介面磁光效應在多層超薄膜系統表面的先進應用

Advance Application of Interface Magneto-Optic Effect on Ultrathin Multilayered Films

蘇炯武(Chiung-Wu Su)

《真空科技》 27卷2期 (2014/06) Pp. 5-13

摘要

近期筆者在超薄膜磁光量測領域之研究中發現，不論是表面磁光科爾效應或是表面磁光法拉第效應，對於超薄膜表面或位於內層之超薄膜介面系統上都具有極佳的探測靈敏度，這也是為何磁光效應技術可以廣泛應用於探測磁性表面系統的主要原因，近期更在二維超薄膜磁性樣品進行不同角度的法拉第效應觀測中發現有趣的週期性磁光訊號震盪現象，推究其原因為磁性超薄膜成長於具有單光學異向軸的光學透明晶體而產生，經由不同入射光的觀測下磁與光兩種行為引致分別不同的反應，而分屬相依於超薄磁膜之厚度以及光學基板之厚度，由於磁光效應需要經由基礎光學原理進行反射或透射以取得反應後之訊號，因此針對不同量測技術以及角度之全面掃描下預期能擷取到光與材料於交互耦合路徑內的所有資訊。我們借重此想法，預期在調整不同入射角度進行磁光科爾效應時，利用光的穿透性，能夠進一步應用此實驗方法於探討磁性超薄膜內部介面之先進量測技術，文中所舉之待測樣品為近期熱門的垂直自旋閥其中的上結構（Ta/CoFeB/MgO）進行系列研究，從對照CoFeB/MgO系列樣品上層表面於製程時覆蓋不同奈米厚度的Ta時發現，確立了此上層Ta之覆蓋也可能同時影響隔了一層鐵磁膜底下MgO光學膜的結構，從完成退火前後之實驗對照，發現了不同光入射角的磁光探測技術展現了鑑別多層超薄膜介面結構變化的功能，也同時驗證了筆者之想法，的確在探討磁性超薄膜結構時必須將鄰近基板或介面間接影響的光學效應一併納入考慮，因為上層Ta層之覆蓋以及退火後所引致晶格應力間匹配的程度可能影響了CoFeB鄰近光學膜MgO（001）結晶結構之形成，而此結構之光學性可以從角度的週期性震盪函數來判定晶軸可能受到影響，這震盪函數所對應之主角來自於光學基板或是鄰近光學層的厚度，並不是針對鐵磁膜厚度，若此光學層具有單軸結構異向性的話，會引致雙折射現象的產生，讓光學函數中屬於此部份的週期性震盪行為產生，而因為此光學波動性之幅度改變會讓多角度下所觀察的磁光效應發生連帶性的變化，這也就是為何會觀察到磁滯曲線反轉的有趣物理現象，甚至在特定的角度磁光訊號會完全消失。藉由本實驗提昇量測精密程度後，許多單從特定方向所決定的單軸磁異向性變成不是絕對，而可能有多軸異向性的可能。經由分析研究，所得到磁光旋轉角，似乎與磁性質的單一性相違背，然而本磁光效應在超薄膜介面中的發現可以進一步應用於磁性顯微影像之對比度，讓日後磁性顯微技術之發展能夠進一步提昇空間解析度。

關鍵字

磁光效應 ； 超薄膜 ； 介面 ； 磁滯反轉

並列摘要

Recently, we found an excellent magneto-optical sensitivity on probing ultrathin surfaces or ultrathin magnetic interfaces no matter the surface magneto-optic Kerr effect or the surface Faraday effect measurements were used in the study of magnetic properties. This may be the reason of the wide application of magneto-optic technique to any kinds of magnetic surface systems. In the various angle of incidence on the Faraday effect measurement, we found a periodic oscillation of magneto-optic signal on an ultrathin magnetic film. The reason originates from the magnetic film grown on the uniaxial optical crystal. The different responses are induced from the magnetic effect and the optical effect to the thickness of the ultrathin magnetic film and the optical substrate, respectively. Through full angle scan experiments, we can obtain most information of optical signals from the material. Based on this idea, we perform more advanced experiment on the magneto-optic Kerr effect measurement. In this article, a perpendicular spin structure CoFeB/MgO with capping Ta layers was studied. From the comparison of different capping thickness, we guess the capping layer may induce different structure even the underneath of CoFeB layer. The MgO corresponds to an uniaxial optical layer. Its optical axis may be altered by the surface layers due to different strain on the surface. We observed a similar behavior of periodic oscillation of angle-dependent Kerr intensities. Thus, we can not just determine the magnetic properties from only two field direction, such as out-of-plane and in-plane magnetic vectors. The birefringence of the optical crystal or layer makes the oscillation of optical signal possible. The magneto-optic effect then follows the behavior to exhibit an interesting reversal behavior even completely disappears. Through delicate angle measurement on the magneto-optic effect measurement, we obtain more about the magnetic and optical properties of related materials. It can help to develop high-resolution magnetic microscopy and has a potential of improving the contrast of magnetic images in the future.

並列關鍵字

Magneto-optic Effect ； Ultrathin Film ； Interface ； Magnetic Reversal