鑭鍶錳氧鐵磁薄膜成長與特性之研究

The growth and properties of (La0.9,Sr0.1)MnO3 ferromagnetic thin films

陳燕華(Yen-Hua Chen)

指導教授 ： 吳泰伯

國立清華大學/工學院/材料科學工程學系/博士(2006年)

https://doi.org/10.6843/NTHU.2006.00083

摘要

鑭鍶錳氧化物因為具有優越的磁/電特性，例如：高居里溫度(Tc）、巨磁阻（CMR）、相分離（Phase separation）等，所以特別引人注目。本論文特針對結晶構造、成分組態、鐵磁性、導電性及表面奈米特性加以研究。其結果分述如下：
利用R.F.磁控濺鍍製作x~0.1之(La1-x,Srx)MnO3(LSMO)薄膜，結果發現以不同的Ar/O2比例可鍍製出具有及不具有(100)優選指向的薄膜。三種薄膜皆呈現鐵磁與順磁之相轉變，其居里溫度約為250K，此值大大高於塊材的居里溫度。不具(100)優選指向的薄膜，其飽和磁化量及導電率皆明顯高於具有(100)優選指向的薄膜，推究其原因為熱應力致使Jahn-Teller distortion，此機制使薄膜產生了不同的鐵磁與電阻特性，並且可能存在著金屬與絕緣相共存的現象。
為了確定應力對鐵磁、導電特性之影響，我們選定具有(100)優選指向（Ar/O2=2/1），及不具有(100)優選指向（Ar/O2=1/1）二種薄膜，並利用超高真空系統之導電性原子力顯微術（C-AFM in UHV system）加以觀測其導電機制，結果顯示：在室溫下，的確有金屬/絕緣二相共存之情況，且具有(100)優選指向的薄膜其導電性在同一晶粒上分佈不均，但不具(100)優選指向的薄膜其導電性則是跟晶粒分佈成正相關。使用C-AFM所得到之金屬-絕緣相轉變溫度(TM-I)幾乎與變溫電阻量測的結果一致。
接著，同樣以具有(100) 和不具(100)優選指向的兩種薄膜，改變其厚度（~350nm, ~160nm, 及~ 50nm），探討膜厚對鐵磁性與電導特性之影響。實驗顯示：二種不同優選指向的薄膜隨著厚度增加，其鐵磁特性/居里溫度與電導度/金屬-絕緣相轉變溫度亦隨著提升。其原因是隨著厚度增加，其應力鬆弛，即應力相對變小，導致於鐵磁性及電導特性變好。
在LSMO薄膜中埋入白金奈米粒子，結果發現：其居里溫度會下降。原因為：添加白金奈米粒子在其界面產生了磁性不均勻或自旋失序等狀況，導致居里溫度降低。而白金奈米粒子之添加確實會改變在垂直膜面方向的自旋狀態；且此種設計也提供電子一種更便捷的路徑進行傳導，所以導致薄膜的導電度變好。

關鍵字

鑭鍶錳氧

並列摘要

The manganites La1-xSrxMnO3 (LSMO), exhibiting a high Curie temperature (Tc), a good canonical double exchange system, and a good colossal magnetoresistance (CMR), have attracted much attention for their magnetic, electrical, and structural properties. The crystal structure, chemical configuration, ferromagnetic property, electrical transport, and nanoscale characterization are investigated in this study.
The LSMO thin films were deposited on SiO2(200 nm)/Si(100) substrate by R.F. magnetron sputtering at 6000C in an atmosphere of 5 mtorr with Ar/O2 ratio ranging from 4/1, 2/1 to 1/1, which is fabricated to different preferred orientation. The diversity of ferromagnetotransport is attributed to the alleviation of Jahn-Teller distortion induced by the thermal stress in films having different preferred orientations. And the polycrystalline films of (La0.91,Sr0.09)MnO3 with and without (100)-preferred orientation have spatial inhomogeneities with conducting and insulating domains coexisted in submicrometer scale, which is observed by conductive atomic force microscopy (CAFM). The domains undergo a percolative metal-insulator transition, and the transition temperature (TM-I) observed from CAFM is consistent to the result of magnetoresistance measurement. Different preferred orientation of LSMO thin films with various thickness were also studied. The ferromagnetic property and electrical transport increase with increasing film thickness. It is considered as the result of small deformation of the Mn-O bond length/bond angle by the relaxation of thermal strain. We fabricate the metallic Pt nano-crystals embeded in the LSMO matrix for the enhancement of ferromagnetotransport. The downshift of Curie temperature with LSMO/Pt(NP)/LSMO thin films is due to the magnetic inhomogeneity or spin disorder, which is caused by the addition of Pt nano-crystals in the films. The spin alignment in the normal direction of the films is significantly affected by the insertion of the ultrathin layers of Pt nanocrystals. The sub-layer of Pt nanocrystals provides an easier pathway for electron transport between LSMO grains, and thus the resistivity becomes significantly reduced.

並列關鍵字

LSMO

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