On magnetization relaxation in ferrofluids and acousto-electromagnetic conversion

Abstract

A theoretical picture of electromagnetic oscillations excitation by an acoustic wave in a magnetic fluid and an experimental testing of it are presented and analysed. It is shown that the main cause of the excitation of electromagnetic oscillations is the disturbance of fluid magnetization, due to its density and its temperature oscillations in the acoustic field. The amplitude of this disturbance is evaluated for two geometries: when the wave vector is parallel to the magnetic field intensity, the magnetization disturbances are compensated by those of the magnetic field intensity, while when these two vectors are perpendicular the annihilation does not take place. Experimental data are obtained by means of a two-coil system in which an electromotive force is induced. The experimental data confirm qualitatively the theoretical prediction. The increase of applied magnetic field intensity leads to a monotonic increase of the induced voltage. For magnetic field intensities exceeding 10 3 A m −1 a saturation tendency is noticed, the induced voltage amplitude saturation occurring in the same range of applied magnetic field intensity as the saturation of the magnetic fluid magnetization. As an explanation for the data for the first configuration, it is shown that the elastic deformation of a magnetic fluid column can induce disturbances of the demagnetizing factor and hence of the demagnetizing field, these being able to excite electromagnetic oscillations.