Mahmood Niad

In this study, the magnetostructural behavior of nickel ferrite nanoparticles with varying nickel content (x) was systematically examined at annealing temperatures of 250, 500, and 1000 ◦C. The nanoparticles were synthesized via a PVA-assisted sol–gel method and characterized using XRD, FTIR, FESEM, and HRTEM analyses. Rietveld refinement confirmed the formation of a well-defined spinel structure. Particular emphasis was placed on first-order reversal curve (FORC) analysis to probe the intrinsic coercivity, interparticle interactions, and domain-state evolution. The FORC distributions revealed a distinct transition in magnetic domain configuration with increasing Ni content and annealing temperature. Notably, the samples annealed at 500 ◦C exhibited a pronounced single-domain behavior accompanied by low coercivity, indicating enhanced magnetic uniformity and reversibility. The FORC results, together with hysteresis loop analysis, demonstrated that increasing the annealing temperature improved crystallinity and saturation magnetization, whereas higher Ni substitution led to a gradual reduction in total magnetic moment. Furthermore, the distribution of cations was analyzed, revealing a presence of nickel cations in A sites. Due to surface spin disorder, particle size, and interparticle interactions, coercivity increased, and FORC analysis confirmed single-domain behavior. Superparamagnetic particles, due to their high reversibility, don’t prominently influence the FORC diagram. However, particles close to a critical size exhibit quasi-reversible behaviour. The FORC diagram results indicate that the Ni1.5Fe1.5O4 sample annealed at 500 ◦C is well-suited for use as a core material in power transformers.