The Magnetic and Transport Properties of CrO2 and La2/3(Sr, Ca)1/3MnO3-y Films
|Keywords:||半金屬;自旋極化;過渡金屬氧化物;絕緣-金屬相變;順磁-鐵磁相變;自旋玻璃;X光吸收光譜;自我吸收效應;half metal;spin polarization;transition-metal oxides;insulator-metal transition;paramganetic-ferromagnetic transition;spin glasses;x-ray absorption spectra;self-absorption effects|
|Abstract:||由於電荷載子的自旋極化特性，半金屬(half metal)鐵磁材料被預期將在自旋相關的電子元件扮演重要的角色。而在這些材料中，由兩種元素所組成的CrO2是最簡單的半金屬材料同時其自旋極化的程度也最高(90%~98%)；而電洞摻雜(hole-doped)的LaMnO3 的自旋極化程度居次(85%~90%)，且因為超巨磁阻(colossal magnetoresistance, CMR)效應而受到矚目。在本論文中我們以這兩種材料為研究對象。
首先，我們發展了利用將TiN 氧化成TiO2 作為緩衝層，在Si 基板上成長高品質CrO2 與La2/3Sr1/3MnO3 薄膜的特殊技術，同時也量測這些薄膜的磁性與傳輸性質，並探討其半金屬特性與磁阻效應的機制。
色還未完全了解。我們在SrTiO3 單晶基板上製備不同氧含量的La2/3Ca1/3MnO3-y(LCMO)薄膜，並量測其磁性與電阻行為，研究氧空缺對LCMO 薄膜傳輸性質與磁性相變的影響，並進一步探討絕緣-金屬相變與磁性相變之間的關係。
最後，X 光近邊吸收結構(x-ray absorption near-edge structure, XANES)光譜是研究鈣鈦礦(perovskite)結構氧化物之電子結構與未填滿軌域的有利工具。然而所謂的自我吸收效應(self-absorption effect)會引起螢光產率(fluorescence yield)光譜的變形，我們以(110)軸向的YBa2Cu3O7-y 薄膜為例，詳細說明自我吸收效應的修正程序，並探討自我吸收效應對CrO2 與La2/3(Sr, Ca)1/3MnO3 薄膜的螢光產率光譜的影響，並比較修正前後的光譜。|
The half-metallic ferromagnets are expected to play a signicant role in the spin-based electronic devices by virtue of the spin-polarized characteristics of the charge carriers. In particular, among these materials, CrO2 with the highest polarization (90%~98%) is the simplest half-metallic ferromagnet, consisting of only two elements. Hole-doped LaMnO3 with the second highest polarization (85%~90%) has attracted much attention recently as a result of the colossal magnetoresistance (CMR) features. Therefore, CrO2 and hole doped LaMnO3 were regarded as the study objects in this dissertation. Firstly, we developed a novel technique to fabricate high-quality CrO2 and La2/3Sr1/3MnO3 films on silicon substrates by means of oxidizing TiN into TiO2 as buffer layer. Simultaneously, the magnetic and transport properties of the as-grown?films were also measured. Similarly, the half-metallicity and mechanism of the magnetoresistance were intensively investigated. Secondly, the role played by the oxygen ions in the hole-doped LaMnO3 is still not completely understood. We prepared La2/3Ca1/3MnO3-y films with various oxygen contents on SrTiO3 single-crystalline substrates and measured the resistivity and magnetic properties. The influence of oxygen vacancies on the transport and magnetic phase tranisitions was investigated. Furthermore, the relationship between the insulator-metal and magnetic transitions was also studied. Finally, x-ray near-edge structure (XANES) is one of the powerful tools to probe the electronic structure and unoccupied states of the perovskite oxides. However, the so-called self-absorption e?ects will distort the uorescence-yield XANES. We took a (110)-oriented YBa2Cu3O7-y film as an example to illustrate the correcting technique of self-absorption e?ects in detail. The influence of the self-absorption effects on the fluorescence-yield XANES of CrO2 and La2/3(Sr, Ca)1/3MnO3 films was investigated. The comparison between the spectra before and after correction was also presented.
|Appears in Collections:||Thesis|