Landau theory gives the magnetic states of matter near their critical temperature. However, it neglects the effects of fluctuations. This hypothesis seems justified for temperatures much smaller than the critical temperature. If we neglect Gaussian fluctuations in the order parameter, it is in fact possible to construct a microscopic theory of magnetism generalising Landau theory to all temperatures. This is mean or molecular field theory. Despite certain limitations, the method can describe qualitatively almost all types of magnetic order occurring in three-dimensional magnets. In addition, many refinements have been made to this theory, extending it to effects of reaction fields and Gaussian fluctuations. It has thus become an essential tool in condensed matter physics.
KeywordsPartition Function Spontaneous Magnetisation Molecular Field Spin Density Wave Landau Theory
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