Structural phase change in Co_2.25Fe_0.75O₄ spinel oxide by vacuum annealing and role of coexisting CoO phase on magnetic properties

Abstract

The spinel oxideCo_2.25Fe_0.75O₄ has been prepared by co-precipitation route. The as prepared material has been annealed at 800 °C under vacuum. Properties of the samples annealed under vacuum have been compared with the sample annealed in air. Synchrotron X-ray diffraction, Raman spectroscopy and X-ray absorption near edge structure spectroscopy have been used for structural phase characterization. The cubic spinel structure in air annealed sample has been splitted into Co and Fe rich phases. The Co rich spinel phase has transformed into Cobalt monoxide (CoO) after vacuum annealing. This has resulted an increase of Fe:Co ratio in the Fe rich spinel phase. Raman spectra indicated a transformation of the distribution of Co and Fe ions from normal spinel like structure for the sample annealed in air to mixed spinel structure for the samples annealed under vacuum. The existence tetravalent Fe⁴⁺ ion has been confirmed in all the samples and it is new information for Co doped spinel ferrite system. All the samples exhibited ferrimagnetic behavior and exchange bias effect. The focus point is that vacuum annealing has been used as an effective route of material processing to control the structural phase and also, enhancement of magnetic properties in Co rich ferrite.