Contribution to the

Theory of Ferromagnetism

1925

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Contribution to the Theory of Ferromagnetism. ¹⁾

by Ernst Ising in Hamburg.

(received on 9 December 1924).

This is essentially an investigation of the thermal behavior of a linear body consisting of elementary magnets whereby it is assumed, in contrast to Weiss' theory of ferromagnetism, that there is not a molecular field, but only a (non-magnetic) mutual effect of neighboring elementary magnets. It is shown that such a model does not yet have ferromagnetic properties and that this statement also extends to the three-dimensional model.

1. Assumptions.

The explanation which P. Weiss 2) gave for ferromagnetism is, it is true, formally satisfying, but it leaves open particularly the question of a physical explanation of the hypothesis of the molecular field. According to this theory, an inner field which is proportional, in each case, to the intensity of magnetization, influences each elementary magnet, except for the outer magnetic field. It proposes electrical dipole effects for the effects of the individual elements (= elementary magnets). But then very considerable electrical field strengths would result through the summation of very slowly decreasing dipole fields which would be destroyed by the conducting power of the material. Therefore, we propose, in contrast to Weiss, that the forces which the elements exert upon each other quickly fade with distance so that, in a first approach, only neighboring atoms influence each other.

Second, we propose that the elements take up only a few distinct energy level orientations corresponding to the crystal structure. As a result of the movement of heat, the elements change from one possible position to another. We propose that the inner energy is the smallest when all elements are oriented in the same direction. Essentially, these propositions were first set up and argued in detail by W. Lenz. 3) 

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1) Excerpt of the Hamburg dissertation 

2) P. Weiss, Journ. de phys. (4) 6, 661, 1907, and Phys. ZS. 9, 358, 1908. 

3) W. Lenz, Phys. ZS. 21, 613, 1920.