| Summary: | 碩士 === 國立臺灣師範大學 === 物理研究所 === 90 === The structural patterns in a magnetic fluid film under a static magnetic field have been studied for several years and are well understood. However, the structural pattern in a magnetic fluid film under ac magnetic fields Hac is still unclear. Thus, in this work, the structures in a magnetic fluid film subjected to a sinusoidly ac fields are investigated and some significant results are explored.
First of all, in order to understand the evolution process of magnetic column with time under Hac, we observe the instant column size of a stable structure as a sinusoidly ac field is applied. The diameter a of a column under an ac magnetic field varies periodically with the same frequency as that of the field. But, the column distance d remains fixed. And the periodically variation of a has a retarded phase difference with respect to the external Hac.
Secondly, for a fixed frequency fH, when the amplitude AH of the ac field with is raised, the magnetic particles which are originally monodispersed in the magnetic fluid start to agglomerate and form columns as the Hac reaches to a critical strength H0. As Hac increases over another critical value Hh, the structure pattern evolves from a disordered state to an ordered structure. With the raising amplitude AH, the ordered structure column distance d decreases.
We also study the effect of frequency fH on diameter a and retard phase in an ordered structure state of magnetic column. With the increasing frequency fH, the maximum diameter amax decreases and the minimum diameter amin increases. According to the results of the phase difference, the column relaxation time more than the period of Hac under higher fH.
Finally, in order to clearly show the structural patterns in a magnetic fluid film under alternating magnetic fields, the structural phase diagram for the
magnetic fluid film is constructed in the Hac-fH configuration. The results reveal that the H0 and Hh become higher for a higher fH.
|
|---|
