The formation of manganese and cobalt ferrites by the air oxidation of aqueous suspensions and their properties.

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Manganese and cobalt ferrites were prepared by the air oxidation of aqueous suspension of Fe(OH)2 and either Mn(OH)2 or Co(OH)2 at 50–80 °C. In alkaline suspensions, all the metal ions are transformed into spinel ferrites, MxFe3−xO4 (M=Mn or Co), by a suitable combination of the oxidation temperature and the concentration of the excess NaOH. Both manganese and cobalt ferrites consist of ferromagnetic particles (ferrite(A)) when x is smaller than 1.3, nonferromagnetic particles (ferrite(B)) when x is larger than 2, and mixtures of the two in the intervening range. The contents of Mn or Co, both in a solid solution of hydroxides, Mx⁄3Fe1−x⁄3(OH)2, and in ferrite, increase with the progress of the formation of the ferrite(A) caused by oxidation. The formation of the ferrite(B) begins when x in the solid solution reaches 2. Each particle in the ferrite(A) is less homogeneous in comparison with that in the ferrite prepared by a solid-state reaction, although with the same composition. The difference in the homogeneity causes the essential difference in magnetic properties.
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