ABSTRACT
In this chapter, the author presents a few standard methods to study a family of simple magnetic systems where the loss of translational invariance is caused by the existence of surfaces or interfaces such as semi-infinite crystals, thin films, or multilayers. He provides a theoretical framework to understand microscopic mechanisms which lead to macroscopic surface effects such as low surface magnetization, low transition temperature, surface phase transition and surface spin-configuration instability. The density of states shows often surface states which modify the filling of electronic bands, the position of the Fermi level and the magnetic moment of surface atoms. In addition, the surface anisotropy and surface exchange interaction can be very different from those of the interior atoms. Phase transition is a collective phenomenon which takes place when the system changes its symmetry. At the transition, the system spins become strongly correlated at a macroscopic scale.
