The relation between the micromagnetic and microstructural properties of HDDR-processed Nd - Fe - B-type materials

Transmission electron microscopy has been used to study the magnetic domain structure and its evolution under an external applied field in thin sections of Nd - Fe - B-type magnets produced by the solid-HDDR process. Grain sizes in the recombined material were typically 200 - 800 nm and the orientation of the c axis varied by about over distances in an aligned region. Mean relative orientations of adjacent aligned regions within the sample varied to a much greater extent. In samples with the c axis inclined with respect to the plane of a thin section, an irregular periodic domain structure was observed. Boundaries between oppositely magnetized domains tended to lie along grain boundaries of the smaller grains. Domain walls were observed within the larger grains when a substantial disturbance of the domain structure would be required to reach a grain boundary. In situ magnetizing experiments showed that domain walls in the centres of grains tended to move until they reached grain boundaries, where they remained pinned; with higher fields occasional jumps from one grain boundary to the next were observed until an applied field that appeared to remove all the domain walls in that region had been reached. Under the application of a reverse field, such that domain structures were observed, it was rare to find domain boundaries and grain boundaries which did not coincide. Restoration of the original state, with walls passing through the centres of grains, was only obtained by thermally demagnetizing the sample.