A REFINEMENT OF THE HEISENBERG THEORY OF FERROMAGNETISM, APPLICABLE TO SIMPLE CUBIC CR YSTALS

By CHARLES H. FAY

RESEARCH LABORATORY OF PHYSIcs, HARVARD UNIVERSITY

Communicated June 17, 1935

 Introduction.-

The Heisenberg theory of ferromagnetism,' in what we shall term its "simple" form,2 involves the replacement of the energy levels of the spin multiplets of the micro-crystal for a given total spin quantum number S' by the average energy for S'. As a supposedly better approximation Heisenberg has used the assumption of a Gaussian distribution of energies for S'; however, this approximation is in some respects worse than the simple approximation, for it has the effect of introducing energies lower than that of the state of maximum S' for states of small spin. Hence, even though ferro magnetism may be obtained in a certain temperature range, the crystal becomes paramagnetic at very low temperatures, instead of approaching its true saturation magnetization as it should. The work of the present paper was undertaken largely in order to get a better idea of the validity of the simple approximation described above.

https://www.pnas.org/content/pnas/21/9/537.full.pdf

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