探討覆蓋層在二維鐵磁薄膜上對電性傳輸與異向性磁阻的影響

Abstract

鐵磁性多層膜在近幾年中，被廣泛的研究和實用在現實生活的磁性讀頭中，這是因為電子不再只是帶電粒子，更可以利用其本質的自旋性質與整體展現的磁性作許多基礎研究與相關的元件應用。一般鐵磁材料與其他材料的介面導致的電磁性質改變問題未完全清楚，這方面的題材仍吸引著科學家持續的研究與探討，本論文主要是探討覆蓋層對鐵磁性薄膜電子傳輸的影響，即藉由覆蓋層和氧化機制探討異向性磁阻的改變。 我們利用濺鍍系統製作10nm的鐵磁性鈷膜，再以熱蒸鍍系統製作不同面積比例的覆蓋鋁層，藉由低溫系統和四點量測技術，量測室溫(~300K)到低溫(1.5K)電阻對溫度關係，由弱局域效應和電子－電子作用力的擬合，得知樣品的維度和無序程度，推論是二維的弱無序鈷膜，鈷表面的氧化會增加樣品的無序性，另外阻隔與覆蓋鋁層的電性直接接觸，因此鈷膜樣品的電性與磁性，在有覆蓋鋁層下並未對應改變，量測得到的結果可以並聯鋁膜模型解釋。 早期在鐵磁材料上證實因s-d scattering在電流與磁化方向不同夾角時，有不同的散射機率，造成電流平行磁場時會展現inverse MR，而電流垂直磁場時是normal MR，即所謂的異向性磁阻﹔二維鐵磁鈷膜，因為表面容易氧化且此氧化層對鐵磁材料的特性有所影響，為了了解此氧化層造成的影響，因此我們藉由曝氧時間的改變製作表面不同氧化程度的樣品，作一系列低溫磁電阻與磁化率測量﹔由我們的實驗數據發現氧化程度較小者，確實符合以上所說，但當氧化程度越強，其原本預期電流平行磁場是inverse MR的結果，會改變成normal MR，相同的電流垂直磁場會由原本預期的normal MR變成inverse MR。推論可能是因為氧化鈷的反鐵磁特性，使得鐵磁/反鐵磁的交互作用力造成鈷膜中磁區耦合形式的改變，因而使得異向性磁阻造成改變﹔也可能是因為氧化鈷在鈷膜中形成許多的邊界，使得電子在其中流動時，會遇到許多界面問題，而界面的不對稱能態密度可能因為鐵磁/反鐵磁的交互作用力造成改變，因而導致磁阻方向的改變。

In the recent year , the ferromagnetic multilayer systems are being investigated popularly and intensively. It was utilized for magnetic read-head by IBM in 1991. People have realized that the intrinsic property, spin, of electron can play an important role in many aspects. Many spin-dependent mechanisms are valuable topics in the basic researches and have profound potentials in industrial application. The interfacial scatterings between ferromagnetic material and others are quite complicated but important attracting many scientists’attentions. In the dissertation, we discuss the influence of the covering layer on the electrical transport and the influence of oxidation on anisotropic magneto-transport in 2D magnetic film. 10nm thick Co films were made by DC sputtering and the covered by thermal evaporated 8nm Al/AlOx. Temperature dependent resistances were measured using four probe technique in low temperature system. Data can be well described by combination of the electron-electron interaction mechanism and the parallel connection model. Hence, we believe that the covering layer (Al/AlOx) does not affect the electrical transport in the two dimensional magnetic Co layer. For most magnetic materials, inverse MR occur when current is applied along magnetic field direction and normal MR occur when current is applied perpendicular to field direction. This scenario can be attributed to the s-d scattering effect is usually referred as anisotropic magneto-resistance (AMR). In a very thin magnetic film, surface oxidation can become very severe and affect its intrinsic properties. Here, we have performed low temperature magneto-resistance and magnetization measurements for a series of Co thin film with different degrees of oxidation to investigate the role of surface oxidation. For thicker films and thin films with less oxidation, magneto-transports behave as theoretical expectation described above. However, for thin films with severe oxidation, the magneto-transports are completely reversed. The inverse MR is replaced by normal MR when current is applied along magnetic field direction. The normal MR is changed to inverse MR when current is applied perpendicular to field direction. It has been known that CoO is an antiferromagnetic material. Hence we suggest that there are two possible reasons for such reversed behaviors in MR. One is the magnetic domains change due to the ferromagnetic/antiferromagnetic coupling and the other is asymmetric density of state change due to the additional interfacial scatterings to CoO.