Generation of Low-Frequency Elastic Waves in Magnetic Fluids

The purpose of the study is to analyse the mechanisms of generation of elastic waves in magnetic fluids in a constant magnetic field by applying an alternating magnetic field.  

Metods. The theoretical calculation of the dependence of the relative amplitude of the excited oscillations on the strength of the magnetizing field (constant component) is carried out under the assumption of a rigid connection between a magnetic nanoparticle and its magnetic moment (Brownian mechanism of magnetization) and the possibility of the magnetic moment to rotate independently of the particle itself (Néel mechanism of magnetization). The work provides a comparison of theoretical conclusions with previously published experimental data.

Results. In the case when the constant and alternating magnetic fields are perpendicular to each other, the function of the amplitude of the magnetoacoustic effect on the magnitude of the constant magnetic field has a linearly increasing character at first, then turns into saturation. The frequency dependence of the magnitude of the magnetoacoustic effect can exhibit several maxima. Theoretical analysis shows that the number of maxima is four (three were found experimentally). At a fixed magnetic field, the maximum value of the elastic vibrations generated in the magnetic fluid increases in proportion to the square of the frequency. In the case of parallelism of alternating and constant magnetic fields, the dominant mechanism for the generation of elastic waves is the ponderomotive mechanism. The dependence of the magnetoacoustic effect on a constant magnetic field has the form of the Langevin function.

Conclusion. The paper proposes mechanisms for the generation of elastic waves in a magnetic fluid in a constant magnetic field by applying an alternating magnetic field, which in one case is perpendicular to the alternating field, and in the other parallel to it. 

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