Classical Heisenberg model

The Classical Heisenberg model, developed by Werner Heisenberg, is the formula_1 case of the n-vector model, one of the models used in statistical physics to model ferromagnetism, and other phenomena.
Definition.
It can be formulated as follows: take a d-dimensional lattice, and a set of spins of the unit length
each one placed on a lattice node.
The model is defined through the following Hamiltonian:
with
a coupling between spins.
Properties.
Three and higher dimensions.
Independently of the range of the interaction, at low enough temperature the magnetization is positive.
Conjecturally, in each of the low temperature extremal states the truncated correlations decay algebraically.