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 route, Large aligned regions of submicron grains cause an irregular, periodic magnetic domain structure, In situ magnetising experiments showed that domain walls in the centres of grains tended to move until they leached grain boundaries where they remained pinned; with higher fields occasional jumps from one grain boundary to the nest were observed until an applied field was reached that appeared to remove all the domain walls in that region. Under the application of a reverse field, where domain structures were observed, it was rare to find domain boundaries and grain boundaries which did not coincide.
Thompson, P., Gutfleisch, O., Chapman, J. N., & Harris, I. R. (1998). Domain studies in thin sections of HDDR-processed Nd-Fe-B-type magnets by TEM. Journal of Magnetism and Magnetic Materials, 177-181(2), 978-979. https://doi.org/10.1016/S0304-8853(97)00780-4