垂直磁化鐵鉑與鈷鐵硼薄膜之磁異向性研究

Study of perpendicular magnetic anisotropy in FePt and CoFeB thin films

蔡武璋(Wu-Chang Tsai)

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(2013年)

https://doi.org/10.6843/NTHU.2013.00027

摘要

近年來，由於自旋電子元件如磁性記憶體、邏輯元件蓬勃發展，其中利用電場控制磁性材料特性的方式備受矚目，主要是利用電場的方式可以大幅降低元件中的功率消耗，加上這一類元件製備的方式完全相容於現有的半導體製程，相關的研究因此而變得熱門。早期在自旋電子元件中，鐵磁層的磁矩都是沿著膜面排列，因應使用者對記錄密度要求的提升，為了提升記錄層的熱穩定度及降低寫入電流，鐵磁層材料的選擇也從水平式發展成垂直式。因此，如何利用施加電場的方式來操控垂直式自旋電子元件變成了一門重要的課題。
所以本論文前兩個部分著重在材料的選擇。第一個部分我們選用鉻鉬錳合金當底層材料，利用升溫鍍膜的方式成長序化相的鐵鉑合金。具有體心立方(002)織構的鉻鉬錳底層可以促進鐵鉑合金在較低的溫度完成序化，序化之後具有良好的垂直磁異向性。另外，升溫鍍膜的方式讓鐵鉑合金產生大小不一的晶粒，造成過大的表面粗糙度。為了改善此一現象，我們利用電漿的方式來處理鐵鉑合金的表面，並且利用處理過後的鐵鉑合金來測試垂直式穿隧磁阻元件的可行性。
第二個部分是探討具有垂直磁異向性的鈷鐵硼合金薄膜。近幾年，很多研究團隊都發現鈷鐵硼合金厚度很薄的時候會具有垂直磁異向性，根據理論推測，此垂直磁異向性來自於鐵原子與鄰近氧化物的氧原子產生混成軌域；這部份我們的貢獻在於利用實驗的方式來驗證此一說法。利用同步輻射光源備有的X光磁性圓偏振二相性光譜取得鈷鐵硼合金薄膜中鐵的訊號，加上求和規則計算鐵原子的軌域磁矩，發現不同的鐵、氧軌域混成產生的軌域磁矩變化和鈷鐵硼合金薄膜磁異向性的變化一致，驗證了鈷鐵硼合金薄膜的垂直磁異向性來自於鐵、氧軌域的混成。
第三個部分則是選用特殊的壓電基板來成長鐵鉑合金磊晶薄膜。透過電場的施加讓壓電基板產生應變，利用磁伸縮的特性讓鐵鉑合金薄膜產生磁異向性的變化。另外我們也發現一個有趣的現象，當試片經過正、負偏壓回到零場的時候，鐵鉑合金具有非揮發性的磁異向性變化；透過鐵鉑合金薄膜厚度調整、插入中介層阻隔介面效應，驗證了此一現象來自於基板的鐵電電場效應所造成。利用電場控制鐵鉑合金的磁翻轉行為，同時提供一個非揮發性的異向性改變，讓成果更具有實際應用的價值。

關鍵字

鈷鐵硼合金 ； 鐵鉑合金 ； 垂直磁異向性 ； 壓電基板

並列摘要

Electric field modulation of magnetic characteristics has drawn much attention due to its potentials for novel spintronic or magnetoelectric devices, such as voltage-driven magnetic random access memories, logic circuits, and so on, with lower power consumption and higher processing speed. In addition, spintronic devices with perpendicularly magnetized ferromagnetic layers have been extensively studied recent years because of their advantages of low critical current density and high thermal stability for current-induced magnetization switching. Therefore, this dissertation focuses on growth of ferromagnetic thin films with perpendicular magnetic anisotropy (PMA) and electric field controlled anisotropy change by using single crystalline piezoelectric substrates.
In the first topic, we fabricated perpendicularly magnetized ordered L10-FePt thin films with a buffer layer of CrMoMn/Pt bi-layers, which were out-of-plane (002) pre-ferred orientation and prepared by in-situ heating deposition on thermally-oxidized silicon substrates. For the purpose of being an electrode in perpendicular magnetic tunnel junc-tions, the plasma treatment with O2/Ar mixture was applied to reduce the surface rough-ness of the L10-FePt thin films. Combined with a MgO barrier and Co/Pt multilayers, the typical magnetic characteristics of pseudo spin valve structure were well demonstrated.
In the second topic, we fabricated a single CoFeB layer with PMA on thermal-ly-oxidized silicon substrates or MgO buffer layers. After annealing, the PMA of the CoFeB layer strongly depended on the oxidation degree of the MgO buffer layers. To in-vestigate the origin of PMA in the CoFeB thin films, x-ray magnetic circular dichroism and sum rule calculation were used to extract the orbital moments per 3d hole (morb/Nh) of Fe in annealed CoFeB layers. The correlation between morb/Nh, depending on the oxidationn time of MgO, and the PMA variation of CoFeB revealed that the PMA of CoFeB was mainly contributed from the interfacial orbital hybridization of the Fe-3d and O-2p.
In the third topic, a perovskite-type single crystalline piezoelectric substrate, (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(x: 0.28~0.32), was used to grow epitaxial L10-FePt and polycrystalline CoFeB thin films. Since strain would be propagated from the piezoelectric substrate, the out-of-plane coercivity (Hc) of L10-FePt showed a typical butterfly-like loop, companying with an abnormal asymmetric phenomenon, related to the applied electric fields due to the magnetostriction. The thickness dependence of FePt on the out-of-plane Hc revealed that the asymmetric behavior resulted from the ferroelectric field effect of the electric polarized substrate. The Hc difference at zero electric field, back from opposite direction of electric fields, showed a non-volatile behavior and was of significance for applications of voltage-driven solid state devices.

並列關鍵字

CoFeB ； FePt ； perpendicular magnetic anisotropy ； piezoelectric substrate

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