以第一原理研究Fen/(BaTiO3)m的磁電耦合效應

The First Principle Study on Magnetoelectric Coupling in Fen/(BaTiO3)m

柯宗良(Zong-Liang Ke)

指導教授 ： 梁贊全

國立中正大學/理學院/物理學系/碩士(2014年)

摘要

鐵電性和鐵磁性是凝態物理中的兩個重要性質，當兩者共存於同一材料上時，就形成所謂的多鐵材料。在鐵電與鐵磁共存的多鐵材料中，電性的改變可能影響磁性，反之磁性的改變也可能影響電性，此種現象稱為磁電耦合現象，磁電耦合現象使材料中不同鐵性間可以互相影響而有較高的應用價值。
但在一個材料中出現鐵電性和鐵磁性所需的對稱性並不相同，這使得同時存在鐵電性和鐵磁性的單相多鐵材料非常稀少，而碩果僅存的少數幾種多鐵材料的磁電耦合效應又相當微弱，造成在應用上難有突破性的進展。因此有人改以研究鐵電與鐵磁的多相複合型材料來尋找耦合強度較高的多鐵材料，而其研究成果發現複合型多鐵材料的耦合強度確實比單相材料要好。
本論文以第一原理研究複合型多鐵材料的磁電耦合性質，選用Fen/(BaTiO3)m（鐵磁/鐵電）的層狀二相複合材料為研究對象，藉著採用不同的鐵電層極化方向與外加不同的外加電場，來改變材料中的電性，藉此觀察並研究材料中的磁性隨著電性改變的變化情形，希望能以改變電性的方式來調控整體的磁矩和磁性異相能的大小。
研究結果顯示，鐵電層極化方向的改變對整體磁性的影響主要發生在Fe與TiO2的界面附近的原子，其中最靠近Fe層的Ti原子磁矩變化最大；而對於磁性異相能的調變，僅施以外加電場的方式對磁性異相能的影響效果不彰，翻轉鐵電層的極化方向對磁性異相能的影響較大。由此推測磁電耦合效應的來源主要是Fe層和鄰近TiO2層間原子的距離的改變所造成，因此外加電場必須大到足以改變Fe與BaTiO3界面間的原子距離，才能有效的用來調控磁性。

關鍵字

磁電耦合 ； 第一原理 ； 多鐵

並列摘要

Ferroelectricity and ferromagnetism play an important role in the condensed matter physics. When the two properties coexist in a material, it is a so-call multiferroic material. In those materials, the electric property may be affected by the change of magnetism property, and the magnetism property may also be affected by the change of electric property. These effects are called electromagnetic coupling, which let the material becomes a highly promising material for applications.
Since the conditions for existence of Ferroelectricity is different from that of ferromagnetism, the multiferroic materials in which ferroelectricity and ferromagnetism coexist in a single-phase are very rare. The electromagnetism coupling is always very weak in these materials. Therefore, there is no big breakthrough in application for single-phase multiferroic materials. Thus some people turn the research direction to the multi-phase composed multiferroic material. They found that the electromagnetic coupling in multi-phase multiferroics is stronger than that in single-phase multiferroics.
In this thesis, we used first-principle calculation to study electromagnetic coupling in Fen/(BaTiO3)m (ferromagnetic/ferroelectric) multilayer. The effect of the polarization in the ferroelectric on the magnetic properties in the ferromagnetic will be studied.
We found that the orientation of the polarization in the ferroelectric affects the size of the magnetic moments of interface Fe and Ti atoms moderately. On the other hand, the effect of external electric field on the magnetic anisotropy energy is ineffective. These findings suggest that the electromagnetic coupling is majorly due to the change of atomic distances between Fe and TiO2 on the interface, so, to modulation magnetic anisotropy energy by external electric field, the external electric field should be large enough to induce significant displacement of the atoms at the interface.

並列關鍵字

multiferroics ； electromagnetic coupling ； first-principle

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