La0.85Zr0.15MnO3 薄膜各向異性磁電阻之研究

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Title:	La0.85Zr0.15MnO3 薄膜各向異性磁電阻之研究
Other Titles:	Anisotropic magnetoresistance in La0.85Zr0.15MnO3 thin films
Authors:	李佳璘;Li, Chia-lin
Contributors:	淡江大學物理學系碩士班林大欽;Ling, Dah-chin
Keywords:	各向異性磁電阻;epitaxial growth;magnetic inhomogeneity;Anisotropic magnetoresistance
Date:	2009
Issue Date:	2010-01-11 02:17:48 (UTC+8)
Abstract:	利用射頻磁控濺鍍法，我們成功地在(001)方向的SrTiO3 (STO)、LaAlO3 (LAO)與MgO基板上成長La0.85Zr0.15MnO3(LZMO)薄膜。由x-光數據分析得知所有的LZMO薄膜都是沿著c-軸方向成長，其中LZMO/STO為高品質的磊晶薄膜。受到壓縮應力的影響，LZMO/STO與LZMO/LAO薄膜的c-軸長度會隨著厚度的增加而遞減；相反地，受到拉撐應力的影響，LZMO/MgO薄膜的c-軸長度會隨著厚度的增加而遞增。當應力逐漸地被釋放掉時，薄膜的c-軸長度則會逐漸地趨近於塊材樣品的c-軸長度值3.92Å。磁性量測數據顯示應力會抑制雙交換機制產生的長程鐵磁性；在低溫下，反鐵磁性與鐵磁性互相抗衡，薄膜具有多重不均勻的磁結構。原子力顯微鏡觀察薄膜的表面形貌發現LZMO/STO與LZMO/LAO薄膜的成長方式為tower-like growth；LZMO/MgO薄膜的成長方式為island-like growth。在定溫下，三種成長在不同基板上的薄膜於I⊥H (out of plane)組態下的電阻值皆大於I//H (in plane)組態下的電阻值。在定磁場下，LZMO/STO磊晶薄膜於I⊥H (in plane)與I⊥H (out of plane)組態下電阻的差異性與溫度的關係與磁阻隨溫度的相依性十分地類似，顯示異向磁阻的產生與磁場誘發的磁化強度方向改變所導致錳離子3d軌域與氧離子2p軌域的重疊程度調變有關係。另外，在定溫、電流與磁場垂直的條件下，LZMO/STO與LZMO/LAO薄膜電阻隨著磁場方向改變作~sin2 We have successfully grown c-axis oriented La0.85Zr0.15MnO3 (LZMO) films on SrTiO3 (001), MgO (001) and LaAlO3 (001) substrates by an off-axis rf sputtering system. X-ray analysis, reciprocal space mapping, and the narrowness of the rocking curve give a clear evidence for epitaxial LZMO growth on the STO substrate. The out-of-plane lattice constant of the strained LZMO/STO and LZMO/LAO film decreases with increasing thickness, whereas the out-of-plane lattice constant of the strained LZMO/MgO film increases with increasing thickness. As the stress is released for the thicker films, the out-of-plane lattice constant approaches to the c-axis lattice constant of bulk sample with a value of 3.92 Å. Magnetic measurements show that the double-exchange-mediated ferromagnetic ordering is suppressed by lattice-mismatch-induced strain. The competition between FM and AFM through superexchange interaction gives rise to magnetic inhomogeneities at low temperatures. The surface profile taken by AFM indicates that the grain growth of LZMO/STO and LZMO/LAO film is 3D tower-like, whereas the grain growth of LZMO/MgO film is kind of 2D island-like. When T < TMI, magnetoresistance observed in I⊥H (out of plane) is consistently larger than that observed in I//H (in plane) for all three kinds of LZMO film. The temperature dependence of anisotropic magnetoresistance (AMR), defined as RI⊥H (in plane) -RI⊥H (out of plane), is similar to that of magnetoresistance ratio. Its magnitude is peaked near TMI and becomes small at low temperatures, indicating that the change of overlapping between Mn 3d and O 2p orbitals induced by rotation of the magnetization through local spin-orbit interaction is responsible for the distinct AMR observed in LZMO films. In addition, the angular dependence of MR of LZMO/STO (LZMO/LAO) and LZMO/MgO film exhibits a simple sin2θ and cos2θ dependence, respectively in the I⊥H configuration, where θ is angle between the applied field and current. The periodicity is smeared out at low temperatures and in low filed regime, suggesting that the angle-dependent MR is intimately related to the pinning of magnetization caused by strain-induced uniaxial magnetic inhomogeneity in the vicinity of grain boundaries or the presence of inhomogeneity of magnetic phase.
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