以磁電阻量測探討微米級鎳平板線之磁異向性

Abstract

本研究工作主要在以磁電阻量測，探討鎳微米平板線之磁異向性，及微結構中的磁矩翻轉性質。樣品的尺度和形狀在鎳平板線之磁性行為中有重要的影響性，因此實驗中以微影技術製作三種系列線長分為20、30，及60微米的樣品，線寬範圍由0.18微米變化至10微米，厚度則為25或30奈米。 鎳微米平板線之飽和磁電阻可用考量s-d軌域自旋軌道耦合的異向性磁阻描述，因而針對三種系列的鎳平板線，可以平行膜面方向之磁場量測其磁電阻，並透過磁力顯微影像掃描樣品在室溫下，外加磁場平行鎳平板線的殘磁態磁區影像，以了解樣品內部之磁異向性及磁矩翻轉行為。令人驚訝的是，實驗結果顯示根據異向性磁電阻效應，微米級鎳平板線之磁易軸趨近於樣品線之短軸，而非隨樣品線之形狀異向性而位於其長軸方向。 線寬大於2微米之鎳平板線呈現較混亂的典型多磁區結構，其磁區翻轉形態為已知的磁區擴張。而樣品線寬在1微米以下之樣品依據磁電阻變化形式可分為另外兩種類型；線寬在0.6微米以上之鎳平板線，其在兩高磁場往零磁場方向掃取之磁電阻曲線時在大磁場下呈現同調翻轉（Coherent rotation），於殘磁態則展現一不可逆之交錯形式，在曲線之不可逆部分形成四個區間；線寬在0.6微米以下之鎳平板線，在大磁場下也同樣呈現同調翻轉的形式，在殘磁態附近之磁電阻展現出類似蝴蝶翅膀的圖形。 為了解線寬在1微米以下之鎳平板線其樣品形狀異向性的貢獻，文中分析樣品之殘磁態電阻以及Coherent rotation部分磁電阻形態與其長寬比之關係；並對一系列不同線寬（w=0.2~4.9μm），長度約為21μm的鎳平板線陣列，在殘磁態進行室溫磁力影像（Magnetic Force Microscope, MFM）掃描。透過掃描影像確定磁易軸偏向短軸，證實本系列鎳平板線的形狀異向性無法將磁易軸轉向長軸。推論鎳平板線磁易軸趨近於短軸的可能原因為磁彈性異向能，其來自於樣品與SiN/Si基板間因晶格常數不同而產生的晶格錯位。

The main purpose of this research is to investigate the magnetic properties of patterned sub-micrometric Ni planar wires, including the magnetic anisotropies and microstructure inside. The geometry of the wire plays an important role in these magnetic behaviors. Therefore, in this work, individual Ni wires with different geometries (width w=0.2~10.0μm, length L=20~60μm, and thickness t=25 or 30nm) were systematically investigated to explore their magnetic properties. We have measured the in-plane magnetoresistance of a series of sub-micrometric nickel planar wires at T＝10K to study the magnetic anisotropy, and scanned the magnetic force microscopic images to find out the microstructure of the wires. Strikingly, demonstrated by anisotropy magnetoresistance effect, our results indicate that samples have the strong transverse magnetic anisotropy despite the enhancement of longitudinal shape anisotropy. The wires of large width (w＞2μm) have typical multi-domain structure. The magnetization reversal is via the well-known domain expansion and all behaviors are under expectation. Furthermore, the narrow wire with w＜1μm, according to their behaviors near the remnant state, are cataloged into two regions. For larger width with w＞0.6μm, the magnetoresistance（MR）curves exist coherent rotation background at large field, but the curve scanned from positive to negative magnetic field and the opposite ones cross near the remnant states, forming four packets for the irreversible part; for wire width with w＜0.6μm, however, existing the butterfly shoulder-shaped near the remnant states. To find out the influences of shape anisotropy in the narrow wires with w＜1.0 μm, the configurations near remnant state and the form of the coherent rotation, namely, the reversible part of the MR curves, are analyzed in details. Also, a series of magnetic force microscope images of Ni planar wires（L＝21μm, w=0.2~4.9μm）were taken at the remnant state at room temperature. Our results demonstrate that the easy axis tends to lie along the short axis of wire. The strong shape anisotropy is not able to turn the easy axis to the long axis of the wire. Moreover, the transverse direction of the easy axis can be resulted from magnetoelastic anisotropy due to the lattice mismatch between Ni and SiN substrate.