The ferromagnetic phase transition in ultra thin films is studied by the many body Green's function method based upon the Heisenberg model with the different surface exchange couplings with respect to bulk one plus a surface second order uniaxial anisotropy. The critical temperature of the system is studied as a function of the ratio S S J J ∆ = / of the surface exchange couplings to the bulk ones and as a function of the surface single ion anisotropic strength K (cross over points). If, the critical temperature of the thin film is larger than the bulk one. We analyse various possible configurations for the surface exchange couplings S J and the surface single ion anisotropic strength

A Heisenberg model is solved within the spin-wave theory for thin films in which ferromagnetic monolayers are separated by nonmagnetic spacer layers. The interface interaction is assumed to be ferromagnetic. We have included also a magnetic anisotropy in each monolayer. The temperature dependence of the spin-wave contribution to the heat capacity C m in such composite systems is derived for several sets of material parameters and for different film thicknesses.

Within the framework of many-body Green's function theory there are studied the properties of the quantum Blume-Capel model for ferromagnetic films with an alternating single-ion anisotropy on the odd atomic layers and on the even ones. We analyse various possible phase diagrams for the surface exchange couplings and the single-ion anisotropy parameters. Model and methodThe ferromagnetic Blume-Capel-Ising (BCI) model has been studied within the mean field approximation [1], the effective field theory [2], the two-spin cluster approximation in the cluster expansion method [3,4], Monte Carlo simulations [5], a thermodynamically self-consistent theory based on an Ornstein-Zernike approximation [6], the exact solution based on the Bethe lattice by means of the exact recursion relations [7]. Most of the studies mentioned above displays also the existence of a tricritical point at which the phase transition changes from second order to first order when the value of K 2 becomes negative. Our work represents the first atempt to consider a quantum version of BCI model. Within quantum Blume-Capel (QBC) model we will study the influence of the enhancement of the surface exchange coupling and the alternative single-ion anisotropy K 2 (1) on the odd atomic layers and K 2 (2) on the even ones on the critical behaviour of thin ferromagnetic films.The Hamiltonian of the considered system consists a Heisenberg exchange interaction with strength J ij > 0 between nearest neighbour lattice sites, an exchange anisotropy with strength D > 0, and a second-order single-ion anisotropy with strength K 2 > 0: (197)

A Heisenberg model is solved within the spin-wave theory for thin films in whictJ ferromagnetic monolayers are separated by nonmagnetic spacer layers. The interface interaction is assumed to be ferromagnetic. We have included also a magnetic anisotropy in each manolayer. The temperature dependelme of the spin-wave contribution to the heat capacity Cm in such composite systems is derived for different interlayer couplings and for different fihn thickness. : 75.30.Ds, PACS

We consider an anisotropic Heisenberg model on a ferromagnetic thin film (FTF) within the linear spin-wave theory. We analyze various possible configurations for the surface and bulk anisotropy parameters. We determine different configurations for the spin ordering of the system.