The Present Status of Ferromagnetic Theory * 



By R. M. BOZORTH 



T~^ 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 im- 

 portant 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 magneto- 

 striction, 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 prop- 

 erties) were invented; and although these served to stimulate those 

 interested in the theoretical aspects of ferromagnetism, still there was 

 little progress. 



Beginning between 1915 and 1920 and extending to the present, 

 there has been a rapid development on both the experimental and 

 theoretical sides of ferromagnetism. To illustrate the progress that 

 has been made in the improvement of magnetic materials. Table I 

 has been prepared. The improvements made during the last 20 years 

 have resulted from new methods of purification of the materials, new 



* This paper as herein published contains a few revisions and additions to the 

 paper as it appeared in the November 1935 issue of Electrical Ettgweering. It is 

 scheduled for presentation at the A. I. E. E. Winter Convention, New York, N. Y., 

 January 28-31, 1936. A subsequent paper in the same field of endeavor is planned 

 in which entirely new material will be presented. 



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