It is strange that the material magnetization is in linear proportion to external field because I see a positive feedback: The external field orientates the atoms. This increases the magnetic field, and, in turn, the increased field forces more and more atoms to align. The process goes on until saturation - all molecules end up in line. So, which force resists the alignment development ensuring the linearity? I suspect that this force must be elastic like the Hooke's law. Honestly, I do not distinguish very well between B, H and M but I know that the current applied to a ferrite core polarizes its domains increasing some magnetic field (which of the three?).