熱退火效應對Co/SiO2磁性顆粒結構電性質的影響

Abstract

本論文的目的是要研究熱退火效應對顆粒狀結構樣品在金屬-絕緣相變附近的影響。利用兩支磁控濺鍍槍（DC與RF）分別以Co與SiO2為靶材，用共鍍方式製作一系列三維Co/SiO2顆粒狀結構樣品。由控制兩者的相對鍍膜速率，可以調變樣品內的體積分率，使樣品無序程度介於金屬-絕緣相變之間。 實驗上，在電阻量測部分，發現隨著樣品無序程度減弱，樣品電阻率與溫度關係，從非金屬行為（高溫時為負TCR）漸漸變為金屬行為（高溫時，TCR為正）。低溫時，在金屬-絕緣相變附近的樣品電阻率與溫度有 ρµ lnT的關係，且無序程度的加大，在低溫時電阻率隨溫度下降增加的越快速。經過熱退火效應會使得樣品電阻率變化達到一至兩倍，但其傳輸性質的機制並不會因為電阻率的變化而有所改變，即在低溫時仍有ρµ lnT的關係。且隨著熱退火溫度的增加，高溫時的TCR及低溫時電阻率對溫度lnT的線性斜率展現出越高的金屬行為。 磁電阻展現類似磁阻因磁場造成翻轉的磁滯曲線，可以用巨磁阻效應(GMR effect)來解釋樣品的磁電阻行為。在未做退火處理的樣品中，我們發現，隨著樣品無序程度的減弱，磁電阻率變化率變小；而針對同一組樣品，我們發現樣品的磁電阻率變化率隨熱退火溫度增加也有變小的趨勢。而從金屬性較強的樣品中可看出，熱退火溫處理達到一定的溫度之後，其磁電阻變化率就已漸漸趨近飽和，我們發現電阻率與磁電阻率變化率則有相同的趨勢。此與其他系統的觀察結果一致。 在低溫時，在金屬-絕緣相變附近的樣品仍有弱局域效應存在，隨著溫度的降低，弱局域效應所貢獻磁電阻影響就越大，所以在溫度5K，磁電阻的磁滯曲線經零磁場時，會另外出現一個低點dip，我們推測此為自旋軌道耦合造成反弱局域效應的影響。 在霍爾效應量測方面，由於磁性顆粒結構造成異常霍爾效應，在高磁場下，橫向電阻率為一飽和值不隨場變化，而在低磁場部分，橫向電阻率近似線性地隨磁場增加，且線性擬合的霍爾係數遠較一般因Lorentz force 所造成的正常霍爾係數大。隨著熱退火效應的溫度增加，樣品的霍爾係數有越來越小的趨勢，且與電阻率隨熱退火效應的變化相關。

The purpose of this work is to study the annealing effect on granular samples near the metal-insulator transition. The composite three-dimensional granular samples were prepared in a magnetion cosputtering system with Co and SiO2 targets mounted separately on DC and RF guns respectively. By controlling the relative sputtering rates, we can adjust the metal volume fraction and make a series of samples near the metal-insulator transition. At low temperatures, sample in this region demonstrates a lnT dependent resistivity. As the sample becomes more disorder, the resistivity increases more rapidly with decreasing temperature. Annealing process change the sample’s resistivity, about 1~2 times of the original value, however, the temperature dependence is still the same, a lnT dependent resistivity at low temperatures. The higher annealing temperature the sample has been through, the more metallic the sample is, that is, the smaller of the slope of resistivity vs. lnT. Magnetoresistances demonstrate hysteresis loops analogy to the magnetic moments in these magnetic granular samples. The behavior can be attributed to spin dependent scattering similar to the mechanism in GMR. The MR value increases with increasing disorder, not only for as deposited samples but also for samples with annealing processes. Weak localization effect is present at very low temperatures. The effect becomes clear with decreasing temperature. At T=5K, an extra dip at H=0 in the hysteresis MR curve appears due to spin-orbital interaction induced anti-weak localization effect. Due to the magnetic granular structure, sample demonstrates anomalous Hall effect. Transverse resistivity saturates at high magnetic fields and increases with magnetic field nearly linearly at low fields. The Hall coefficient obtained from the linear fit at low fields is much bigger than the ordinary Hall coefficient that come from the Lorentz interaction.