Weiss molecular field

This review will be concluded with some speculative remarks on possible sources of the several extrinsic field effects. There is one kind of behavior that appears to have a simple explanation. The ferromagnetic catalysts all show zero field effect below Tc The reason for this must be that in all such substances the Weiss molecular field is of the order of 10s to 107 Oe. An extrinsic field of a few kOe could produce little additional effect. 

A. 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... 

Recent theoretical treatments of the soft-mode behaviour include a detailed study by Onodera using classical mechanics, and a theory of hydrogen-bond mechanics, including tunnelling effects, by Stamenkovic and Novakovic. ° Onodera assumes a quartic potential function for his individual oscillators, with a bilinear interaction which reduces to c x, where x is the displacement, under the Weiss-molecular-field approximation. The model is soluble without further approximation (in series of elliptic functions), yielding the temperature variation of frequency and damping. If the quartic potential has a central hump larger than kTc,... 

As could be expected, it follows from the results of this model that the phase transition region broadens with increasing temperature and disappears at high temperatures. The same picture was obtained for 2D structures in the paper (26) in the Weiss molecular field approximation. 

In Nd pnictides which have a Fg ground state, quadrupole-quadrupole interactions appear to be dominant. For NdSb the tetragonal distortion below the N6el point (Levy, 1969) due to the magnetoelastic effect has been interpreted with a Hamiltonian including crystal field, Weiss molecular field and a magnetoelastic term (Bak and Lindgdrd, 1973 Furrer et al., 1976 Wakabayashi and Furrer, 1976)... 

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