Role of Cu-substitution on microstructural, magnetic, magnetostrictive and dielectric properties of sintered NiFe2O4

Abstract

This study examines the impact of Cu-amalgamation on the microstructural, magnetic, magnetostrictive, and dielectric properties of sintered nickel ferrite (NiFe2O4) ceramics. Without post synthesis calcination, the series of Ni1-xCuxFe2O4 (0 ≤ x ≤ 1) samples were synthesized via an autocombustion process with glycine (1 Mole/mole of metal ion) as a fuel. XRD analysis of the as-synthesized samples revealed that compositions up to x = 0.75 were phase pure. However, impurity CuFeO2 was detected in the x = 1 (CuFe2O4) sample, leading to its exclusion from further processing and testing. After sintering the compacted pellets (12 mm diameter and 3 mm thickness) at 1200 °C for 2 h, compositions up to x = 0.5 were confirmed as phase pure, prompting further characterization of these samples' structural, microstructural, magnetic, and dielectric properties. The unit cell dimension increased from 8.3268 Å for NiFe2O4 (x = 0) to 8.3560 Å for Ni0.5Cu0.5Fe2O4 (x = 0.50) due to the substitution of larger-sized ion (Cu2+) for the smaller-sized ion (Ni2+) in NiFe2O4. Notably, the addition of Cu facilitated substantial grain growth, as observed in the SEM micrographs, where the grain size increased from 556 nm for x = 0 to 13 μm for x = 0.5. As the Cu-content increased in Ni1-xCuxFe2O4, magnetic parameters such as TC (Curie temperature), K1 (Magnetocrystalline anisotropy constant), HC (Coercivity), and MS (Saturation magnetization) decreased. This decrease can be attributed to the replacement of the high-magnetic cation (Ni2+, 2 μB) with the low-magnetic cation (Cu2+, 1 μB) in the octahedral coordination site of NiFe2O4. The x = 0 and 0.5 samples exhibited a maximum magnetostriction strain (λmax) of ∽-35 ppm, and for x = 0.25 sample the value is −24 ppm. In terms of dielectric properties, the permittivity count of the x = 0 sample was higher than that of x = 0.25 and 0.5 samples in the lower frequency regime, likely due to a larger surface area (smaller grain size) contributing to significant surface polarization. Conversely, in the higher frequency regime, the samples with x = 0.25 and 0.5 demonstrated higher permittivity than the parent sample. The changes in dielectric behavior in the Cu-substituted samples were explained based on the microstructure of the samples.