Bulletin of the Chemical Society of Japan
Online ISSN : 1348-0634
Print ISSN : 0009-2673
ISSN-L : 0009-2673
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Hard Magnetic Ferrite: ε-Fe2O3
Shin-ichi OhkoshiHiroko Tokoro
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2013 Volume 86 Issue 8 Pages 897-907

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Abstract

Iron oxide Fe2O3 has four polymorphs: α-, β-, γ-, and ε-phases. α-Fe2O3 (hematite) and γ-Fe2O3 (maghemite) are abundant in nature, whereas β- and ε-Fe2O3 phases are very rare and must be artificially synthesized in the laboratory. Pure ε-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 (Hc) among metal oxide-based magnets of 20 kOe at room temperature. Successively, several kinds of metal-substituted ε-iron oxides, ε-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 Hc are attractive from industrial application viewpoints, e.g., magnetic recordings and electromagnetic wave absorbers. A series of ε-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 ε-Fe2O3, (ii) generation mechanism of ε-Fe2O3, the origin of the gigantic coercive field, and theoretical analysis of magnetic ordering, (iii) metal-substituted ε-iron oxides, ε-MxFe2−xO3, and (iv) electromagnetic wave absorption in the millimeter wave range.

ε-Iron oxide (ε-Fe2O3) and metal-substituted ε-iron oxides (ε-MxFe2−xO3), which exist as nanoscale size particles, demonstrate gigantic magnetic coercive fields of up to 20 kOe and high-frequency electromagnetic wave absorption. Fullsize Image
 
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© 2013 The Chemical Society of Japan
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