Nucleation-controlled magnets

Fig. 15. Schematic magnetization curves for nucleation controlled magnets, type A, like SmCo5 and for pinning controlled magnets, type B, like Sm(Co, Cu, TM)X.

Most experimental observations made on the Nd-Fe-B type materials indicate that the coercivity in the corresponding sintered magnets is nucleation-controlled (fig. 47a or b) rather than pinning controlled (fig. 47c). 

Fig. 16. Comparison of the virgin magnetization curves for typical magnets of three basic behavior types A is nucleation controlled B is pinning controlled, and C is mixed (or pinning with very nonuniform pin...

The transformation of the jointly precipitated (Step I) mixture of ferrous and ferric hydroxides to magnetite (Step II) was studied by the continuous measurement of the magnetic moment. It was found that at large excess ammonia, added in Step I, magnetite forms at a fast rate and the transformation is complete. At small excess ammonia the transformation is slow and incomplete. The application of Avrami s theory to the data indicated grain boundary nucleation in the former case and grain boundary nucleation after saturation followed by diffusion controlled growth in the latter case. Mechanisms are proposed for both cases. 

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