  A scalar model is built, as a quantum field theory defined on toroidal
topologies, to describe phase transition in films subjected to periodic
boundary conditions and influenced by an external and constant magnetic field.
Criticality is studied and the relations between the critical temperature, the
film thickness, the magnetic field strength and the chemical potential are
investigated. Since the model describes a second-order phase transition a
comparison with the Ginzburg-Landau theory is made.
