Properties Of Ferromagnetic Films In Variational Cumulant Expansion

Recent development of the technique of molecular-beam expitaxy has allowed us to fabricate different kinds of crystalline materials in the form of thin films, heterostructures and superlattices. Beacause of the small thickness, the properties of thin films can differ drastically from the bulk ones. With the strange properties, the materials have been applied extensively. Among them, the magnetic film consisting of a few parallel atomic layers is of particular importance, for it has been used as a conventional model for theoretical and experimental analysis of a variety of magnetic phenomena.The mean-field theory has been always a simple and effective method for analyzing magnetic properties of all material. The analysis to various models indicates, however, that it is difficult to treat the restricted systems as magnetic films in the mean-field theory, and there exist obvious difference between the mean-field results and those of Monte Carlo simulation. As a systematic approach, the variational cumulant expansion, which is first proposed for discussions of lattice gauge field models, can give higher-order corrections to mean-field analyses, and it has been proved to be a very effective method in treating film system. Therefore, using variational cumulant expansion, we investigate the thermodynamical and magnetic properties of ferromagnetic films.