ALBERTO BOLLERO REAL

4.3 PrFeB
would explain the higher saturation moment exhibited for low Co contents, in comparison
with Nd 2 Fe 14 B.
The substitution of Fe by Co has a small effect on the magnetocrystalline anisotropy
because the main contribution is from the rare-earth sublattice, but it is a negative effect
having a decrease of H a with Co addition. This effect was expected from the shrinkage of
the lattice mentioned previously, i.e. of the decrease of the easy c axis. Figure 4.3 shows
the decrease of the anisotropy field with increasing temperature of Nd 2 Fe 14 B and
Nd 2 Co 14 B. It can be seen that the decrease of the latter is faster, reflecting the weaker R-Co
interactions.
H a
( T )
T ( K )
Fig. 4.3: The temperature dependence of the anisotropy field in Nd 2 Fe 14 B and Nd 2 Co 14 B
(adapted from Coey [3]).
In conclusion, addition of Co constitutes an effective way of improving T C , but a too
high Co-content leads to a detriment of the magnetic properties besides an increase in the
final cost of the magnet due to the higher Co price.
4.3 PrFeB
The Pr 2 Fe 14 B compound also exhibits a tetragonal structure. The Pr-Fe-B phase
diagram is very similar to that of Nd-Fe-B as it can be observed in Fig. 4.4. In terms of
intrinsic magnetic properties, Pr 2 Fe 14 B has a slightly lower saturation polarisation value (J s
= 1.56 T) and a lower Curie temperature (T C = 297°C) than Nd 2 Fe 14 B [83]. However,
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