Air annealing effects on lattice structure, charge state distribution of cations, and room temperature ferrimagnetism in the ferrite composition Co_2.25Fe_0.75O₄

The ferrite composition Co_2.25Fe_0.75O₄ has been prepared by chemical coprecipitation route. The asprepared material has been annealed at different temperatures to investigate thermal activated changes in structural phase, charge states of cations, and room temperature magnetic properties. Synchrotron x-ray diffraction (SXRD) patterns have shown splitting of cubic spinel structure into Fe-rich and Corich phases for annealing temperature up to 800 °C. Single phase cubic spinel structure has been stabilized at annealing temperature 900 °C. The existence of Fe⁴⁺ ions, as confirmed from x-ray absorption near edge structure spectroscopy (XANES), is the new information for spinel ferrite. Raman spectra indicated normal spinel structure. The results of SXRD, XANES and Raman spectra have been used to estimate distribution of Co and Fe ions in spinel structure. The variation in population of Co and Fe ions and phase instability has affected the magnetic properties. The sample annealed at 800 °C shows maximum coercivity (∼567 Oe) and squareness (∼0.38), whereas the single-phased sample showed lowest values of ferrimagnetic parameters. The tuning of magnetic parameters by thermal activated structural phase variation of a hetero-structured magnetic system appears to be a new technique for the development of magnetic materials.