https://scholars.lib.ntu.edu.tw/handle/123456789/31954
標題: | 三層節系統Co/CoO/Fe3O4及Fe3O4/AlOx/Co之磁性穿隧效應的研究 Magnetic Tunneling Effect in Co/CoO/Fe3O4 and Fe3O4/AlOx/Co Junctions |
作者: | 簡崇欽 Jian, Chung-Chin |
關鍵字: | 磁性穿隧效應;Magnetic Tunneling Effect | 公開日期: | 2004 | 摘要: | 摘要 以半金屬作為電極的磁性三層節(MTJ)系統在磁性記錄方面有很大的應用潛力,因為半金屬具有很高的自旋極化率,預期可提高磁性三層節的TMR值。然而,在半金屬的材料當中,又以Fe3O4最具應用價值,因為它具有很高的居理溫度( ~ 856 K)。但是至今仍未發現以Fe3O4當電極而得到很高的TMR值的磁性三層節系統 。可能是帶有自旋的電子在穿隧過程當中在接面處發生了自旋翻轉,以及MTJ系統中間的氧化絕緣層和Fe3O4之間在接面處的作用,降低了Fe3O4本身的自旋極化率。這些都很有可能造成降低MTJ系統 的TMR值。 本實驗中,嘗試以Fe3O4作為MTJ系統中的電極,所研究的MTJ系統是Co/ CoO / Fe3O4 及Fe3O4 / AlOx / Co,兩系統為相同的電極,但不同的中間絕緣層,分別是反鐵磁性絕緣體CoO及一般非磁性絕緣體AlOx,目的是拿來做比較。首先對MTJ兩邊的電極做基本電性和磁性的研究,之後主要工作就是研究中間氧化絕緣層的形成條件,這部份同時也是實驗中最具困難度的地方。另外,使用四點量測法量測MTJ時,發現量測到負電阻現象,這是因為在Junction接面處電極的電阻比MTJ的電阻來的大或是兩者電阻數量級一樣大時,所造成在接面處電流分佈不均勻的效應。要解決這個問題,就必須提高中間絕緣層的性質,如提高氧化的緻密程度、減少雜質比例及提高熱穩定度等,或是降低電極的電阻來拉開電極和MTJ電阻之大小差距。利用兩點量測法可基本先測得電子在MTJ的穿隧情況,若是有非線性三次方的I – V曲線結果,就表示中間有緻密的氧化絕緣層而形成電子穿隧的位勢障。另外,可將I - V結果fitting之後,透過Simmons’s rule而得知位勢障的寬度和高度。本實驗中,為了要降低內層接面的不平整所造成的氧化層難以覆蓋緻密對MTJ所造成的重大阻礙,所以嘗試將MTJ的接面面積縮小希望能到μm2-size的大小,如此預測可提高內層接面之平整度,同時亦可減少絕緣層中的雜質比例。實驗過程先學習使用電子微影術(E-Beam lithography)的技術,過程中先研究磁性Co薄膜的顆粒陣列性質,而Co薄膜顆粒直徑最小可達450nm。 Abstract Magnetic trilayer junctions(MTJs) based on half metallic oxides have attracted much attention because of their potential in memory and logic device applications. Among half metallic materials, the magnetite(Fe3O4) is the most promising candidate due to its high ferrimagnetic Curie temperature(~856K). And an enhanced TMR effect has been expected for this sort of MTJ. However, up to now, the MR effect of MTJ with Fe3O4 as an electrode has been found to be negligibly small. Several causes have been speculated to explain the reduction. For examples, the spin flip processes on the interface would have a deleterious effect on the MR. A less-than ideal insulating barrier containing impurities and defects would also lead to spin scattering. In order to explore the major origins of reduction in MR, we have systematically investigated the TMR of MTJs prepared under various conditions. Our MTJs consist of Fe3O4 and Co as the electrodes sandwitched with cobalt oxide and AlOx thin layers as the tunneling barriers for comparison. The choice of Fe3O4 and Co as the electrodes is due to a large difference of coercivities from these two magnetic layers. Besides, a thin oxide layer can be directly obtained from oxidation of the metal layer right after deposition of the metal, where metals are Co and Al, respectively. We have adopted different oxidation time to acquire tunneling barrier of various thickness. Thus, the impact of tunneling barrier on TMR can be evaluated. Furthermore, since the polarization of Fe3O4 has been predicted to be negative, the inverse MR will be also studied. In Our MTJs, the resistance of electrode Fe3O4 is too large compared to the junction’s, then it would cause nonuniform current distribution over the junction area. Thus, negative resistance will be obtained by using four-probe measurement. We try to reduce the junction area from mm2-size to um2-size to pursuit a uniform coverage of the barrier on the electrode in MTJs. For this purpose, we learn to use E-beam lithography technique to make um2-size magnetic films. We also fabricate hundreds of nm size Co dot arrays to investigate its properties. |
URI: | http://ntur.lib.ntu.edu.tw//handle/246246/54477 | 其他識別: | zh-TW |
顯示於: | 物理學系 |
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