以變溫磁電阻探討次微米級鎳平板線之磁異向性

Abstract

本論文研究利用電子束微影與熱蒸鍍法於Si3N4/Si[100]基板上製作出一系列長30μm，寬0.24~5.4μm，厚25nm的鎳平板線。分析於溫度範圍100~300K時的磁阻行為，以探討磁異向性(形狀、晶格、磁彈)於磁矩翻轉過程扮演的角色。 首先由室溫平行膜面磁阻證實鎳平板線的磁阻來源為異向性磁阻效應，此外曲線特徵行為於趨近Aharoni模型所界定的臨界尺寸Lc分為兩類，當線寬大於0.92μm時磁阻呈現多磁區的行為；然而小於0.92μm時其背景磁阻值隨著磁場減小而單調性的增加，顯示磁矩因形狀異向性逐漸趨向長軸，此外於小磁場範圍下呈現明顯的磁滯且反隨磁場減弱而有減小，由其殘磁態附近的磁阻推論磁矩受到磁晶異向性的主導而偏向[111]方向。當溫度下降，磁彈異向性隨之增強且低於260K時足以抗衡形狀異向性與磁晶異向性，此時若膜面短軸有磁場分量則磁矩由原磁場方向往膜面短軸翻轉，於殘磁態達近乎平行於膜面短軸，此外藉由LMR(磁場平行長軸磁阻)與TMR (磁場平行膜面短軸磁阻)的飽和磁場可計算磁彈能常數K⊥並估算各溫度下對應的應力值，而該應力隨溫度的變化關係符合理論的預測為Ni與SiN基板間的熱膨脹係數差異所致。 我們推論此應力所感應出的異向性為磁彈異向性的主要貢獻，主導了鎳平板線於低溫時的磁矩翻轉。

The main purpose of this work is to investigate the magnetic properties of patterned sub-micron planar Ni wires by using magneto-transport measurements ranging from 100K to 300K. The samples with 30μm in length, 0.24~5.4μm in width(W), and 25nm in thickness, are fabricated by e-beam lithography and thermal evaporation on Si3N4/Si substrate. At first, we confirm that the origin of magnetoresistance(MR) of planar Ni wires is attributed to anisotropy magnetoresistance effect(AMR effect). According to the MR characteristics, samples are roughly categorized into two parts with the transition occurs near the critical size predicted by Aharoni model. For W>0.92μm, MR curves present the behavior of multi-domain state. However, when W<0.92μm, the reversible MR background increases with decreasing magnetic field indicating that the moments dominated by the shape anisotropy rotate toward the wire axis. At low fields, MR curves exhibit obvious hysteresis loops and surprisingly, MR decreases with decreasing magnetic field. We suggest that the magnetocrystalline anisotropy dominates the hysteresis process and forces moments to align with [111] direction near the remanent states based on the nearby MR values. When temperature decreases, the magnetoelastic anisotropy is enhanced. As temperature is less than 260K, the magnetoelastic anisotropy overcomes the shape anisotropy resulting in that the magnetic moments prefer to align transverse to the wire axis in the presence of magnetic field parallel to in-plane short axis. Furthermore, we can also estimate the magnitude of stress by the magnetoelastic anisotropy energy deduced from saturation fields of LMR(magnetic field along the wire axis) and TMR(magnetic field along the in-plane short axis). The relation of obtained stress and temperature is in good agreement with the theoretical expectation of the thermal expansion mismatch between Ni and SiN substrate. We conclude that this stress-induced anisotropy contributes mainly to the magnetoelastic anisotropy, dominating the magnetization reversal in our planar Ni wires at low temperatures.