A more realistic model of magnetism in that it treats the spins quantum-mechanically, replacing each spin by a quantum operator acting on the tensor product (C²)^⊗N.
For positive coupling J the ground state is always ferromagnetic; at negative J the ground state is antiferromagnetic in two and three dimensions.
Introduced in Werner Heisenberg's 1928 paper 'Zur Theorie des Ferromagnetismus' in Zeitschrift für Physik 49, 619–636.