Thermodynamic properties of ferromagnetic films in variational cumulant expansi on

Abstract

The thermodynamical properties of quantum Heisenberg films are studied in variat ional cumulant expansion. The spontaneous magnetization，internal energy and spe cific 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 de creased. The internal energy increases with decreasing number of atomic layers at any given temperature, and the specific heat increases below the critical tem perature and decreases beyond the critical temperature with decreasing number o f atomic layers.