Until now, we have treated the case of magnetic body not subject
to external field. Therefore, all the energy terms introduced in
the previous sections can be regarded as parts of the Helmholtz
free energy functional. When the external field is considered, it
is convenient to introduce the Gibbs free energy functional. In
this respect, the additional term (see
Eq. (1.17)) related to the external field
a,
is itself a long-range contribution too. In fact,
it can be seen as the potential energy of a continuous magnetic
moments distribution [15] subject to external field
a:

aa

(1.51)

This energy term is referred in literature to as Zeeman
energy.
Massimiliano d'Aquino
2005-11-26