Crystallite Size Effects on Electrical Properties of Nickel Chromite (NiCr2O4) Spinel Ceramics: A Study of Structural, Magnetic, and Dielectric Transitions

The effect of sintering temperature on the structural, magnetic, and dielectric properties of NiCr2O4 ceramics was investigated. A powder X-ray analysis indicates that the prepared nanocrystallites effectively inhibit the cooperative Jahn–Teller distortion, thereby stabilizing the high-temperature cubic phase structure with space group Fd-3m. Multiple transitions are confirmed by temperature-dependent magnetization M(T) data. Moreover, the magnetization value decreases and the Curie temperature increases with a decrease in the crystallite size. The low-temperature-dependent real permittivity (ε′-T) for a NiCr2O4 crystallite size of 78 nm exhibits a broad maximum at 40 K that is independent of frequency. This establishes a correlation between electric ordering and the underlying magnetic structure. The temperature dependency of the dielectric constant at fixed frequencies for both NiCr2O4 crystallite sizes rises with temperature for a certain range of frequencies. A significant improvement is evident: the dielectric constant (ε’) at room temperature reaches approximately 5738 for the sample with 28 nm crystallites, while the 78 nm crystallite sample shows a noticeable drop to ε’~174. The frequency-dependent conductivity curves for both types of NiCr2O4 nanocrystallites have different conductivity values. The lower-crystallite-size sample demonstrates higher conductivity values than the 78 nm crystallite size one. This observation is attributed to the decrease in crystallite size, which increases the number of grain boundaries and, consequently, scatters a higher number of charge carriers.