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A ferrite is a type of ceramic compound composed of iron oxide (Fe2O3) combined chemically with one or more additional metallic elements. They are both electrically nonconductive and ferrimagnetic, meaning they can be magnetized or attracted to a magnet. Ferrites can be divided into two families based on their magnetic coercivity, their resistance to being demagnetized. ''Hard ferrites'' have high coercivity; Hence they are difficult to demagnetize. They are used to make magnets, for devices such as refrigerator magnets, loudspeakers and small electric motors. ''Soft ferrites'' have low coercivity. They are used in the electronics industry to make ferrite cores for inductors and transformers, and in various microwave components. Yogoro Kato and Takeshi Takei of the Tokyo Institute of Technology invented this material in 1930. ==Composition and properties== Ferrites are usually non-conductive ferrimagnetic ceramic compounds derived from iron oxides such as hematite (Fe2O3) or magnetite (Fe3O4) as well as oxides of other metals. Ferrites are, like most of the other ceramics, hard and brittle. Many ferrites are spinels with the formula AB2O4, where A and B represent various metal cations, usually including iron Fe. Spinel ferrites usually adopt a crystal motif consisting of cubic close-packed (fcc) oxides (O2−) with A cations occupying one eighth of the tetrahedral holes and B cations occupying half of the octahedral holes. If one eighth of the tetrahedral holes are occupied by B cation, then one fourth of the octahedral sites are occupied by A cation and the other one fourth by B cation and it is called the inverse spinel structure. It is also possible to have mixed structure spinel ferrites with formula ()()O4 where δ is the degree of inversion. The magnetic material known as "ZnFe" has the formula ZnFe2O4, with Fe3+ occupying the octahedral sites and Zn2+ occupy the tetrahedral sites, it is an example of normal structure spinel ferrite. Some ferrites have hexagonal crystal structure, like Barium and Strontium ferrites BaFe12O19 (BaO:6Fe2O3) and SrFe12O19 (SrO:6Fe2O3). In terms of their magnetic properties, the different ferrites are often classified as "soft" or "hard", which refers to their low or high magnetic coercivity, as follows. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ferrite (magnet)」の詳細全文を読む スポンサード リンク
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