A Fluctuating Field Theory for Systems of Localized Magnetic Moments
Thermodynamics of Heisenberg ferromagnets is described by means of a fluctuating exchange field operating on the atomic magnetic moments. A self-consistent method for calculating field characteristics is developed at arbitrary temperatures. It is shown that transitions in a paramagnetic state may pass both continuously and stepwise on temperature subject to magnitude of single-site susceptibility.
V. I. Grebennikov "A Fluctuating Field Theory for Systems of Localized Magnetic Moments", Solid State Phenomena, Vols. 152-153, pp. 563-566, 2009