在外加穩定垂直磁場強度下，磁性流體薄膜之結構型態已經被研究了解，但對於其在外加交流垂直磁場下的結構型態並未有相關之研究，故在外加交流垂直磁場下磁性流體薄膜之結構演變將是本論文的研究重點。
首先為了解磁性流體薄膜結構在外加交流磁場下，隨時間的動態演化機制，我們待磁性流體薄膜結構呈現穩定變化後，觀察其隨時間的演化。發現磁性流體薄膜中的磁束會在原地做大小週期性的改變，其週期性的變化頻率與外加交流磁場頻率相同，而且磁束尺寸大小週期性的變化有一個落後外加交流垂直磁場的相位差存在。
接著我們固定外加交流磁場頻率，改變不同的外加交流磁場強度振幅，觀察磁性流體薄膜在不同的外加交流磁場強度振幅下之結構變化，隨著外加交流磁場強度振幅的增加，磁性流體薄膜結構由沒有磁束生成的型態，磁束無序排列結構，到有序結構。並藉由量測有序結構時之兩鄰近磁束間的平均距離，以理解外加交流磁場對磁性流體薄膜結構之影響，得知隨磁場強度振幅的增加，兩鄰近磁束間的平均距離會漸漸變小。
並研究在有序結構下，外加交流垂直磁場頻率對有序磁束結構的磁束截面積直徑大小及相位差之影響，實驗結果顯示隨著交流磁
場頻率增加，隨時間變化之最大磁束截面積直徑將減小，而最小磁束截面積直徑將漸漸增大，當交流磁場頻率大於某一臨界頻率時，磁束截面積將不隨時間變化。由相位差的變化得知，在較高的頻率下磁束的鬆弛時間大於磁場的週期。
最後，我們將建立在不同薄膜厚度下外加交流垂直磁場強度振幅和磁場頻率之結構相圖，以釐清磁性流體薄膜在外加交流垂直磁場下結構行為之控制機制。

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.

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