The theory of spin models is a far reaching and unifying topic that cuts across many fields.
In ordinary materials, the magnetic dipole moments of individual atoms produce magnetic fields that cancel one another, because each dipole points in a random direction.
Ferromagnetic materials below their Curie temperature, however, exhibit magnetic domains in which the atomic dipole moments are locally aligned, producing a macroscopic, non-zero magnetic field from the domain.
The study of the behavior of such "spin models" is a thriving area of research in condensed matter physics.
Numerical implementation of these models has led to several interesting results, such as quantitative research in the theory of phase transitions.