Concerning the Theory of Ferromagnetic Resonance Absorption

The first part of this paper gives a simple quantum-mechanical derivation of Kittel's formula for the resonance frequency of ferromagnetic materials. The interactions between the elementary dipoles are handled directly, rather than through the ad hoc introduction of macroscopic demagnetization factors, as is usually done. In Section 3, Kittel's corrections for the effect of anisotropy on frequency are derived from the microscopic standpoint with a model having quadrupolar coupling between atoms. Section 4 discusses qualitatively the effect of exchange narrowing on the line-width. In Section 5 it is shown that Kittel's relation $g-2=2-g^{\prime }$ connecting the spectroscopic splitting factor $g$ and the gyromagnetic ratio $g^{\prime }$ is a general consequence of first-order perturbation theory. Throughout the paper it is stressed that ferromagnetic bodies may have important short-range forces of dipolar structure which arise from anisotropic exchange rather than from true magnetic coupling. It is shown that in the first approximation, inclusion of these anomalous forces does not modify Kittel's results.