Ferrite

(inorganic chemistry) An unstable compound of a strong base and ferric oxide which exists in alkaline solution, such as NaFeO₂.
(metallurgy) Iron that has not combined with carbon in pig iron or steel.
(petrology) Grains or scales of unidentifiable, generally transparent amorphous iron oxide in the matrix of a porphyritic rock.
(solid-state physics) Any ferrimagnetic material having high electrical resistivity which has a spinel crystal structure and the chemical formula XFe₂O₄, where X represents any divalent metal ion whose size is such that it will fit into the crystal structure.

Any of the class of magnetic oxides. Typically the ferrites have a crystal structure which has more than one type of site for the cations. Usually the magnetic moments of the metal ions on sites of one type are parallel to each other, and antiparallel to the moments on at least one site of another type. Thus ferrites exhibit ferrimagnetism. See also Ferrimagnetism; Magnetic materials.

There are three important classes of commercial ferrites. One class has the spinel structure, with the general formula M²⁺Fe₂³⁺O₄, where M²⁺ is a divalent metal ion. So-called linear ferrites used in inductors and transformers are made of Mn and Zn (for frequencies up to 1 MHz) and Ni and Zn (for frequencies greater than 1 MHz). MgMn ferrites are used in microwave devices such as isolators and circulators. Until the late 1970s, ferrites with square loop shapes held a dominant position as computer memory-core elements, but these gave way to semiconductors. See also Computer storage technology.

The second class of commercially important ferrites have the garnet structure, with the formula M₃³⁺Fe₅³⁺O₁₂, where M³⁺ is a rare-earth or yttrium ion. Yttrium-based garnets are used in microwave devices. Thin monocrystalline films of complex garnets have been developed for bubble domain memory devices.

The third class of ferrites has a hexagonal structure, of the M²⁺Fe₁₂³⁺O₁₉ magnetoplumbite type, where M²⁺ is usually Ba, Sr, or Pb. Because of their large magnetocrystalline anisotropy, the hexagonal ferrites develop high coercivity and are an important member of the permanent magnet family.

Another magnetic oxide, γ-Fe₂O₃, also has the spinel structure, but has no divalent cations. It is the most commonly used material in the preparation of magnetic recording tapes. See also Ferrite devices.