Physics of Magnetism

118 CHAPTER 12. PERMANENT MAGNETS

It can be seen in Table 12.5.1 that the advantage of and over

is their much higher Curie temperature. These materials are preferred as permanent magnets

in electrical machines having a high use temperature, as in several automotive and aircraft

applications with use temperatures in the range 200–300°C. The reason why, of the

and series of compounds, only the compounds with R = Sm qualify for permanent-

magnet materials can be sketched along the same lines as given above for Because

is also negative, high total magnetization values are only obtained when R belongs

to the light rare-earth elements. In contradistinction to the crystal structure of the

compounds, one has for the and series that Using again Eq. (12.5.2)

and the data listed in Table 5.2.1, one finds

This leaves R = Sm as the only possible rare-earth element that can be used in and

permanent magnets.

At temperatures below room temperature it is no longer legitimate to ignore the fourth-

at room temperature the

value of

order term in Eqs. (12.5.2) and (12.5.3). Although, in
is only approximately 1% of the fourth-order term will dominate at low
temperatures. This leads to a temperature dependence of the anisotropy constants and
as shown in Fig. 12.5.1. It can be seen in this figure that changes sign at the spin-
reorientation temperature Below this temperature, the preferred magnetization
direction starts to deviate from the direction and for each temperature has a direction given
by Eq. (11.4) introduced in Chapter 11:

It can be seen in Fig. 12.5.2 that the tilt angle reaches about 30° at 4.2 K. The results
shown in Fig. 12.5.2 make it also clear that permanent magnets based on
their usefulness at cryogenic temperatures.

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