Hard Magnetic Ferrite: ε-Fe2O3

Iron oxide Fe2O3 has four polymorphs: alpha-, beta-, gamma-, and epsilon-phases. alpha-Fe2O3 (hematite) and gamma-Fe2O3 (maghemite) are abundant in nature, whereas beta- and epsilon-Fe2O3 phases are very rare and must be artificially synthesized in the laboratory. Pure epsilon-Fe2O3 phase was first synthesized in 2004 using a combination of reverse-micelle and sol-gel techniques, and it shows the largest coercive field value (H-e) among metal oxide-based magnets of 20 kOe at room temperature. Successively, several kinds of metal-substituted epsilon-iron oxides, epsilon-MxFe2-xO3 (M = In, Ga, and Al), have been synthesized, and their magnetic properties are controlled by the degree of the metal substitution. Such iron oxides with a high H-c are attractive from industrial application viewpoints, e.g., magnetic recordings and electromagnetic wave absorbers. A series of epsilon-MxFe2-xO3 shows high-frequency electromagnetic wave absorption due to zero-field ferromagnetic resonance at 35-182 GHz in the millimeter range, which is useful for next-generation high-speed wireless communications. In this article, we describe (i) the synthesis, crystal structure, and magnetic properties of epsilon-Fe2O3, (ii) generation mechanism of epsilon-Fe2O3, the origin of the gigantic coercive field, and theoretical analysis of magnetic ordering, (iii) metal-substituted epsilon-iron oxides, epsilon-MxFe2-xO3, and (iv) electromagnetic wave absorption in the millimeter wave range.