Domain wall pinning by fine precipitates

The addition of Nb to the Nd-Fe-B-type magnet causes precipitates to form within the hard magnetic grain. Domain-wall pinning effects of the precipitates were investigated by measuring an initial magnetization curve and by domain-wall observation in a magnetic field using Lorentz microscopy. Precipitates which were 500 A in size and 10/sup 15/ nuclei/cm/sup 3/ in density were produced in an Nb-doped (Nd,Dy)-Fe-B sintered magnet and found to contain Nb. A Lorentz micrograph showed that the domain walls pass through the precipitates. The initial magnetization increased abruptly, which indicates that the magnetization process is still nucleation-type. It was observed that almost all walls disappear under application of an external field of 5 kOe. It is concluded that the pinning effect is not strong enough to produce a significant increase in the coercive force of the permanent magnet and that the suppression of grain growth by niobium addition is the cause of the improvement of the coercivity.<
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