We have synthesized the Co1.75Fe1.25O4 spinel ferrite by chemical reaction of Co and Fe nitrates at alkaline medium. We have annealed the as-prepared material up to 1100 °C to study the changes in lattice structure and magnetic properties. We have used synchrotron X-ray diffraction and Raman spectroscopy to identify the lattice structure. The material formed a composite spinel structure of Co rich phase (20–30%) and Fe rich phase (80–70%) for annealing temperature range 200–800 °C, and single phased spinel structure at 900 °C. At annealing temperature 1100 °C, the sample showed minor amount of hematite phase, in addition to two spinel phases. All the samples are ferrimagnetic in nature, but ferrimagnetic properties in mixed-phased samples are distinctly different from single phased sample. The analysis of the temperature and magnetic field dependence of magnetization data revealed a wide distribution of exchange interactions, anisotropy barriers, blocking of magnetic domains, and exchange bias for the samples having multi-phase components. On the other hand, single phased sample has shown the highest magnetization and the smallest coercivity with lowest distribution of magnetic exchange energy and anisotropy.