Absence of a Magnetic Phase Transition in Heisenberg, Hubbard, and Kondo-lattice (s-f) Films

The possibility of a magnetic phase transition in Heisenberg, Hubbard, and s-f (Kondo-lattice) films is investigated. It is shown that, for any finite temperature and any finite number of layers, the magnetization within every layer must vanish. Thus, the Mermin-Wagner theorem is extended to a variety of system geometries. We also comment on the microscopic interpretation of the transition from two to three dimensions in the limit of an infinite number of layers.