磁性記憶體寫入效益之研究

Study of writing efficiency for magnetic random access memory

黃勝煌(Sheng-Huang Huang)

指導教授 ： 賴志煌

國立清華大學/工學院/材料科學工程學系/博士(2010年)

https://doi.org/10.6843/NTHU.2010.00661

摘要

利用磁阻元件來做為位元紀錄的記憶體稱為磁性記憶體，也因為它不僅具有非揮發性的特性外，讀寫速度也可媲美目前市面上的記憶體，因此又被稱為夢幻記憶體。在本論文中，主要是針對磁性記憶體的主要課題進行研究－磁性記憶體的寫入效益。
首先，在利用自旋極化電流來驅動之磁性記憶體的研究中，發現藉由改變自由層鈷鐵硼的覆蓋層可以用來調變它的飽和磁化量以及自旋阻尼係數。當覆蓋層為鉭的時候，自旋阻尼係數會增加，主要原因是來自於鉭與鈷鐵硼之間的膜層混合。而當我們在鉭與鈷鐵硼之間插入一層金屬層(銅，銀)時，不僅鉭與鈷鐵硼之間的膜層混合可以有效的被抑制，且自旋阻尼係數也因為自旋幫浦效應的關係而降低。此外，由微磁學的結果得知，當插入層為銅的時候，臨界翻轉電流可以有效的被降低。
第二個部份裡，研究的主角為利用磁區壁來做為紀錄的磁性記憶體。在此研究中，磁性奈米線上的刻痕是用來控制磁區壁停留的位置，然而在該記憶體中，如何使磁區壁脫離刻痕的位置，是此記憶體目前主要的研究課題。在本研究中，當施加的電流密度高於臨界電流密度時，磁區壁的結構會開始做橫切狀與反渦漩狀之間週期性的轉換。然而研究發現磁區壁脫離刻痕的關鍵與磁區壁的形狀有很大的相關性。當磁區壁接近刻痕的位置，如果磁區壁的形狀可以轉換成反渦漩狀，則磁區壁即可順利的脫離刻痕，其主要原因為當磁區壁形狀轉換成反渦漩狀時，自旋極化電流所注入的能量將被儲存，然而當其通過刻痕的位置的時候，所儲存的能量開始轉換成磁區壁移動的動能，所以脫離的機率即可大幅的增加。
在最後一個部分的研究中結合了磁區壁的優勢，來探討並設計增加磁性記憶體寫入效益的方法。本研究藉由離子佈植的方法來侷部控制磁性薄膜的交換異向性，研究中發現當樣品經過離子佈植後，因為佈植區域與非佈植區域的翻轉場不同而導致在翻轉的過程中，磁區壁會在佈植區域與非佈植區域之間形成，而當磁區壁形成後，將會輔助其他位置磁性層的翻轉。因此，利用此效應可以設計出具有高寫入效益的新型的磁性記憶體，經由微磁學模擬的結果發現，此新型記憶體無論是利用磁場驅動或自旋極化電流驅動，其寫入效益均可大幅的被提升。

關鍵字

磁性記憶體 ； 自旋阻尼係數 ； 磁區壁 ； 微磁學 ； 離子佈植 ； 自旋轉換磁矩

並列摘要

A novel memory device based on the magnetoreisitive effect is known as magnetic random access memory (MRAM) and is also called “dream memory”. In this dissertation, we investigate the key issue of the MRAM － writing efficiency.
Firstly, in the concept of spin torque transfer MRAM, we can manipulate the damping constant and saturation magnetization of CoFeB by simply adjusting the capping layers. We found that the Ta capping layer caused the intermixing, and increased the damping constant of CoFeB. By inserting various metals, the intermixing can be improved and the lower damping constant with Cu/Ta capping layers was obtained. Moreover, the results of micromagnetic simulation indicate that the optimal switching current density can be achieved by reducing the damping constant with a Cu capping layer. In our simulation results, we demonstrate that by changing the capping layer from Ta to Cu/Ta, we can effectively reduce the critical current density by 27 %.
Secondly, in the concept of magnetic domain wall RAM (so called “race-track memory”), we demonstrate that the depinning probability of the transverse-type domain wall strongly depends on the domain wall configuration when the domain wall moves to the notch. We found that when the current density is larger than threshold current density, the domain wall structure periodically changes between transverse-type domain wall and antivortex-type domain wall. Due to this transition behavior, part of the spin torque energy contributes to the transformation of the domain wall structure. Since the antivortex-type domain wall stores more energy, the stored energy can help the domain wall to be depinned from the notch and increases the depinning probability.
Finally, we take advantage of the magnetic domain wall and we propose a method which combines either conventional current- or field- driven MRAM with the magnetic domain wall. We demonstrate that the domain wall can be artificially created in patterned antiferromagnetic/ferromagnetic exchange bias system using ion irradiation. The magnetic domain wall was formed by the different switching fields in the irradiated and non-irradiated areas and could assist the magnetization switching. Furthermore, this study proposes a new type of MRAM design. A comparison of the proposed magnetoresistive device with the conventional ellipse one shows that the new design reduces the switching field and the critical current density by about 90.0 % and 68.8 %, respectively, in field- and current- driven cases. In both of these cases, we demonstrated the domain wall assisted magnetization reversal. This study provides an effect way to increase the writing efficiency of MRAM.

並列關鍵字

magnetic random access memory ； damping constant ； magnetic domain wall ； micromagnetic ； irradiation ； spin transfer torque

參考文獻

[4] Sun J. Z., Phys. Rev. B, 62, 570 (2000).

[5] L. Berger, J. Appl. Phys., 55, 1954 (1984).

[12] S. Glathe, R. Mattheis, and D. V. Berkov, Appl. Phys. Lett., 93, 072508 (2008).

[13] D. Atkinson, D. A. Allwood, G. Xiong, M. D. Cooke, C. C. Faulkner and R. P. Cowburn, Nature Mater., 2, 85 (2003).

[20] M. Julliere, Phys. Lett. A, 54, 225 (1975).