New insight into the thermodynamics of Heisenberg ferromagnets as inferred from high-temperature series
Résumé
In search of a suitable equation of state for ferromagnets, we revise the information about the Heisenberg model obtainable from high- temperature series. Special attention is paid to the ratio chi(3)/chi(4) (where chi and chi(3) are the linear and cubic susceptibilities) related to Landau's quartic coefficient b. It is found in particular that both chi(3)/chi(4) and b tend to a finite limit as T. TC. This limit is small - an order of magnitude smaller than predicted by Weiss's molecular field and similar theories - but contrary to the common belief, nonzero. This implies a rejection of the generally accepted critical- point exponents and a return to those of Landau: alpha = 0, beta = 1/2, gamma = 1, etc.