Soft Magnetic Ferrites

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.

Spinel ferrites, isostmctural with the mineral spinel [1302-67-6] MgAl204, combine interesting soft magnetic properties with a relatively high electrical resistivity. The latter permits low eddy current losses in a-c appHcations, and based on this feature spinel ferrites have largely replaced the iron-based core materials in the r-f range. The main representatives are MnZn-ferrites (frequencies up to about 1 MH2) and NiZn-ferrites (frequencies 1 MHz). 

The soft magnetic spinel ferrites, by far the most important cubic ferrites, were first introduced by Philips under the trade name Ferroxcube (14) and are now widely commercially available under various trade names. The world market for soft magnetic ferrites amounts to about one biUion dollars (1991), about 350 million dollars of which is in the United States. 

Soft magnetic materials are characterized by high permeabiUty and low coercivity. There are sis principal groups of commercially important soft magnetic materials iron and low carbon steels, iron—siUcon alloys, iron—aluminum and iron—aluminum—silicon alloys, nickel—iron alloys, iron-cobalt alloys, and ferrites. In addition, iron-boron-based amorphous soft magnetic alloys are commercially available. Some have properties similar to the best grades of the permalloys whereas others exhibit core losses substantially below those of the oriented siUcon steels. Table 1 summarizes the properties of some of these materials. 

The most commonly used ferrites, the so-called soft ferrites, are used ia soft magnet and low field telecommunication appHcations, low power... 

Soft magnetic spinel ferrites, 11 57 Soft oils, in toilet soap making, 22 734 Soft repulsions, 23 94 Soft rot... 

Energy losses in soft magnetic materials arise due to both hysteresis and eddy currents, as described in the previous section. Eddy current losses can be reduced by increasing the electrical resistivity of the magnetic material. This is one reason why solid-solution iron-silicon alloys ( 4% Si) are used at power frequencies of around 60 Hz and why iron-nickel alloys are used at audio frequencies. Some magnetically soft ferrites (see Section 6.2.2.1) are very nearly electrical insulators and are thus immune to eddy current losses. Some common soft magnetic materials and their properties are listed in Table 6.19. Soft magnetic alloys are described further in Section 6.2.1.6. 

Soft magnetic ferrites are oxides and they are electrical insulators. Because of their exceptionally higher resistivities, ferrites are particularly suitable for high frequency applications, of about 100.000 cycles 110 kHz). 

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. 

The magnetic properties of these compounds and hence their applications are closely coupled to their crystal structure. This is the basis of their classification into ferrites with soft magnetic properties (cubic) and hard magnetic properties (hexagonal). 

Ferrites with the cubic spinel structure (see Section 5.3.10) are soft magnetic materials (see Section 12.4.3), widely used in electronic circuitry. The formula of all ferrites can be written as A +Fe +04, where can be chosen from a large number of medium-sized cations, for example Ni or Zn. The majority of the important ferrites are inverse spinels, in which the cations occupy the octahedral sites, together with half of the Fe + ions. The other half of the Fe cations is found in the tetrahedral sites. Thus nickel ferrite would be written (Fe +)[Ni Fe ]04, where the cations in octahedral sites are enclosed in square brackets and those in tetrahedral sites in parentheses. Lodestone, or magnetite, Fe304, described in Section 12.3.4, is an example in which the cations are Fe and Fe, and the cation distribution is (Fe +)[Fe2+Fe +]204. 

Spinel (cubic ferrites) 1 MeO IFesOa MeO = transition metal oxide, e.g., Ni,Co, Mn, Zn Soft magnets... 

Table 33.10 compares some of the relevant properties of metal and ceramic soft magnets. Spinel ferrites based upon the (Mn,Zn,Fe)04 system are examples of commercially important soft magnets. The market for soft ferrites is about 50,0001 per year. These are usually marketed under the trade name Ferroxcube. 

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