Remarques sur la théorie des propriétés magnétiques des couches minces et des grains fins

Abstract
The author shows, first, that in the case of powdered iron, the variation of the coercitive field against the grains'size, is correctly interpreted by supposing that the energy spent during a hysteresis cycle is only used for the irreversible production of the system of "walls" (produced twice in a cycle in the region where the resulting magnetisation nears zero). Agreement with theory is already less good for magnetite and becomes very bad for Mn-Bi : for the latter, and generally for the substances with only one direction of easy magnetisation, the coercitive field is rather determined by the difficulties in forming the nuclei of inversion of the magnetisation. No satisfactory theory is now available for these phenomena of "nucleation": serious theoretical objections can be raised in fact against the theory proposed by Kittel. In the second part, the author deals again with the theory of the coercitive field of thin continuous layers, and shows that the superficial energy of Bloch's "walls "has no constant value but increases as soon as the thickness of the wall is not negligible when compared with the thickness of the layer : let 0,1 μ be the thickness of an iron layer, the "wall" energy is 7 erg/cm2, the normal value being 1,4 erg-cm 2. It is perhaps an explanation of the increase of the coercitive field of thin layers when their thickness decreases.