Spontaneous and Induced Magnetisation in Ferromagnetic Bodies

Abstract

According to Weiss's well-known theory, a ferromagnetic body in the absence of an external magnetic field must be spontaneously magnetised in such a way that the direction of magnetisation varies in an irregular manner in different portions of the body. It was originally assumed by Weiss that these portions coincide with the minute crystals of which the body is built up. That this is not so is clear from the fact that spontaneous magnetisation (as revealed by the existence of a Curie temperature) is present also in single crystals. We are thus forced to assume that a moderate-sized single crystal of a ferromagnetic body consists of a number of ‘elementary magnets’ the resultant magnetic moment of which vanishes. This spontaneous subdivision of a ferromagnetic body into elementary magnets can be interpreted both qualitatively and quantitatively in the following manner.