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Summary Kittel's classical theory of resonance in a magnetized ferromagnetic material is generalized; a phenomenological theory is obtained in which the dependence on the shape and the dimensions of the specimen no longer occurs. As an application the propagation of a plane electromagnetic (micro-) wave in a magnetized ferromagnetic medium is discussed. A quantum theory of the resonance phenomenon is given on the basis of a generalized Bloch spin-wave theory. An ambiguity in the use of the term gyro magnetic ratio is pointed out. It is suggested that the anomalous g-value found by resonance experiments may be explained by the combined effect of an electric crystal field and spin-orbit interaction.
D. Polder (Sat,) studied this question.