多鐵性材料之金茲伯格-藍道理論

Abstract

在本文中，我們提出一個模型來描述磁化強度及自由能在外加磁場前、後的行為，藉由以對稱性為基礎的藍道理論推衍出該自由能的形式。我們的機制顯示出外加場不僅能降低系統能量，並且會在外加場方向誘導出一均勻的磁化強度。此外，當外加場達到某一臨界值，在該方向原本與位置相關的磁化強度將會消失，並使得自由能轉變為另一種形式，使得整體系統的平衡能量更為減低。此模型對於預測外加磁場後磁化強度的變化將是相當有助益的。

In this thesis, we present a phenomenological model to describe the behavior of magnetization and free energy with and without the participation of an external field. The derivation of the free energy is developed in the frame of Landau theory based on symmetry requirements. Our mechanism demonstrates that the external field can not only decrease the free energy but induce the emergence of the uniform magnetization in the same direction of the external field. In addition, when the external field enhances to a critical value, the original spiral component vanishes and the formation of the free energy changes into another form that can decrease the energy much more. This model would be useful to predict how the magnetization affected by an external field.