Brown's "fundamental theorem" revisited

Brown's method of calculating an upper and a lower bound of the "critical size" for a single domain behavior is recalled, with a particular emphasis on the case of soft ferromagnets. The upper bound for a sphere is very slightly improved. Any further improvement, if at all needed for isolated particles of soft materials, should be done in the same direction. This improvement is used to demonstrate that this method can be extended to the case of interacting neighbors, but not to the study of very many grains, for which micromagnetics is inadequate and has to be superimposed by domain theory. (C) 2001 American Institute of Physics.