Theory of dipole-dipole interactions in ferromagnets. I. The thermodynamic properties

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 1/z, where z is the number of spins interacting with any given spin. The lowest order (1/z) 0 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 (1/z) 2 . They are valid for all temperatures below T c , and the results may also extend into the paramagnetic region if there is a suitable applied magnetic field. In a low temperature limit the (1/z) 1 results reduce to linear spin wave theory, and the (1/z) 2 contributions give a leading order description of spin wave interactions.