Ferrites: Ferrites are mixed metal oxides of magnetic nature in which iron is the main component. In general, ferrites show four different types of crystal structures namely, 1] Ferrospinel Structure. 2] Hexagonal Structure. 3] Garnet Structure. 4] Orthoferrite structure. We are going to discuss about spinel structure as Co Ni ferrite is a spinel ferrite. Ferrospinel Structure. They have the general formula MeFe2O4, where Me is divalent metal ion or a mixture of ions having average valence of two. The unit cell is cubic. The oxygen ions forms a nearly close-packed face centered cubic structure and the metal ions are distributed over tetrahedral and octahedral holes. Normal ferrites:In which all-divalent metal ions occupy A sites and all the Fe3+ occupy B sites. e.x. Zn 2+ [Fe23+] O4 Zn 2+ ions have a very low octahedral preference; therefore they enter the A-sites of the lattice, resulting in normal ferrites. Inverse ferrites:In which all divalent metal ions and half the Fe3+ ions occupy B sites while remaining Fe3+ occupy A sites. e.x. Fe3+[ Fe3+ Ni2+] O4 Mixed ferrites:In which all divalent metal ions and Fe3+ ions are uniformly distributed over the tetrahedral and octahedral sites. Co - Ni ferrite is a mixed spinel ferrite, which has general formulae AII x BII 1-x Fe2O4