Magnetic Properties of Heisenberg Thin Films in an External Field**The project supported in part by National Natural Science Foundation of China under Grant No. 10247208 and the Foundation of the Committee of Science and Technology of Chongqing under Grant No. 2002-7310

Abstract

The magnetic properties of Heisenberg ferromagnetic films in an external magnetic field are investigated by means of the variational cumulant expansion (VCE). The magnetization can be in principle calculated analytically as the function of the temperature and the number of atomic layers in the film to an arbitrary order of accuracy in the VCE. We calculate the spontaneous magnetization and coercivity to the third order for spin-1/2 Heisenberg films with simple cubic lattices by using a graphic technique.