鎳鐵圖形化微結構中自旋傳輸及磁化翻轉行為之研究

Abstract

本論文的研究以電子束微影及舉離技術，製作次微米級和奈米級的圖形化磁性薄膜樣品。並藉由磁力顯微儀的量化分析及磁阻量測，來研究該圖形化磁性薄膜的磁化翻轉以及自旋傳輸的現象，商業化的磁性穿隧界面元件也在溫度變化的條件下量測其穿隧磁阻的變化。 在一個由兩段不同線寬所組成的磁性長線中，磁力顯微儀的分析結果顯示，不同區域的磁化翻轉現象能被各別地分開並互相比較。寬度越寬的線段，其端點雜散磁力越強；相反地，越細的線段其端點的矯頑場越強，這導因於形狀異向性的效應，也因此在頸部(粗細線段交接處)的矯頑場易受影響，此影響來自於兩線段的磁化彼此相向時的競爭，在頸部處兩同極性磁極的互斥結果使得此處的矯頑場變弱。在磁性穿隧界面元件的熱效應研究中，磁阻變化率在140℃時，降到約為在室溫的87%。這意味著該元件在一般使用狀況中能適應儀器或電器所產生略高於室溫的溫度(60–80 ℃)，而數據顯示在該溫度此元件之磁阻變化率仍能保有在室溫的90-95%。我們也製作了三層的環形自旋閥及單層的磁性環並量測其磁阻，在單層環中，由電流所驅動的磁區壁移動，其所需的臨界電流密度約1.5 ~ 3.5 × 107 A/cm2；在三層環形自旋閥的磁化翻轉過程中，所研究的元件尺寸極可能落在渦旋態和洋蔥態臨界的邊界上。因為觀察到的數據顯示某一暫穩態隨機地出現，該暫穩態是軟磁層處於渦旋態，而硬磁層處於洋蔥態。由於單層及三層的量測方式皆無法提供明確辨認渦旋方向的方法，故我們接著製作橫向的自旋閥，並利用其非局域的量測方式來判斷渦旋態的旋向。此非局域的橫向自旋閥直接探測銅導線中，來自鐵磁環因自旋注入所產生的自旋堆積訊號，也因此能得知鐵磁環的旋向。經由數據推算，該橫向自旋閥在銅質擴散通道裡的自旋級化率約為2%。而在橫向自旋閥的另一種量測中，局域的量測方式產生一個反對稱於零磁場的訊號，幾乎是等同於一般磁滯曲線的訊號。該訊號顯然來自於磁性跟非磁性交接介面的電阻訊號。在一系列有系統的尺寸變化研究下，數據結果顯示該訊號來自於磁性跟非磁性交接介面的異常霍爾效應。

Submicron- and nano-sized magnetic patterns were fabricated by e-beam lithography with lift-off techniques. The magnetoresistance (MR) measurements and quantitative analysis of magnetic force microscopy (MFM) were using to investigate the spin transports and magnetic reversal behaviors of these magnetic patterns. The commercial magnetic tunnel junctions (MTJs) were also measured at various temperatures to observe its thermal effects. The MFM were used to investigate a permalloy (Py) strip including two parts with different widths. The results indicated that the magnetic behaviors in different sections of the strip can be separated. The intensity of the phase-shift in the wider end is stronger than that in the narrower one. In contrast, the coercive force in the narrower end (9 Oe) is larger than that in the wider one (8 Oe). This is due to a strong anisotropic effect, and thus the Hc in the neck section could be strongly affected by the competition of the head-to-tail magnetic configurations in the two parts of the strip wire. This results in a small Hc in the neck section. For the thermal effect measurement of the MTJs, the MR ratio at 140 ℃ remained roughly 87% of that at room temperature. The operational temperature of electronic equipments is generally around 60–80 ℃ and the MR ratio of the MTJs at such temperatures be preserved in a considerable portion (90–95%) of that at room temperature. Single-layered and tri-layered spin valve rings were investigated by MR measurements. The critical current density of current-induced domain wall motion in the single-layered Py ring is about 1.5 ~ 3.5 × 107 A/cm2. With the present size, the tri-layered spin valve was possibly in the critical boundary between the formations of vortex and onion. Within this state, the soft ring was in vortex state and the hard one was in the forward onion state. The nonlocal lateral spin valve (NLSV) devices were also constructed to detect the vortex chirality of the ring. The spin polarization induced in Cu diffusive channel of the NLSV devices in the present work was estimated at about 2%. The spin signals were also enhanced by shortening the distance of the diffusive channel. Finally, we investigated the Cu-Py cross structure through which charge current flows and the resistance of the contact region. The concept of this structure was from the NLSV studies mentioned above. When choosing one voltage electrode as the spin injector itself, the probe arrangement was no longer the nonlocal geometry, and hence the signal from the local contact region was sensed. This signal exhibited a magnetic hysteretic loop, i.e., odd-asymmetric roughly equals to the zero field. We found the variation of the odd-asymmetric signals directly related to the switching of the spin injector at the contact region (the Cu-Py cross). It was attributed to the anomalous Hall effect of the injector at the contact region and the argument was supported by the results of size dependent investigation.