Hard ferrites

Ferrites can be classified according to crystal stmcture, ie, cubic vs hexagonal, or magnetic behavior, ie, soft vs hard ferrites. A systematic classification as well as some appHcations ate given in Table 1 (see also Magnetic materials, bulk Magnetic materials, thin film). 

Iron oxides and hydroxides are used as the starting material in the production of ferrites. Hard ferrites (high coercivity) e. g. BaFei20i9 and SrFei20i9 are made from hematite by a sintering process, i. e. 

Uses. Hard ferrites are used widely in electromechanical devices, e.g., generators, relays, motors, and magnetos electronic applications, e.g., loudspeakers, traveling-wave lubes, and telephone ringers and receivers aniitheft tags, holding devices such as door closers, seals, and latches ... 

Synthetic ferrites also exhibit ferrimagnetism. Ferrites are magnetic ceramic materials which are made by mixing iron (III) oxide with, for instance, powdered oxides and carbonates and subsequently compressing and sintering them. They are produced in large quantities and in many different shapes. We distinguish soft and hard ferrites. 

Hard ferrites in general have the formula M0.6Fe203, where M = Ba2+, Sr2+ or Pb 2+. They are mainly used as permanent magnets. 

For hard ferrites for use as permanent magnets relative permeability is unimportant and is usually near to unity in the magnetized state. The important parameters are a high coercive field, a high remanent magnetization and the maintenance of a suitable combination of these properties over the operational... 

Fig. 9.59 Hard ferrite components (a) windscreeen wiper motor and ferrite segments (b) loudspeaker and field magnets (c) hard ferrite-polymer composites. (Perspective size distortion identification letters are the same actual size.)...

Generally speaking the higher quality hard ferrites are based upon Sr-hexaferrite and those for less demanding applications on the cheaper Ba-hexaferrite. 

The pre-reacted substance is ground and formed after adding an organic binder. The firing temperatures lie in the range 1000-1400 X both temperature and atmosphere have to be chosen with respect to the stability of the substances. With the exception of hard ferrites the firing is taken to complete sintering. The properties of the product depend on the character of the polyciystalline structure. A coarser... 

Commercially produced ferrites are divided into soft and hard ones. In the former case, the area enclosed by the hysteresis loop is small, the branches being close together. With hard ferrites, the shape of the hysteresis loop approaches that of a rectangle. 

In the manufacture of magnetic memory cores, use is made of ferrites which represent a transition between soft and hard ferrites, e.g. 7 MnO, 52 MgO, 4l Fe203 (mole %). 

A guide to the quality of hard ferrites can be obtained from the maximum energy product (B which can... 

The crystallographic structure of hard ferrites is such that the magnetically preferred (i.e., easy) orientation is the c axis, i.e., perpendicular to the basal plane. Consequently, hexagonal ferrites have been further classified as being either isotropic or anisotropic depending on whether the grains... 

Kools, F. (1991). Hard ferrites. In Concise Encyclopedia of Advanced Ceramic Materials. Ed. R. J. Brook. Pergamon Press, Oxford, pp. 200-6. 

Permanent magnets, made of hard ferrites, are used in applications where a constant magnetic field without electric current is needed. A permanent... 

Table 4.17. Remanence, coercivity and energy product of some hard ferrites.

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