Fine Powders of Co-Mn Cation Deficient Spinel Ferrites for Magneto-Optical Pigments

Abstract
Very small particles of Co-Mn defect spinel ferrites with acicular and spherical shape were prepared. Their magnetic and magneto-optical properties were measured in function of their oxidation state and their size. It was shown that the coercivity of acicular (AP) and spherical (SP) particles with a mean crystallite size close to 20 nm, reached 3800 and 2950 Oe respectively. The SP coercivity decreases when the grains sizes become lower than 20 nm, but remains higher than 1000 Oe for particles having crystallites close to 10-15 nm. The Curie temperature decreases also sligthly when the grains are smaller than 20 nm, while the remanent Faraday rotation falls from 0.6 to 0 deg/µm when the crystallite size is reduced from 40 to 7-8 nm. The Co-Mn ferrite particles studied have interesting properties, even they have not, for now, all the required properties to be high performance pigments for magneto-optical recording.