New insight into the thermodynamics of Heisenberg ferromagnets as inferred from high-temperature series

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 χ3/χ4 (where χ⁢and⁢χ3 are the linear and cubic susceptibilities) related to Landau's quartic coefficient b. It is found in particular that both χ3/χ4 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: α=0, β=12, γ=1, etc.