以磁電傳輸研究次微米鎳鐵/銅/鎳鐵三層結構平板線之磁矩翻轉

Abstract

本論文製作一系列次微米尺度下的鎳鐵/銅/鎳鐵三層結構平板線樣品，並利用磁電阻量測的方式來研究此三層結構中鐵磁層的磁矩翻轉機制。 在高長寬比的鐵磁平板線內部因形狀異向性能主導磁矩分布，呈現單一磁區結構磁矩主要是以curling的形式瞬間翻轉，我們利用這樣的機制來製造具有高低電阻態快速切換的磁電阻表現的三層結構（tri-layers）磁閥，並研究各單層鐵磁層磁矩翻轉機制。樣品涵蓋了三種不同鐵磁層厚度的組合，其中一層鎳鐵層皆為35nm，另一層則分別為20nm、25nm、30nm。而中間的非鐵磁層 (銅)為了避開因為厚度較薄時所產生的RKKY震盪效應所以將其固定在8nm。使用微影製程與熱蒸鍍，製作長寬比高的平板線結構，其中線長固定為20微米，線寬則從0.2微米依序變化到2微米m，來研究不同長寬比以及鐵磁層厚度對磁性表現的影響。 在三層結構樣品展現出其應有的巨磁阻特性，低溫下磁場平行樣品長軸的高低電阻變化率約是3%~4%，即便在室溫的磁電阻曲線也能明顯觀察到curling model所展現的瞬間磁矩翻轉行為，室溫電阻變化率約為1%~2%，此外三層結構樣品磁電阻在改變外加磁場方向時會有受到異向性磁阻的影響，低溫下磁場垂直樣品長軸量測的異向性磁阻變化率約2%~3%，室溫約是0.5%~1%。 三層結構樣品中因為有兩層不同厚度鐵磁層，在磁電阻曲線中會呈現兩個轉換場（Hsw1,2）。其中Hsw1符合curling 所預期轉換場對應外加磁場與樣品長軸夾角的關係。而Hsw2的值則是高於單層平板線以curling model所預期，我們認為Hsw2的值受到兩鐵磁層間所產生的反鐵磁耦合的影響，使得其轉換場的值增加，另外Hsw2的值似乎沒有受到該層厚度的影響。 當外加磁場垂直樣品長軸時，兩鐵磁層的磁化方向互相平行且內部磁矩翻轉依循coherent rotation的機制，擬合計算出樣品的異向性能Ku與飽和磁場都與樣品的厚度以及寬度的倒數近似正比關係，然而在線寬較窄樣品計算出Ku值受到鐵磁層間的交換耦合影響而變得比預期的小，推論當兩鐵磁層的磁化方向互相平行作coherent rotation時，鐵磁層間的交換耦合影響三層結構樣品中Ku值的大小。

The main purpose of this workis to investigate the magnetic properties of patternedNiFe/Cu/NiFetri-layersby using magneto-transport measurements. For patterned ferromagnetic planar wiresof high aspect ratio, magnetic momentsare dominated by shape induced anisotropy andmagnetization reversal follows the curling mechanism.Thus,based on the properties, the tri-layers in planar wire configuration are expected to have a rapid switching between two resistance states in magneto-transport. Samples are made of two NiFe layers of different thicknesses. One is fixed to be 35nm thick. The other layer for three series is 20, 25, and 30nm in thickness, respectively. The spacer layer Cuis 8nm thick such that RKKY effect can be neglected. Samples were fabricated by lithographytechnique and thermal evaporation with sample length being kept at 20um and its width varying from 0.2 to 2.0um. We can investigate the influence of sample geometry on magnetic properties. All ofour tri-layer samples demonstrateGMR (Giant Magneto-Resistance) effect and the resistancevariationratiois about 3% ~ 4% at 10K.The abrupt switching due to curling is clearly present atroom temperature with about 1% ~2%in the resistancevariation ratio. In addition, when the magnetic field deviates the wire axis, the AMR effect appears. Such theresistancevariationratio in transverse magneto-resistance (TMR) is about 2% ~3%at 10K and about 0.5% ~1% at room temperature, respectively. The switching fields can be observedin the magneto-resistance curve with two NiFe layers of different thicknesses. The smaller switching field (Hsw1)of the thick layer increases with increasing the angle between the magnetic field and the wire axis, in coincidence with the result of a single wire of about the same thickness. It can be well described by the curling model. Theswitching field of the thin layeris larger than the expectationof the curling model. We suggest that exchange coupling between both layers tends to keep both moments in anti-parallelorientation and hence,the switching field is enhanced. Meanwhile, the switching field is insensitive to the sample thickness. TMR curves indicate thatall moments of both NiFe layers in tri-layers are in parallel and coherently rotate with magnetic field. Anisotropic energy Ku is obtained by fitting TMR curves of all samples. Ku and saturation fields are roughly proportional to sample thickness and the inverse of samplewidth as expected for a single NiFe wire. However, the value of Ku is smaller in tri-layers than a single NiFe wire of the same thickness. We suggest that whenboth moments of two layers are in parallel and coherentlyrotate with magnetic field, Ku is slightly affected by the exchange coupling.