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Concept
Ferrite (magnet)
Summary
A ferrite is a ceramic material made by mixing and firing iron(III) oxide (, rust) with one or more additional metallic elements, such as strontium, barium, manganese, nickel, and zinc. They are ferrimagnetic, meaning they are attracted by magnetic fields and can be magnetized to become permanent magnets. Unlike other ferromagnetic materials, most ferrites are not electrically conductive, making them useful in applications like magnetic cores for transformers to suppress eddy currents. Ferrites can be divided into two families based on their resistance to being demagnetized (magnetic coercivity).
Hard ferrites have high coercivity, so are difficult to demagnetize. They are used to make permanent magnets for applications such as refrigerator magnets, loudspeakers, and small electric motors.
Soft ferrites have low coercivity, so they easily change their magnetization and act as conductors of magnetic fields. They are used in the electronics industry to make efficient magnetic cores called ferrite cores for high-frequency inductors, transformers and antennas, and in various microwave components.
Ferrite compounds are extremely low cost, being made of mostly iron oxide, and have excellent corrosion resistance. Yogoro Kato and Takeshi Takei of the Tokyo Institute of Technology synthesized the first ferrite compounds in 1930.
Ferrites are usually ferrimagnetic ceramic compounds derived from iron oxides. Magnetite (Fe3O4) is a famous example. Like most of the other ceramics, ferrites are hard, brittle, and poor conductors of electricity.
Many ferrites adopt the spinel structure 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, i.e., A2+B23+O42−.
Ferrite crystals do not adopt the ordinary spinel structure, but rather the inverse spinel structure: One eighth of the tetrahedral holes are occupied by B cations, one fourth of the octahedral sites are occupied by A cations.
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