Emission of light from a strip waveguide with periodically varying parameters

Abstract

The magnetization and free energy of a ferromagnet with dipole-dipole coupling between spins are evaluated approximately using the drone-fermion perturbation method. The contributions are classified by means of a high density parameter l/z. where z is the number of spins interacting with any given spin. The lowest order (l/z)O results lead to molecular field theory, and fluctuation effects are introduced in higher orders. The calculations have been carried out up to and including order (liz)'. They are valid for all temperatures below T,, and the results may also extend into the paramagnetic region if there is a suitable applied magnetic field. In a low temperature limit the (l/z)' results reduce to linear spin wave theory, and the (1,'~)' contributions give a leading order description of spin wave interactions.