Ferromagnetic phase transition in ultra-thin films

摘要

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 Delta(S) = J(S)/J of the surface exchange couplings to the bulk ones and as a function of the surface single ion anisotropic strength K-2(S). A model calculation shows thatexist critical values Delta(C)(S) and K,s' (cross over points). if Delta(S) > Delta(C)(S) or K-2(S) > K-2(SC), the critical temperature of the thin film is larger than the bulk one. We analyse various possible configurations for the surface exchange couplings J(S) and the surface single ion anisotropic strength K-2(S). (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.