Weiss Molecular Field and Heisenberg Exchange Hamiltonian

WEISS MOLECULAR FIELD AND HEISENBERG EXCHANGE HAMILTONIAN 

For the case of ferromagnetism, i.e. all interactions favoring a parallel alignment of the atomic moments, introduction of a simple phenomenological coupling term has proven extremely successful for describing several important attributes. This term is called the Weiss molecular field Hw, after its inventor Pierre Weiss (655). It is assumed that the interatomic coupling can be represented by an 

However, if W = 47r/3, ferromagnetism would occur only below 1°K. 

In the discussion of the hydrogen molecule, it was pointed out that there are two energy levels, Ei corresponding to a symmetrical coordinate wave function (singlet spin function) and En corresponding to an antisymmetrical coordinate wave function (triplet spin functions). If the part of the Hamiltonian labeled H in equation 21 is treated as a perturbation, the energies of the singlet and triplet states are 

Early workers assumed Jab = Je, neglecting terms to order d2 and higher. Heisenberg suggested that although Je 0 in the hydrogen 

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A. WEISS MOLECULAR FIELD AND HEISENBERG EXCHANGE HAMILTONIAN... 

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