自組裝多鐵性奈米材料之介面應力引發之磁電耦合研究

Abstract

在本論文中，我們利用光學偵測方式，研究在以雷射濺鍍法製備的自組裝多鐵性奈米結構材料中，由介面應力所引發的磁電耦合。它包含了四個主題:導電薄膜釕酸鍶(SrRuO3)的成長、氧化鐵鈷及鈦酸鉛(CoFe2O4-PbTiO3)奈米材料的基本特性、氧化鐵鈷及鈦酸鉛(CoFe2O4-PbTiO3)奈米材料中由磁場產生的磁電耦合、以及氧化鐵鈷及鈦酸鋇(CoFe2O4-BaTiO3)奈米材料中異常的磁自旋及聲子的動態行為。 第一部分，我們研究成長的氧氣氛對於濺鍍在鈦酸鍶(SrTiO3(001))基板上SrRuO3薄膜的結構及導電性的影響。在高真空中所成長的SrRuO3 導電度的下降及結構膨脹的原因與銣(Ru)元素不足有關。在高氧壓中則可獲得高品質及高導電度的SrRuO3 薄膜。第二部分，我們研究多鐵性奈米材料的基本特性，此奈米材料是以CoFe2O4 為基底而鑲嵌著自組裝PbTiO3 奈米柱並沉積於氧化鎂(MgO(001))基板上。由於在平面方向的張應力，顯示了在平行於平面的極化及垂直於平面的磁性優選異向性。除此之外，由於磁致伸縮，CoFe2O4 的A1g 聲子頻率隨著外加磁場而藍移，這顯示了以應力為媒介的磁致伸縮的可行性。第三部分，我們研究成長在以SrTiO3(001)為基板並以SrRuO3 為緩衝層的CoFe2O4-PbTiO3 奈米材料隨著厚度而變化的特性。其磁異向性與聲子頻率的改變與應力的變化息息相關。由於未釋放的介面應力使得較薄的奈米材料顯示出較大的磁電耦合。最後，在CoFe2O4-BaTiO3 奈米材料中，我們觀察到CoFe2O4 的平面磁化量隨著BaTiO3相變而異常增加。而增加的磁有序排列分別干擾了在尖晶石上A1g 聲子與在尖晶石與鈣鈦礦介面上A1(2TO)聲子的能量。這個奈米結構顯示出磁自旋、聲子、及晶格應力的強耦合作用。

In this thesis, we investigate the interfacial stress-driven magnetoelectric (ME) coupling in self-assembled multiferroic nanostructures deposited by pulsed laser deposition. It includes four subjects: the growth of conductive SrRuO3 thin film, the basic properties of CoFe2O4-PbTiO3 nanostructure, the magnetic field generated ME effect in CoFe2O4-PbTiO3 nanostructure, and anomalous spin and phonon dynamics in CoFe2O4-BaTiO3 nanostructure. First, we investigate the influence of growth oxygen ambience on the structure andconductivity of SrRuO3 thin films deposited on SrTiO3(001) substrates. The reduced conductivity and expansion of structure grown in high vacuum is associated with the deficiency of Ru element. High quality conductive SrRuO3 film is obtained under high oxygen pressure. Second, we study the basic properties of nanostructure, which consists of self-assembled PbTiO3 nano-rods embedded in CoFe2O4 matrix deposited on MgO(001) substrate. It shows the simultaneous in-plane polarization and vertical preferred magnetic anisotropy due to in-plane tensile strain. Furthermore, T-site (A1g) phonon frequency of CoFe2O4 blue shifts under the magnetic field because of magnetostriction, which reveals applicability of stress mediated magnetostriction. Third, from the thickness-dependent properties of CoFe2O4-PbTiO3 nanostructures grown on SrRuO3 buffered SrTiO3(001) substrates, we found that the strain variations are correlated with changes of magnetic anisotropy and phonon frequency. The thinner CoFe2O4-PbTiO3 nanostructure reveals the stronger ME coupling due to the unreleased interfacial strain. Finally, in CoFe2O4-BaTiO3 nanostructure, we observed the anomalous increase of in-plane magnetization during phase transformation of BaTiO3 matrix. The increased magnetic ordering perturbs the CoFe2O4-A1g and BaTiO3-A1(2TO) phonon modes in spinel and between spinel-perovskite interfaces, respectively. This nanostructure shows a strong coupling between spin, phonon, and lattice strain.