Impact of Copper Substitution on the Structural Refinement, Magnetic Behaviour, and Dielectric Response of Ni–Zn Spinel Ferrites

Ni0.5Zn0.5−xCuxFe2O4 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) spinel ferrite nanoparticles were synthesized via the sol-gel auto-combustion method and sintered at 700°C for 5 h. Thermogravimetric and differential thermal analysis (TG-DTA) revealed the thermal decomposition behaviour. Single phase ferrite with Fd‑3m space group was confirmed by the Rietveld refinement of X-ray diffraction data. The distribution of cation among octahedral B and tetrahedral A-site was estimated by the computational method. With increasing copper substitution, the lattice parameter decreased while X-ray density increased. Crystallite sizes ranged from 22 to 24 nm, consistent with the Williamson–Hall method, and strain decreased. Fourier-transform infrared (FTIR) spectroscopy confirmed the spinel structure. High-resolution transmission electron microscopy (HR-TEM) and field emission scanning electron microscopy (FE-SEM) showed grain sizes between 70 and 130 nm. Energy dispersive X-ray (EDAX) analysis confirmed chemical purity. Magnetic studies showed an increase in saturation magnetization and coercivity (42 to 57 Oe) with copper substitution, while dielectric behaviour followed the Maxwell–Wagner model.