多鐵性正交結構鈥錳氧薄膜之磁電特性研究

Abstract

本論文主要研究具有E-type反鐵磁有序磁結構之多鐵性正交結構鈥錳氧薄膜之磁有序誘發鐵電特性等物理性質。我們首先以固態燒結法製備出具有六方結構的稀土元素錳氧化物（離子半徑較小之鈥元素）；並利用X-ray繞射儀及超導量子干涉儀(SQUID)來分析靶材之晶體結構與磁相變等特性。另一方面，為了探索其正交結構磁電耦合之基本物理特性，本研究乃以製備具良好晶軸取向之薄膜為初期之首要目標。實驗結果顯示藉由選擇鈦酸鍶(110)、鑭鋁氧(110)、與鈦酸鍶(100)三種基板，我們可分別製備出具單一軸向之a、b、與c軸薄膜；除此之外，亦藉由上述匹配之基板來調變鈥錳氧薄膜正交結構之晶格常數，進而利用這些薄膜來進行相關磁電特性的量測以確認磁電耦合的機制是否存在，並與理論預測做比較。

This dissertation is aimed primarily at exploring the manifestations of magnetic ordering induced ferroelectricity expected in the E-type multiferroic orthorhombic HoMnO3. We first prepare the hexagonal HoMnO3 bulk synthesized by conventional solid-state reaction processes. The structural and magnetic characteristics of these samples were examined by x-ray diffraction and superconducting quantum interference device (SQUID) to ensure the stoichiometry of the material. Since within the frameworks of various microscopic mechanisms the magnetic ordering and associated ferroelectricity are intimately related to specific crystallographic orientations, it is thus essential to obtain well-aligned orthorhombic HoMnO3 (o-HMO) thin films capable of revealing the relevant properties along respective crystallographic orientations. To this respect, we have carefully calculated the in-plane lattice mismatch and chose SrTiO3(110), LaAlO3(110), and SrTiO3(100) as the substrates for preparing the a-, b-, and c-axis-oriented o-HMO thin films. Moreover, the external strain force originated from the epitaxial relation between these substrates and thin films during deposition can also serve as an excellent method in slightly modifying the lattice parameters with desired crystal structures. Finally, the magnetic ordering anisotropy and the ferroelectricity in these epitaxial o-HMO thin films were investigated to check whether or not the predicted marked enhancement of the magnetoelectric effect does indeed exists in these E-type multiferroic perovskite manganites.