On the electromagnetic characterization of ferromagnetic media: Permeability tensors and spin wave equations

Various constitutive equations applicable to ferromagnetic and ferrimagnetic media are discussed systematically, the emphasis being on a formulation and analysis of the underlying assumptions. A distinction is made between the "ordinary" (Maxwellian) and certain "average" field vectors. The latter are useful in the presence of domain structure; they include appropriately defined spatial averages, and , of the time-dependent components of the ordinary and , respectively. In cases where and are connected by a "point relation", the general form of Polder\´s permeability tensor is extended to nonsaturated media; the special tensors due to Polder, the writer, and Wangsness, are then reviewed. In cases where and are not so connected, the "exchange effect" and the "spin wave equation" are discussed. Following Ament and Rado, three consequences of this equation are treated: the new boundary conditions, and the triple refraction and "equivalent isotropic permeability" in metals.