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  • 學位論文

NiFe與CoFeB自旋閥之磁化翻轉行為研究與自旋極化率測量

Behavior of NiFe and CoFeB magnetization reversal in submicron linewidth pseudo spin valve structures and measurements on the degree of spin polarization of transport electrons

指導教授 : 郭光宇
共同指導教授 : 李尚凡

摘要


在微觀的磁學研究裡,磁性料在侷限的幾何形狀下的磁化翻轉行為以及本身的自旋極化率,不但是很重要的課題,對於下一世代磁性元件的應用也提供新穎的方向,所以我們的實驗就針對這兩個主題做研究。 首先,以自旋閥的結構做磁化翻轉的研究,不但探討其幾何形狀上的影響,也對磁壁(domain wall)的動態變化做時域上的研究。 在NiFe/Cu/NiFe橢圓環自旋閥結構中,磁區翻轉行為隨著樣品尺寸變化,呈現一階段翻轉或兩階段翻轉,經由量測不同橢圓環長短軸比例與線寬的樣品,我們得到具有物理行為的相圖,而其結果和物理機制與理論模擬吻合。 我們也針對邊的粗糙度對次微米自旋閥線做研究,有系統地在線上製造不同大小與不同週期間距的突出物。當粗糙度小於某尺度時(對NiFe而言約 30nm為界線,對CoFeB而言約40nm是界線),磁壁在400nm線寬上的運動速度不會受到此粗糙度影響,而粗糙度大於此界線時,磁壁的運動速度會因此粗糙度所侷限而變慢。由於自旋電流引入之力矩也可導致磁化反轉, 所以我們也探討臨界電流在此結構下的影響,發現具有粗糙度的次微米自旋閥線可藉由一個外加橫向磁場,降低臨界電流密度,而我們的結果可歸因於在空間上局部磁化所造成。 在第二的主題裡,以點接觸安德烈夫反射(Point Contact Andreev Reflection)的方法量測不同磁性材料的自旋極化率,如同大部分的文獻,我們得到較寬的電導對電壓能譜圖,此行為是無法用MBTK模型解釋,但經由引入一個外加電阻項以及等效溫度於MBTK模型中,可成功解釋此現象並得到自旋極化率。另外我們考慮各方向入射角的入射電子貢獻,建立一個三維的BTK模型來解釋一些傳統BTK模型無法描述的特殊能普,這是由於入射電流在某些條件下會衰減,造成電子會在金屬與超導介面上堆積。新的模型很成功的可以擬合我們的量測數據。

並列摘要


The behavior of magnetization reversal in confined geometry and the degree of spin polarization of magnetic materials are fundamental questions in magnetism and important for a wide range of magnetoresistive device application. Therefore we investigate these two topics experimentally. First, we studied the behavior of magnetization reversal on pseudo spin valve sub micron structures. The research includes not only geometric shape effect but also dynamic measurements. We studied nanoscale elliptical ring shaped NiFe/Cu/NiFe trilayer pseudo spin valve structures. The magnetization reversal processes showed simultaneous-reversal single-step transition or double-step transition involving flux closure states. For various aspect ratios (short axis to long axis) and line widths, transition between single-step and double-step magnetization reversals was measured to form a phase diagram. Simulations of the magnetization reversal behavior agreed qualitatively with our results. We also studied edge roughness effect on the magnetization reversal of spin valve submicron wires. We prepared wires designed with periodic 'spikes' as artificial roughness. The height and the pitch of the spikes were varied systematically. No obvious dependence was found between the roughness and the domain wall velocity when the spikes were smaller than a threshold of 30 nm for NiFe (the widths were fixed at 400 nm), 40nm for CoFeB. The average velocity was slowed down when the height of the spikes were larger than the threshold. We also studied the current-induced magnetization switching. In-plane transverse magnetic fields help to reduce the critical current density for current induced domain-wall motion. Our results could be attributed to the space modulation of the local magnetization. In the second topic, the point-contact Andreev reflection technique is employed to determine spin polarization of transport electrons or holes of various ferromagnetic materials. The observed conductance versus voltage spectra exhibited behaviors which were described by the Modified Blonder-Tinkham-Klapwijk (MBTK) model but with significant broadening. We propose a model by introducing a spreading resistance and compare with an existing model which included effective temperature as a parameter. We also established the new three-dimension BTK model to fit some anomalous data. The model considered three-dimension contact and incident current from all angles. The incident current from the ferromagnet was restricted under some conditions which made it decay. The electron would accumulate at the ferromaget/superconductor interface. Not only ideal data but also anomalous spectra are fitted well in this model.

參考文獻


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