impedance study, a clear relaxation process of a non-Debye-type has been noticed with dominant contribution from charge carriers relaxation across grain boundaries. The relaxation frequency follows the Arrhenius law with an anomaly in the neighbourhood of FIM T C indicating possible magneto-electric coupling.