Many-body Green's function theory for the magnetic reorientation of thin ferromagnetic films

The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Green's function theory by co nsidering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigen vectors of the equations of motion matrix fur the set of Green's functions. This formulation allows a straight forward extension of the monolayer case to thin films with many layers and for arbitrary spin and moreover provide s a practicable procedure for numerical computation. The model Hamiltonian includes a Heisenberg term, an external magnetic field, a second-order unia xial single-ion anisotrophy, and the magnetic dipole-dipole coupling. We ut ilize the Tyablikov (RPA) decoupling for the exchange interaction terms and the Anderson-Callen decoupling for the anisotrophy terms. The dipole coupl ing is treated in the mean-field approximation, a procedure which we demons trate to be a sufficiently good approximation for realistic coupling streng ths. We apply the new method to monolayers with spin S greater than or equa l to 1 and to multilayer systems with S = 1. We compare some of our results to those where mean-field theory (RIFT) is: applied to all interactions, p ointing out some significant differences.