探討表面氧化對Co平板線的磁矩翻轉的影響

Abstract

鐵磁性/反鐵磁性雙層薄膜在近幾年磁學的研究中佔有很大的地位，它與我們一般所知道的鐵磁材料的磁性行為有著很大的不同，我們實驗主要是藉由磁電阻的量測來探討反鐵磁層如何影響次微米鐵磁層的磁矩翻轉。 我們利用微影技術及濺鍍的方式做出了數個系列的Co平板線，厚度為30 nm、長度為30 μm，寬度介於0.2~1.1 mm，並刻意將某些系列的Co平板線放在空氣中使其表層氧化產生反鐵磁層CoO，以四點量測技術作低溫的磁電阻測量。 所有樣品在低溫時電阻對溫度的變化展現或多或少的無序性，電阻隨溫度下降而上升，但氧化過的Co平板線在低溫的無序性會比Co平板線要來的大。在磁區分佈方面，線寬為0.5 μm以下的Co平板線展現形狀異向性主導的單磁區特徵行為，但線寬介於0.5~1.2 μm之間的Co平板線似乎因為不可避免的自然氧化導致類似刻意氧化的Co平板線的磁阻行為，影響的程度隨著線寬增加而加劇，使得反鐵磁與鐵磁層的交換耦合抑制了原本簡單的鐵磁層所擁有的異向性主導至單磁區分佈與curling rotation翻轉機制。 除此之外，這些具有不可忽略的CoO外層樣品，其磁電阻展現清晰的偏移、training effect、較大的交換偏壓(exchange bias)，隨著氧化層的增加此三現象越加顯著。

The study of the Ferromagnetic/Antiferromagnetic (FM/AFM) bilayer system is important in the research of magnetism. The magnetic behavior of the FM/AFM system is different from single FM layer. The thesis focus on the influence of the antiferromagnetic layer of cobalt oxide on the magnetization reversal of submicron cobalt planar wires using the magneto-transport properties. We used the techniques of lithography and sputtering to fabricate several series of Co planar wires of 30 nm in thickness and 30 μm in length. The widths range from 0.2 to 1.1 □m. Some of Co planar wires are in situ covered by a 3 nm thick Au layer to prevent oxidation. Others are forced oxidized by exposing in the air for certain amounts of time before covering Au. The magnetroresistance was measured by the four-probe technique. The resistance of all Co samples increases with decreasing temperature below 10 K indicating that all samples are slightly disordered. The disorder of CoO/Co planar wires is worse. For the Co planar wires, when the wire width is less than 0.5 μm, the shape anisotropy dominates the magnetic behavior and reveals all characteristics of a single domain structure. When the width is between 0.5 μm and 1.2 μm, magnetoresisrtance curve is analog to that of the CoO/Co planar wires implying that there always is a naturally oxidized layer. The influence of such a nature oxidation is strong as the wire width increases. The exchange coupling of the AFM/FM bilayer seems suppress the shape anisotropy induced single domain structure in the high aspect ratio FM layer. Besides, for the planar wires with surface oxidation, the magnetroresistance curve is clearly shifted from the remanent field and has the training effect and a big exchange bias. With increasing amounts of CoO, the above phenomena become more notable.