What is Heisenberg model of ferromagnetism?
The isotropic Heisenberg model is a magnetic model in which interaction energy of spins s1 and s2 on the neighboring sites of the lattice is equal to Js1 •s2. In the two-dimensional Heisenberg model the order is absent at T ≠ 0 (see, for instance, Patashinskii and Pokrovsky, 1979).