Mean-field approximation general concept

This quite general concept is used to specify any shift (in time and space) between the instantaneous value of magnetic moments and dieir mean value given by flie molecular field approximation and the Boltzmann statistics. Spin fluctuations can be individual and/or collective. In the latter case processes such as spin waves or paramagnons are considered and many models have been developed to interprete experiments with these concepts. 

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