Effects of several kinds of anisotropy on the coercivity behaviors of iron oxides

Abstract

Coercivity behaviors of iron oxides during phase changes between Fe3O4 and γ-Fe2O3 for granular powders and continuous thin films, which can be interpreted in a unified framework by an intermediate product hypothesis, are investigated. There exists a critical value ccr of Fe3O4 concentration for different samples. The intermediate product Fe3−zO4 in oxidation and reduction processes is a mixture of two phases when Fe3O4 concentration c is smaller than ccr; but it may exist in the form of a homogeneous solid solution (single-phase) (Fe3O4)1−x(γ-Fe2O3)x when c is larger than ccr. The difference in lattice constants will produce a tensive stress (σ≳0) on γ-Fe2O3 and compressive stress (σ<0) on Fe3O4 at interfaces. Superposition of a positive stress anisotropy on the easy axes 〈111〉 of magnetocrystalline anisotropy for Fe3O4 and on the easy axes 〈110〉 for γ-Fe2O3 will cause an increase in the coercivity Hc for spherical particle and thin film samples. The coercivity behavior of our acicular particle sample is contrary to that of thin film and spherical particle samples. Also, the switching field distribution curve showed a peak near c=0.4. There always exist some two-phase intermediate compounds in Fe3−zO4 due to the incomplete reaction of single domain particles. The decrease in coercivity of acicular particles is affected by the shape anisotropy related to changes in effective particle sizes.