ALBERTO BOLLERO REAL

2.2 Magnetic properties
have a critical single-domain particle size of d c = 200-300 nm [7].
Because in R-T intermetallic compounds the T-T interactions dominate over R-T and
R-R interactions, the balance between exchange energy and anisotropy energy, at any
position in the wall, may thus be considered by taking the T moments as a reference
(corresponding angle from the easy axis). The anisotropy energy for these alloys must
include the R anisotropy and exchange interactions in addition to the T anisotropy.
In fine-grained polycrystalline materials with grain sizes sufficiently less than d c the
grains will be single domain and Bloch walls cannot develop. However, due to the
magnetic interaction (exchange as well as magnetostatic) across the grain boundaries,
competitive magnetic phenomena called interaction domains will occur [6,8-11]. The
contrast and the visibility of the interaction domains is increased if the polycrystalline
material is well textured [3,12,13]. Figure 2.5 shows, as an example, the Kerr image
corresponding to a die-upset melt-spun NdFeB sample.
15 µm
Fig. 2.5: Kerr image showing the domains on a die-upset melt-spun NdFeB sample (MQU-
F), with the c-axis perpendicular to the imaging plane (courtesy O. Gutfleisch).
2.2.5 Origin of coercivity
Once the magnetic domains are aligned along the direction of an applied magnetic
field, the formation of reversed domains is necessary in order to demagnetise the material.
An upper limit of the coercivity is the anisotropy field H a (Eq. (2.6)). However, the
coercivity values of real materials are much less than the corresponding theoretical
maximum values (Brown’s paradox). This is because the formation of reverse domains is a
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