  The field-induced reorientation of the magnetization of ferromagnetic films
is treated within the framework of many-body Green's function theory by
considering all components of the magnetization. We present a new method for
the calculation of expectation values in terms of the eigenvalues and
eigenvectors of the equations of motion matrix for the set of Green's
functions. This formulation allows a straightforward extension of the monolayer
case to thin films with many layers and for arbitrary spin and moreover
provides a practicable procedure for numerical computation. The model
Hamiltonian includes a Heisenberg term, an external magnetic field, a
second-order uniaxial single-ion anisotropy, and the magnetic dipole-dipole
coupling. We utilize the Tyablikov (RPA) decoupling for the exchange
interaction terms and the Anderson-Callen decoupling for the anisotropy terms.
The dipole coupling is treated in the mean-field approximation, a procedure
which we demonstrate to be a sufficiently good approximation for realistic
coupling strengths. We apply the new method to monolayers with spin $S\geq 1$
and to multilayer systems with S=1. We compare some of our results to those
where mean-field theory (MFT) is applied to all interactions, pointing out some
significant differences.

Statistical Mechanics;Strongly Correlated Electrons