Basic model of ferromagnetic solids:
atoms are fixed on the vertices of a lattice. Their
dipole vectors (spins) may have varying directions: either
up/down (Ising model) or any direction (Heisenberg model).
: given by
the spins on the lattice.
Two-dimensional square Ising lattice; only the four nearest spins
contribute to the energy of spin
(). (Three dimensions: six nearest neighbors).
The total energy is
( ... coupling constant).
may be determined as a function of temperature.
If a magnetic field is applied,
the additional potential energy is
F. J. Vesely / University of Vienna