Co0.5Zn0.5ErxFe2−xO4 \((x=0.00-0.10)\) nanoferrites were synthesized by a sol–gel auto-combustion route to examine the effect of Er³⁺ on structural, microstructural, vibrational, thermal, and magnetic characteristics. X-ray diffraction with Rietveld refinement confirmed a single-phase cubic spinel (Fd–3 m) with a near-linear Vegard-type increase in lattice parameter from 8.3954 Å \((x=0.00)\) to 8.4118 Å \((x=0.08)\) and a slight relaxation at \(x=0.10\) , consistent with B-site Er³⁺ substitution under a constrained Bertaut distribution. Williamson–Hall analysis showed a reduction of coherent domain size from 38.3 nm to 16.6 nm across the series. Thermal analysis gave a total mass loss of ~ 11.24% with a dominant crystallization exotherm at ~ 318 °C (low end of the typical 300–450 °C range), and negligible mass change beyond ~ 450 °C, indicating thermal stability. FTIR spectra exhibited the characteristic M–O bands near ~ 530 cm⁻1 (A-site) and ~ 420 cm⁻1 (B-site), both blue-shifting with \(x\) ; the corresponding force constants increase modestly, supporting octahedral-sublattice perturbation. FESEM revealed irregular, plate-like agglomerates, while TEM/HRTEM showed clear spinel lattice fringes (with \({D}_{\text{W}\text{H}}\le\:{D}_{\text{T}\text{E}\text{M}}\) , as expected). VSM confirmed soft ferrimagnetic behavior at 300 K with very low coercivity (~ 1.5–2.0 Oe) and a decrease in \({M}_{s}\) from 28.4 to 21.2 emu·g⁻1 due to B-site moment dilution and enhanced surface spin canting. These results establish phase-pure Co–Zn–Er spinels with soft-magnetic behavior, indicating potential for high-frequency uses; however, application-specific validation requires frequency-dependent dielectric/permeability and RF-loss measurements.
