Hard 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.

Fig. 10. Demagnetization curves of hard magnetic materials A, Nd2Fe B B, Sm(Co, Cu,Fe,Zr)2 C, SmCo D, bonded SmCo E, Alnico 5 F, Mn—Al—C G, Alnico 8 H, Cr—Co—Fe I, ferrite , bonded ferrite. To convert T to G, multiply by 10". ...

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. 

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

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). 

Boll, R. Hilzinger, H. R. (1983). Comparison of amorphous materials, ferrites and permalloys. IEEE Transactions on Magnetics, Mag-19, 1946-51. Buschow, K. H. J. (1990). New Developments in hard magnetic materials. Reports of Progress in Physics, 54, 1123-213. 

A magnetic material which is easy to magnetise (and demagnetise) is known as a soft material, and permanent magnets are referred to as hard magnets. The boundary in coercive field value for these two cases is somewhere between 1 kA/m (soft if // < 1 kA/m) and 10 kA/m (hard for k 10 kA/m). In ferrites, Mn-Zn spinels have the lowest values (//< 16 A/m) and BaFe 120,9 particles the highest 420 kA/m, Mee... 

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). 

At the end of the 1960s, a new material with exceptional hard magnetic properties (Table 6.7) was prepared samarium-cobalt, SmCoj. Its most outstanding property is its magnetocrystalline anisotropy reported values are in the range (11-20) x 10 J/m (by comparison, Ba ferrite has an anisotropy of 0.33 x 10 J/m, Table 4.15). Rather than being a serendipitous discovery, the successful synthesis of SmCoj was the result of a systematic effort (Strnat, 1988). 

Hard magnetic materials have rather rectangular, broad, hysteresis curves (Figure 12.16). These materials are used in permanent magnets, with applications from door catches to electric motors. The hexagonal ferrites are hard magnetic materials, as are alloys such as SmCos and Nd2Fei4B. 

Magnetoplumbite (hexagonal ferrites) 1 MeOieFejOa MeO = divalent metal oxide from group IIA e.g., BaO, CaO, SrO Hard magnets... 

Philips introduced ferrite magnets commercially in 1952 under the trade name Ferroxdure. About 550,0001 of hard ferrites are produced annually (>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. 

A hard magnetic ferrite, also eaUed ferrite magnet, is a magnetic material based on iron oxide. The composition of the typical hard magnetic ferrite compounds is shown in Table 4.3-54. M t3tpe material is used most widely and BaO can be replaced by SrO. 

Table 4.3-55 shows the basic magnetic properties of hard magnetic ferrites. The properties of actual products are shown in Fig. 4.3-77. The demagnetization curves of type YBM-9B, which have the best magnetic properties, are shown in Fig. 4.3-78. 

The mineral barium magnetoplumbite has the chemical formula BaFe Oig or BaO -bFeiOs and is perhaps the most important of the hexagonal ferrite since it is a hard magnet with the spins all aligned along the c-axis. This oxide is used in the magnetic stripe on credit cards. 

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