P. J. Jensen

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

Abstract:The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Greens 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 Greens 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 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.

Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory

Abstract:The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Greens function theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire temperature range of interest since interactions between spin waves are taken into account. A Heisenberg Hamiltonian plus a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term. The orientation of the magnetization is determined by the spin components

Enhanced induced magnetization in coupled magnetic trilayers in the presence of spin fluctuations

\langle {S^\alpha }\rangle (\alpha = x,y,z)

Magnetization and susceptibility of coupled ferromagnetic trilayers calculated with a Green’s function type theory

, which are calculated with the help of the spectral theorem. The knowledge of the orientation angle

Many-body Green’s function theory of ferromagnetic Heisenberg systems with single-ion anisotropies in more than one direction

{\theta _0}

Elementary transitions and magnetic correlations in two-dimensional disordered nanoparticle ensembles

allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient. Results for the Greens function theory are compared with those obtained with mean-field theory (MFT). We find significant differences between these approaches.