Annealing-Induced Evolution of Magnetic Anisotropy and Coercivity in Green-Synthesized NiFe2O4 Nanoparticles

Abstract

NiFe2O4 nanoparticles (NPs) were synthesized by a biomass-assisted green route using postconsumer yellow passion fruit waste and annealed between 573 and 1273 K to investigate their structural and magnetic evolution. FTIR, XRD, Mössbauer spectroscopy, TEM, VSM, and FMR were combined to correlate phase formation, crystallite growth, local magnetic ordering, quasi-static magnetization, and dynamic anisotropy. Structural characterization revealed improved phase purity, progressive suppression of residual NiO, and an empirical exponential increase in crystallite size from 3.5 to 140.6 nm with increasing annealing temperature. Magnetization measurements showed that the maximum mass-normalized magnetization increased from 5.6 to 44.7 emu g–1, whereas the coercive field reached a maximum of 155 Oe at an average crystallite size of approximately 43 nm. FMR analysis showed that the effective cubic anisotropy field (H1) evolves from positive (+558 Oe) to negative (−1082 Oe), indicating a crossover in the dominant cubic anisotropy contribution from ⟨100⟩-type to ⟨111⟩-type behavior. Accordingly, the cubic anisotropy constant (K1) is treated as a signed quantity and becomes negative in the high-temperature regime. The fitted effective spectroscopic (g)-factor increases from 2.38 to 2.68, reflecting changes in the local magnetic and electronic environment as the ferrite phase becomes more crystalline and structurally ordered. The FMR response is interpreted within an effective anisotropy framework involving cubic magnetocrystalline anisotropy, surface disorder, particle-shape effects, size dispersion, local structural disorder, interparticle interactions, and dynamic relaxation. The combined VSM and FMR analysis shows that coercivity and dynamic anisotropy do not evolve in parallel, revealing partial decoupling between macroscopic magnetization reversal and the anisotropy probed on the FMR time scale.