鈣參雜鑭錳氧化物薄膜及其雙軸階梯晶界磁電阻性質之研究

Abstract

本文將探討以鈣摻雜錳氧化物薄膜製作的雙軸晶界元件中因自旋相關散射所產生的磁電阻性質以及利用電子穿隧顯微鏡來研究存在於這類材料中的相分離特性，這些性質的瞭解將有助於自旋相關電子元件的開發與應用。 首先將描述以二氧化鈦為緩衝層蒸鍍雙軸La0.7Ca0.3MnO3 (LCMO)薄膜的方法及製作雙軸晶界接面樣品的過程，並量測晶界接面的磁電阻特性。我們發現其溫度和磁場相依的磁電阻特性相似於其他種類的晶界樣品並且展現可比擬的磁電阻率，除此之外，這雙軸晶界接面卻展現著線性的電流電壓曲線，不同於其他種類的晶界樣品。依據這些實驗發現，雙軸晶界接面的磁電阻特性是自旋極化電子傳輸經過一個具有弱磁性及類金屬特性的接面層所造成的結果。 另一方面，我們嘗試以電子穿隧顯微鏡來探討顆粒狀之晶界樣品的微觀電子結構變化，發現在鈣摻雜錳氧化物薄膜樣品中，於絕緣金屬相變溫度之下同時具有類金屬及類絕緣體的相共存狀態，並觀察到在相變溫度附近樣品在約數十奈米的尺度下類金屬叢集會因為外加磁場而變化，這空間分布的變化具有不可逆的現象，而在較低溫度時，則不可逆消失，顯示出該體系處於較穩定狀態。除此之外，我們也發現這些共存的類金屬及類絕緣體叢集的結構和樣品的顆粒結構形狀有著密切的關係。 總之，由上述實驗觀察所得，鈣鈦礦結構之錳氧化物具有多樣化且相依的電特性和磁特性，對於目前無法解釋的晶界磁電阻行為，我們認為晶界樣品的微觀結構性質對於自旋極化電子傳輸時產生的磁電阻特性有著基本重要的影響。

The extensive exploration for the magnetoresistance of the grain-boundary junctions and the properties of phase separation in calcium-doped manganite films are presented in this dissertation, respectively. In particular, the planar structure in a form of the biepitaxial step junction was adopted to investigate various behaviors of the grain-boundary effects. With the aid of scanning tunneling microscopy, the spatial variations of the local electronic structures in nanometer scale can be examined as well. Firstly, the biepitaxial La(0.7)Ca(0.3)MnO(3) thin films were grown on SrTiO(3) substrates using a buffer layer of anatase TiO2 and fabricated as a form of the planar structure of the biepitaxial step junction. The magnetoresistance of the biepitaxial step junction across the boundary layer of the biepitaxial (001)- and (110)-oriented films was investigated. The temperature and field dependence of magnetoresistance for the biepitaxial step junction are qualitatively similar to those of other types of artificial grain-boundary junctions with a comparable magnetoresistance ratio at low temperatures. However, the observed linear current-voltage characteristics across the biepitaxial step junction are in contrast to the commonly reported non-ohmic characteristics. Based on these experimental results, the biepitaxial step junction can be described by the model of spin-dependent transport across a depressed magnetic ordering and metallic-like junction layer. However, an intricate maze of the temperature dependent magnetoresistance behaviors is still unsolved in most types of grain-boundary structure. Furthermore, the attempts to explore the features of the grain-boundary effects in granular samples have been fulfilled by scanning tunneling microscopy in nanometer scale. We have confirmed the fact of phase separation in La(2/3)Ca(1/3)MnO(3) films. It is also worthy noting that the domains of the coexisting metallic-like and insulating-like phases is susceptible to the external magnetic field at the temperatures not far below the insulator-metal transition. The domain distributions vary in a irreversible manner. The observed irreversibility in the domain distribution suggests that the metallic percolation paths could be affected by the magnetic field. However, the magnetic field becomes ineffective on the domain distributions at lower temperatures. In addition, the correlation between the grain structure and the spatial distribution of the coexisting phases was evidently established. Due to the interplay between magnetic and electronic properties in manganites, the spin-polarized transport and the nanometer scale phase separation in manganites could be important to the understanding of the grain-boundary magnetoresistance. The reasons will be described in the contents of dissertation.