Abstract
Studies on the increase in the coercive forces (Hc) of Fe3O4-γFe2O3 intermediate particles and thin films have been summarized. This phenomenon was reported first by M. Namikawa in 1963 and was intermittently followed for years by many researchers. The effective experimental results to approach the origin of the coercive force increase have been pointed out, i.e., (1) increase in He at a particular region of Fe2+/Fe3+, (2) reversible increase in He for the oxidation process of Fe3O4 and for the reduction process of γFe2O3, (3) increase in He during an aging process, (4) large increase in He for thin film samples than for powder samples. It is emphasized that the two phases of cubic and tetragonal crystal structures coexist at the oxidation rate of Fe2+/Fe3+=0.10-0.15 where the coercive forces of the samples increase to their maximum values. The intermediate films are examined with the transmission electron microscopy. The result suggests that the apparent crystallites observed in the intermediate films have fine sub-structures.
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