Equilibrium and non-equilibrium lattice structure, ferrimagnetic spin order and electrical conductivity at high-temperature regime of single-phased and bi-phased Co-rich spinel ferrites

This work investigates the role of stable (sing-phased) and unstable (bi-phased) lattice structures in controlling the magnetic spin order and electronic properties in Co-rich ferrites of compositions Co_1.25Fe_1.75O₄ and Co_2.25Fe_0.75O₄. The material was synthesized via the chemical reaction of metal (Co and Fe) nitrates in an alkaline medium and post-heat treatment. The XRD patterns of Co_1.25Fe_1.75O₄ showed a single-phased structure upon heat treatment in the temperature range of 200–900 °C, whereas Co_2.25Fe_0.75O₄ showed a single-phased structure at 900 °C and bi-phased structure at a low heating temperature range of 200–800 °C. The thermally induced irreversibility (non-equilibrium) effect was observed in the lattice structure, magnetic spin order and electrical conductivity. The lattice structure at the (local) microscopic scale showed a more or less metastable state and coexistence of mixed-charge states of Co and Fe ions, irrespective of the single or bi-phased structure. The magnetic and electronic responses are found to be sensitive enough to detect local level non-equilibrium (metastable) states in non-equilibrium and equilibrium crystalline phases seen from XRD patterns.