Hard and Soft Ferrites

Convention magnetic oxides that contain Fe ions are called ferrites. This terminology does not distinguish between the crystal structures do not confuse it with the chemical name. We often say that ferrites contain FciOs as a principal component. 

Hard ferrites, which are used to fabricate permanent magnets, must have large 

To satisfy these requirements, it is necessary to use materials with crystal structures that exhibit a large magnetic anisotropy and to prevent the growth and rotation of 

Philips introduced ferrite magnets conunercially in 1952 under the trade name Ferroxdure. About 550,0001 of hard ferrites are produced aimually ( 95% of the hard magnet market). This is more than metallic magnets. There are a number of reasons why ferrite magnets are commercially so important, not least of which is that the raw materials are relatively cheap and widely available and the manufacturing processes are simple. 

FIGURE 33.23 Comparison of the size and shape of hysteresis loops for hard and soft magnets. 

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Natural and synthetic iron oxides not possessing pigment properties are used as raw materials in the production of hard and soft ferrites (see Section 5.5.5.2, Electro- and Magneto-Ceramics) for radio, television and telephone technology, for adhesive magnets, for rotors in dynamos, for low-loss magnetic layers, for DC-motors, for transformer cores, for electronic calculators and high frequency furnaces. This amounts to an annual worldwide production of more than 300 10 t/a. 

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

These are developed from magnetic oxides, the ferrites (see section 11.3 and [GOL 87]). They are called hard or soft ferrites depending on the values of the parameters and H. (see Table 11.22). 

Magnetic ceramics represent an important fraction of the magnetic industry in the US, an estimated 40% of the total hard magnetic materials market value is dominated by ferrites, and in spite of the continuous development of new materials, ferrite consumption is still growing. In soft material applications, ferrites participate with an estimated 20% of the market value. In 1990, the estimated world production was 159 500 metric tons of soft ferrites, and 431 100 metric tons of hard ferrites (Ruthner, 1989). In addition to the versatility of ferrites, there are two essential factors which explain this success the low electrical conductivity, and the low production cost. The market value of ferrites ( 3/kg) is very low compared with other electroceramics 33/kg for varistors, 330/kg for thermistors and 1100/kg for ceramic capacitors (Cantagrel, 1986). 

The reader probably knows already that real motors have more coils and poles, and more complex commutators. There is more than one pulse of attraction, and sometimes the current is reversed to also cause repulsion. The coil almost always has a small soft iron core, and the permanent magnet is a large piece of hard steel or ferrite (iron oxide and barium oxide ceramic). Because the permanent magnet is heavier than the coil, it is usually the stationary part, not the way it is shown here. However, this diagram communicates the main ideas. 

Soft ferrites and hard ferrites or hexaferrites are important materials for magnetic uses. Soft ferrites are crystals with a spinel structure whose reference is magnetite Fe304 hard hexaferrites are crystals with a hexagonal structure, whose type is BaFe 2 i9 ... 

More information on noncommercially available ferrites can be found in the Hterature (4,24). Extended reviews on soft (4,24—29) and hard (4,29—32) ferrites are also available. Explanations of magnetic concepts used in this article, such as permeabiHty and coercivity, may be found in magnetism textbooks. 

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