A Survey of the Theory of Ferromagnetism

J. H. Van Vleck

Harvard University, Cambridge, 3fassackusetts 

Rev. Mod. Phys. 17, 27 – Published 1 January 1945

THE WEISS MOLECULAR FIELD 

The outstanding development in the phenomenological description of ferromagnetism is undoubtedly Weiss' theory of the molecular field, ' which he first published in 1907. Shortly before this, Langevin had developed his wellknown theory of paramagnetism, based on the fundamental idea that the orientation of a molecular dipole of moment p in a field H is governed by the Boltzmann distribution law. With this hypothesis it is easy to show' that the moment per unit volume is given by the expression

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