Structural and Mössbauer study of nanoparticles CoFe₂O₄ prepared by sol-gel auto-combustion and subsequent sintering

Abstract

Single-phased nanocrystalline Cobalt ferrite (CoFe₂O₄) have been synthesized by sol-gel auto-combustion method. The prepared sample was divided into four different samples (auto-combustion (S1), 400 ^∘C (S2), 600 ^∘C (S3), and 1000 ^∘C (S4)). The effect of sintering temperature on the structure, and morphology, were subsequently evaluated by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM). The hyperfine Interactions of all samples were studied using Mössbauer Spectroscopy (MS). No significant changes noticed in lattice constant (a), and the crystallite size was found to increase with sintering temperature which can be attributed to the grain growth of the particles. SEM shows an increase in the grain size and crystallinity as the sintering temperature increases, and the particles are cubic-like for S4. The particle size of each sample was determined using TEM and were in the range of 10–70 nm for the first three samples S1, S2, and S3, and increased beyond the nano-sized limit for S4 sample. The ⁵⁷Fe Mössbauer spectra at 295 K and 78 K showed two sets of magnetic hyperfine patterns for all the four samples, indicating the presence of Fe³⁺ in both A and B-sites, with superimposed doublet in each of S1 and S2 samples. This doublet shows a significant reduction in intensity with increasing the sintering temperature. The existence of the doublet at 78 K suggests that the blocking temperature is below 78 K temperature. From the intensity of the Fe³⁺ sites and assumed site occupancies of the Co²⁺ ions, it may be deduced that the paramagnetic doublet is associated with octahedral site.