多鐵性 (Bi, Ln)FeO3之結構、磁性及介電性研究

Abstract

多鐵性材料係指在某一種材料中，具有兩種以上耦合鐵的特性，其隨外在環境改變，而產生自發極化滯留現象。由於（反）鐵磁性及（反）鐵電性同時存在於一種材料中，使其材料的物理性質及機制變得相當複雜而有趣。 其中，BiFeO3更因其高於室溫的居里溫度與尼爾溫度，成為應用於日常生活中的最佳材料。 此論文中，我們利用固態反應法合成Bi1-xLnxFeO3 (Ln = La and Dy, x = 0 ~ 0.40)塊材，以及利用溶膠-凝膠法合成Bi1-xDyxFeO3 (x = 0 ~ 0.40)奈米粒子，並且對於這些化合物進行系統化的結構、磁性及介電性質研究。我們發現摻雜鑭、鏑的系列分別在x ≧ 0.15及x ≧ 0.10時，有晶體結構轉變的現象。其中，由於鑭的摻雜，結構由BiFeO3的菱形晶系轉變為近似立方晶系，而鏑的摻雜則使結構轉變為正交晶系。在室溫下，由固態反應法製成的Bi1-xDyxFeO3及Bi1-xLaxFeO3其磁化率隨著摻雜濃度增加而增大。我們認為這個現象因為摻雜鑭系元素後，BiFeO3原有的旋輪線型 (cycloidal) 自旋排列被壓制或是破壞了。另外，摻雜鏑的系列中，x = 0.10 ~ 0.30樣品的介電常數增大至68 ~ 300，我們認為這是由於在結構轉變中，結構不穩定性所造成的。 最後，溶膠-凝膠法合成的Bi1-xDyxFeO3 (x = 0 ~ 0.40) 奈米粒子具有有限的矯完磁場、交換場及細長型的磁滯曲線，我們認為這些樣品的磁性是似超順磁性。我們更發現在樣品的磁性在低溫下(10 K)大為增加，這是因為旋輪線型自旋排列在低溫下成為非簡諧排列。另一方面，我們證明利用溶膠-凝膠法可改善樣品的漏電問題。Multiferroic bismuth ferrite (BiFeO3) has gained a considerable attention from the aspects of technology and fundamental science due to the coexistence of (anti)ferromagnetic and ferroelectric properties (so called multiferroics), which allows an additional degree of freedom for the design of spintronic devices. Moreover, the high transition temperature of BiFeO3 is the most important advantage for application in practice. In this master thesis, we prepared series of Bi1-xLnxFeO3 (Ln = La and Dy, x = 0 ~ 0.40) polycrystals by solid state reaction method. Moreover, sol-gel method is used to synthesize pure Bi1-xDyxFeO3 (x = 0 ~ 0.40). Their structural, magnetic and dielectric properties are systematically investigated. The crystal structures start transforming from rhombohedral to cubic ones near x = 0.15 for La-series and to orthorhombic ones near x = 0.10 for Dy-series. For samples prepared by solid state method, it is interesting tofind that the room temperature magnetization at 20 kOe (Meff ) for Dy-doped samples are one-order larger compared with that of La-doped samples, which is ascribed to the suppression of cycloidal spin structure. Furthermore, the dielectric constant is enhanced greatly from 68 to 300 with Dy = 0.10 to 0.30, which should be associated with the structural instability during the transformation. Finally, the grain size of Dy- series prepared by sol-gel method is in nanoscale. Those nanoparticles show the possibility of superparamagnetic behavior due to the finite coercivefield (Hc) and exchange bias (He). At low temperature (10 K), the anharmonic cycloidal spin structure is attributed to the larg Meff . The restrained leakage problem in nanoscaled Dy- series proves the ability of sol-gel method to control the leakage problem.