Coupling and surface effects in magnetic thin films

Using the high-temperature series expansions technique, we have analyzed the phase transition and the critical phenomena of a ferromagnetic thin film through three models: Heisenberg, XY and Ising. The critical reduced temperature is studied as a function of the exchange interaction in the bulk and within the surfaces Jb, Js and J⊥, respectively. The dependence of τc on the thickness L of the film has been investigated. A critical value of the surface exchange interaction in the film above which the surface magnetism appears is obtained. The shifts of the critical temperatures Tc(L) from the bulk value [(Tc(∞)/Tc(L))-1] can be described by a power law L-λ, where λ is the inverse of the correlation length exponent. The obtained values are λ=1.34, λ=1.253 and λ=1.21 for Heisenberg, XY and Ising models, respectively. The critical exponent γ associated with the magnetic susceptibility is studied as a function of Jb, Js and J⊥. In a defined range of the exchange interactions, the values of γ are comparable to the universal ones and are independent of the film thickness. The asymmetry of the structure and the competition of the effects of the exchange coupling, are important for the magnetic properties of the system.