Abstract
We consider an Ising model with Kac potential γdK(γ¦x¦) which may have arbitrary sign, and show, following Gates and Penrose, that the free energy in the classical limitγ→0+ can be obtained from a variational principle. When the Fourier transform of the potential has its maximum atp=0 one recovers the usual mean-field theory of magnetism. When the maximum occurs forp 0≠0, however, one obtains an oscillatory or helicoidal phase in which the magnetization near the critical point oscillates with period 2π/¦p 0¦. An example with a potential possessing parameter-dependent oscillations is shown to exhibit crossover phenomena and a multicritical Lifshitz point in the classical limit.
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Pisani, C., Thompson, C.J. Generalized classical theory of magnetism. J Stat Phys 46, 971–982 (1987). https://doi.org/10.1007/BF01011152
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DOI: https://doi.org/10.1007/BF01011152