Thermodynamic properties of ferromagnetic films in variational cumulant expansion

The thermodynamical properties of quantum Heisenberg films are studied in variational cumulant expansion. The spontaneous magnetization, internal energy and specific heat are calculated to the third order. For a thicknesses below a critical value, the magnetization decreases rapidly as the number of atomic layers is decreased. The internal energy increases with decreasing number of atomic layers at any given temperature, and the specific heat increases below the critical temperature and decreases beyond the critical temperature with decreasing number of atomic layers.