ALBERTO BOLLERO REAL

2.2 Magnetic properties
Fig. 2.6: Hysteresis loop for a permanent magnet material indicating by dashed lines the
characteristic initial magnetisation curve for the case of pinning and nucleation
mechanisms (from Skomski and Coey [19]).
single domain size diameter [11,27]. The detailed demagnetisation behaviour of such
materials will depend on the type and the strength of the magnetic interaction between the
small grains. The magnetisation process of a certain class of fine-grained magnets will be
discussed in Section 3.3 and Chapter 8.
The pinning mechanism is typical of materials with pronounced nanoscale
inhomogeneities. The effectiveness of the pinning will depend on the geometry and spatial
distribution of the pinning centers, being necessary a sufficiently high density of such
centers not uniformly distributed [28]. The pinning will be most pronounced when the size
of the pinning centers is comparable to the domain wall width. In addition sharp
boundaries may be very effective pinning centers. Sm 2 Co 17 -based magnets constitute a
good example of this type of magnets where the pinning of the domain walls takes place in
the SmCo 5 boundary phase [28,29].
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