The Present Status of Ferromagnetic Theory

ISCOVERY of the loadstone and some of its magnetic properties is now reputed to be some three thousand years old. During these many years ferromagnetism has resisted very successfully the attack of theorists, and even at the present time theory lags far behind experiment. But advances in theory have been particularly rapid during the last five or ten years; the author describes in this paper what he regards as the high points of this progress. Not until the last quarter of the last century was any considerable work done on magnetic materials. During this period data were gathered rapidly until, just before the close of the century, an excellent book 2 of four hundred pages, containing practically all of the important experimental and theoretical facts, was written by J. A. Ewing, later Sir James Ewing. The shape of the magnetization curves of iron, cobalt, and nickel, the existence of magnetic saturation and the magnetic transformation temperature, the existence and some of the laws of hysteresis, the simpler effects of stress and of magnetostriction, together with the important methods of measurement--all were known then, and silicon steel had just been invented. Strangely enough, during the next fifteen years there was but little advance in knowledge of magnetic materials, but there were many applications of existing knowledge by engineers to electrical machinery, including those in electrical communication. During this period, also, the Heusler alloys (non-ferrous alloys exhibiting ferromagnetic properties) were invented; and although these served to stimulate those interested in the theoretical aspects of ferromagnetism, still there was little progress.