Magnetism in the twentieth century

Abstract

The state of magnetism at the beginning of the century is indicated and the work up to 1913, mainly that of Langevin and Weiss, is briefly described. The outlook during the quantum-principle period (1913-25) and after the reformulation of the quantum theory is sketched. Against this background the theoretical and experimental progress made in connection with dia-, para- and ferromagnetism is considered. The diamagnetic susceptibility of atoms and ions is in good agreement with the charge-distribution indicated by wave mechanics; the susceptibilities of molecules give information as to their sizes, and of crystals as to the sizes and shapes of the molecules in the crystals. Paramagnetic theory applies particularly to ions of the various transition series of elements. For the rare earths the observed magnetic moments agree with those corresponding to the ground states deduced by Hund's method. For the other series the susceptibilities support the view that the effective electrons (those in incomplete groups), being on the outside of the ions, take part in interaction with neighbouring ions. Ferromagnetism is considered in the light of the molecular field hypothesis, and the following topics are discussed: spontaneous magnetization, the nature of the carriers, specific heat, the Barkhausen effect, single crystals, Heisenberg's theory of the molecular field, ferromagnetic materials. The importance of the work on strong magnetic fields is indicated, and the magnetic properties of the elements are briefly discussed.