微結構鈷/氧化鈷雙層平板線交換偏耦合的幾何尺度效應

Abstract

本研究針對一系列不同長寬比的微米尺度鈷/氧化鈷兩層結構平板線樣品，利用磁電阻量測來研究幾何尺度對此兩層結構中的交換偏耦合效應的影響。 在高長寬比的單層的鈷平板線中，由於形狀異向性能，內部呈現單磁區結構，且磁電阻曲線對稱於零磁場；但當有一層氧化鈷層緊鄰鈷層之後，磁電阻曲線並未展現單磁區的特性，且造成磁電阻曲線偏移，產生交換偏耦合(exchange bias coupling)效應。因此我們利用微影技術配合直流濺鍍，製作長度30μm，線寬從0.2μm到1μm的平板線，厚度為鈷(28nm)/氧化鈷(2nm)，及單一鈷(30nm)平板線作對照，來探討幾何尺度對交換偏耦合效應的影響。 在單層的鈷平板線中，在1μm以下的線寬都有單磁區的特性，並符合我們所預期的異向性磁阻效應，及交換場隨著線寬變大有單一雙曲線下降的趨勢；在縱向磁阻曲線的部份，僅有大約10Oe的偏移量，表示側邊氧化對樣品的影響並不大。 兩層結構的樣品在100K的溫度下，皆展現出明顯的鐵磁/反鐵磁交換偏耦合效應，並有隨著線寬越小，交換偏耦合場(exchange bias field,Heb)越小的趨勢。而磁電阻曲線除了往負磁場方向偏移，也發現到正磁場偏移的情況，表示鐵磁/反鐵磁介面在同一批鍍膜中可能出現不一樣的磁矩排列方式。除此之外，相較於單一的鈷平板線，有顯著的矯頑場增益，以及training effect，但兩者與長寬比之間沒有明顯相關性。

The main purpose of this work is to investigate the size dependence of exchange bias coupling in patterned Co/CoO bilayers by using magneto-transport measurement. For patterned Co planar wires of high aspect ratio, because of shape induced anisotropy, these wires should tend towards a single domain states and its magnetoresistance curve might be symmetrical to the magnetic field axis. Meanwhile, for which there is a CoO layer beneath it, these wires seems to be multi-domain state, and exhibited magnetoresistance curve displaced along the magnetic field axis, which is called exchange bias coupling effect. Samples were fabricated by lithography technique and DC Sputtering with sample length being kept at 30 μm and its width varying from 0.2μm to 1μm. The sample thickness is Co(28nm)/CoO(2nm), and single Co layer(30nm) also be fabricated to be contrast. For single Co layer, wires narrower than 1μm tend towards a single domain states, and in line with anisotropy magnetoresistance as we expected. The switching field of wires can be fitted with a single hyperbolic function, and it decrease as width rise. In LMR(Longitudinal magnetoresistance) curve, only 10 Oe shift along the magnetic field axis, so there is only little contribution to exchange bias coupling of edge oxidation. At 100K, we find that Co/CoO bilayer wires have much more significant exchange bias coupling effect than single Co layer, and we also find the reduction of exchange bias coupling field as widths decrease. Except for magnetoresistance curve shift to negative field, a few samples exhibited magnetoresistance curve displaced positively along the magnetic field axis, which means that Co/CoO interface might have different magnetic moment arrangement in the same procedure. Besides, we also observed coercivity enhancement and training effect, but they are insensitive with the aspect ratio.