THERMODYNAMICS OF THIN FERROMAGNETIC-FILMS IN THE PRESENCE OF ANISOTROPY AND DIPOLAR COUPLING

We discuss aspects of the thermodynamics of ultrathin ferromagnetic films, with dipolar couplings and anisotropy present. We present a formalism that allows one to form proper normal-mode spin-wave annihilation and creation operators, from the left and right eigenvectors generated by the non-Hermitian matrix encountered in the spin-wave equations of motion. We present numerical calculations that explore the dependence on magnetic field (parallel to the surface) of the magnetization of an ultrathin film with easy axis normal to the surface. The magnetization has a singularity at the critical field where the magnetization of the film just aligns parallel to the surface. We discuss the nature of the spin correlations near the critical field.