  The thermodynamic properties of a classical d-dimensional spin-S Heisenberg
ferromagnet, with long-range interactions decaying as $r^{-p}$ and in the
presence of an external magnetic field, is investigated by means of the
spectral density method in the framework of classical statistical mechanics. We
find that long-range order exists at finite temperature for $d<p<2d$ with
$d\leq 2$ and for $p>d$ with $d>2$, consistently with known theorems. Besides,
the related critical temperature is determined and a study of the critical
properties is performed.

Statistical Mechanics