多鐵正交結構鐿錳氧薄膜之磁電各向異性研究

Abstract

本論文主要研究基板應力對鐿錳氧薄膜磁電特性之影響。實驗先利用固態燒結法製備六方結構之鐿錳氧塊材，接著採用脈衝雷射濺鍍系統在鈦酸鍶(001)基板上成長出高純相且c軸垂直膜面之正交結構鐿錳氧薄膜，進而使用超導量子干涉儀量測在不同軸向的磁化率與溫度關係。我們發現量測磁場在平行不同晶軸方向時，均可觀測到一反鐵磁相變，其尼爾溫度均在45 K附近，並且不隨量測磁場方向而有所變動。另外在磁場平行平面正交的不同方向上存在另一明顯較低溫的磁性相變，該相變溫度隨量測磁場加大而有顯著降低的情形。同時利用LCR meter與premier II量測電極化與溫度變化之特性，發現該相變應為錳離子磁矩之反鐵磁態不相稱至相稱之相轉變，因在該溫度附近伴隨明顯的極化變化。所得結果並與鈥錳氧薄膜文獻結果做系統性的比較，以了解基板應力和稀土離子尺寸效應在薄膜之磁、電特性中扮演的角色。

The c-axis-oriented orthorhombic YbMnO3(001) thin films were deposited on SrTiO3(001) substrates by pulse laser deposition to investigate the correlation between strain and magneto-electric properties of the rare-earth manganites. In addition to the usual anti-ferromagnetic (AFM) transition around 45 K, which was independent to the orientation of the applied field, we also found a second magnetic transition at 38 K when the applied magnetic field was parallel to the film surface. The 38 K transition can be further suppressed to 33 K when the applied field is increased from 200 to 1000 Oe. We also measured the temperature dependent capacitance of the films by LCR meter. The results indicate that the 38 K magnetic anomaly is corresponding to the incommensurate-to-commensurate phase transition within the AFM state. The remnant polarization measurements also show a drastic increment around 38 K, which is the expected result of magnetic-ordering-induced polarization. In order to delineate the role of the rare earth element in affecting the modulation of Mn-ion moment, we compare our experimental results with that of HoMnO3 thin films published in the literature.