TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preparation (Disclosed): Preparation of (I) comprises simultaneous co-precipitation at constant pH by controlled precipitation of a solution of divalent and trivalent metal salts in adequate proportion of basic solution, where the divalent and trivalent metal salts solutions are prepared by dissolving the metal salt type precursors in deionized water and the basic solution is prepared by dissolving sodium hydroxide in deionized water. Preferred Components: (I) has a crystallization temperature of 450-650 degrees C. The crystal structure is face-centered cubic with the space group Fd3m. The crystallite size of (I) is 4-10 nm. In (I), the cobalt, chromium, iron, erbium, gadolinium and samarium are present at 28.276 wt.%, 19.958 wt.%, 21.436 wt.%, 8.025 wt.%, 15.089 wt.% and 7.2142 wt.%, respectively. (I) has: coercive field at 5, 80 and 300K equal to 3336.89, 3806.96 and 530.71 Oe; magnetization saturation at 5, 80 and 300K equal to 22.16, 4.96 and 2.80 emu/g; remanent moment at 5, 80 and 300K equal to 4.26; 2.71 and 1.10 emu/g; an average blocking temperature equal to 80K; and Mr/Ms ratio of at 5, 80 and 300K equal to 0.19, 0.54 and 0.39 respectively. EXAMPLE - Solutions of cobalt chloride hexahydrate, erbium chloride hexahydrate, iron(III) chloride, gadolinium chloride hexahydrate, samarium chloride hexahydrate and chromium nitrate nonahydrate were prepared in deionized water and were mixed in a flask. The mixture was then refluxed at 60 degrees C for 15 minutes under mechanical agitation at 425 rpm, followed by increasing the temperature to 80 degrees C. Then 6.34 mmol oleic acid was added, and at 80 degrees C, 25 ml of 3M sodium hydroxide solution was added dropwise for 30 minutes. The reaction mixture was allowed to react for 15 minutes, then 25 ml of 3M sodium hydroxide was added and the mixture was stirred for 45 minutes. The mixture was aged at room temperature, cooled and recovered the precipitate by filtration, and the precipitate was washed with hot distilled water to remove by-products, and finally the filtered precipitate was dried in an oven at 80 degrees C overnight. The dried powder was ground and then calcined at 500 degrees C for 2 hours in a muffle furnace to obtain the crystalline spinel phase of substituted iron cobalt chromite doped with rare earth element.