The Grifiths inequalities establishes the existance of the thermodynamic fo
The Grifiths inequalities establishes the existance of the thermodynamic for this model. Mack and Schomerus developed a chiral version of it used in quantum field theories. A symmetry relating the free energy of low temperature and high temperature versions of this model is named for Kramers and Wannier.  Originally given as a problem from Wilhelm Lenz to its namesake, the 2-dimensional case with no external field was solved by Onsager.  In the case of vectors with more than two possible orientations, this model generalizes to the Potts model.  In two dimensions or more, it possess a phase transition, which for the original use of this model is represented by the Curie temperature. FTP, give the name of this nearest-neighbor model of aligning spins originally developed to explain ferromagnetism.
The Grifiths inequalities establishes the existance of the thermodynamic for this model.
Ising model