藉由調控介面之交換耦合控制鐵磁/反鐵磁超薄雙層膜系統垂直方向磁性之研究

Controlling Perpendicular Magnetization in Ferromagnetic-Antiferromagnetic Ultrathin Bilayer Systems by Tunning Interfacial Exchange Coupling

紀乃友(Nae-Yeou Jih)

指導教授 ： 林敏聰

國立臺灣大學/理學院/物理研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.10330

摘要

具有垂直磁異向性的超薄膜在工業界、學術機構及研究人員間引起廣泛注意，最主要由於這類低維度的材料在高密度磁紀錄以及磁性奈米元件具有非常大的應用潛力。在本論文中，我們將先研究鐵磁/反鐵磁交換耦合雙層膜的生長、結構以及磁性，並且著重在藉由調控介面之交換偶合以控制鐵磁/反鐵磁雙層膜在垂直方向的磁性。本文的第一部份將介紹超薄鎳鐵 (permalloy) 以及鎳鐵/錳雙層膜成長在銅三金上的研究及結果。樣品的生長模式以及結構分別藉由中能量電子繞射及低能量電子繞射來監測，磁性性質則藉由磁光柯爾效應來測量。鎳鐵生長在銅三金上呈現面心四方結構 (fct) 以及平面方向上的磁異向性。藉由加入一錳底層，即可發現垂直方向的磁異向性。經由厚度相依性以及溫度相依性的實驗發現，垂直方向磁異向性的增加是由於反鐵磁層及鐵磁層的交換耦合作用。在經過降溫加場的過程後，可以發現鎳鐵/錳/銅三金雙層膜的磁滯曲線偏移現象。
本研究的第二部份將由應用的角度探討控制鎳鐵/錳/銅三金雙層膜在垂直方向上的飽和磁化量、矯頑力以及熱穩定性。飽和磁化量及矯頑力被發現分別隨著鎳鐵厚度及錳厚度的增加而增大，垂直方向磁異向性的熱穩定性也發現隨著鎳鐵及錳的厚度而改變。鎳鐵及錳層的厚度應該是調控垂直方向磁異向性最重要的參數之ㄧ。這些發現提供一個新的方式考量控制磁性穿隧接面 (magnetic tunneling junctions) 中的自由層，使其具有高熱穩定性及低消耗功率。
第三部份，我們進一步改用鐵錳合金作為反鐵磁層材料以研究鐵磁/反鐵磁交換耦合促成自旋轉向的系統。在鐵/鐵錳/銅三金雙層膜也發現了鐵磁/反鐵磁交換耦合促成自旋轉向的現象。和錳ㄧ樣，較厚的鐵錳層可使較厚的鐵層維持在垂直方向的磁異向性。進一步發現鐵錳合金比例對垂直磁異向性的影響，鐵x錳1-x合金當x約0.3時貢獻最大的垂直磁異向性。這個發現建議可以推廣不同的反鐵磁材料到應用鐵磁/反鐵磁交換耦合促成自旋轉向的效應。
第四部份將藉由搭配X光磁圓偏振二項性 (X-ray magnetic circular dichroism)的光電子顯微鏡 (photoemission electron microscopy) 觀察鐵磁/反鐵磁交換耦合系統的磁性微結構 (磁域)。在鎳鐵/錳/銅三金雙層膜中，沒有錳層的範圍呈現大的平面方向上的磁域 (>100 μm)，而在具有21-23錳原子層的地方發現小的不規則磁域 (~20μm)。在鈷/錳/銅三金雙層膜，鈷磁矩的指向隨著錳層厚度而有變化，並且在錳約7原子層的地方，鈷的磁域碎裂成小磁域。由於磁域影像中鈷層與錳層的亮度對比是相反的，可以判斷鈷與錳是反向耦合。由於發現錳層中隨一個原子層厚度週期的條狀圖案，錳層中的自旋電子結構極可能是層狀的反鐵磁結構 (layered AFM)。此外，在鈷層磁矩方向擺動的區域，存在160°鈷與錳非同一直線方向的耦合。

關鍵字

磁性超薄膜 ； 垂直磁異向性 ； 交換耦合 ； 自旋翻轉現象 ； 磁域

並列摘要

Ultrathin films with perpendicular magnetic anisotropy (PMA) have drawn a lot of attention among industries, academic institutions, and researchers due mostly to such materials’ particular magnetic phenomena with low dimensionality and vast potential of applications in the fields of high density magnetic recording and magnetic nanodevices. In this dissertation, we will firstly investigate the growth, structure and magnetism of ferromagnetic (FM)-antiferromagnetic (AFM) exchange coupling bilayers and subsequently focus our study on controlling perpendicular magnetization in FM/AFM bilayers by tuning interfacial exchange coupling. In the first part of this dissertation, investigations and their results associated with both ultrathin permalloy (Py) layers and Py/Mn bilayers grown on Cu3Au(001) will be presented. Modes of growth and structures of specimen have been monitored by means of medium-energy electron diffraction (MEED) and low-energy electron diffraction (LEED) respectively. Magnetic properties, on the other hand have been measured by MOKE. It is found that Py grown on Cu3Au(001) has revealed fct structures and exhibited in-plane magnetic anisotropy. By adding a Mn underlayer, PMA has then been observed. According to findings stemming from these thickness-dependent and temperature-dependent experiments, existence of enhanced PMA in Py/Mn/Cu3Au(001) bilayers could be validated based on AFM-FM exchange coupling. After the process of field cooling, the biased magnetic hysteresis loops in Py/Mn/Cu3Au(001) bilayers could then be found.
In the second part of our study, we will investigate controlling saturation magnetization, coercivity, and thermal stability of perpendicular magnetization in Py/Mn/Cu3Au(001) bilayers from the perspective of applicability. Saturation magnetization and coercivity have both been found to increase as functions of Py thickness and Mn thickness respectively in our experiment regimes. Thermal stability of PMA has been found to vary as a function of Py and Mn thicknesses. Thickness of Py and Mn layers could probably be one of the most important parameters in tuning the strength of PMA. These findings have thus provided us with a new approach in contemplating the control of PMA in magnetic free layers of MTJs with high thermal stability one the one hand while with low power consumption on the other hand.
In the third part, we will further our study focusing on the AFM-FM exchange coupling assisted SRT systems by using FexMn1-x alloy as the AFM layer. It has been found that Fe/Fe0.5Mn0.5/Cu3Au(001) bilayers have also revealed AFM-FM exchange coupling assisted SRT. Just like Mn layer, thicker Fe0.5Mn0.5 layer can also facilitate thicker Fe layer to remain on PMA. Furthermore, the influence of FeMn alloy compositions on PMA has also been observed in our study. It has been discovered that FexMn1-x layer has contributed the largest PMA as x ~ 0.3. This finding suggests that applications derived from the effect of AFM-FM exchange coupling assisted PMA could be further expanded to various other AFM materials.
In the fourth part, microscopic magnetic structures (magnetic domains) of FM/AFM exchange coupling systems will be observed by utilizing photoemission electron microscopy (PEEM) with X-ray magnetic circular dichroism (XMCD). In Py/Mn/Cu3Au(001) bilayers , large in-plane magnetic domains (>100 μm) have been found in areas without Mn underlayer while small irregular domains (~20μm) have been found in areas with 21-23 ML Mn underlayer. In Co/Mn/Cu3Au(001) bilayers, orientations of Co domains have been found to vary as a function of Mn thickness, and the domains have further been found to break into smaller ones when Mn thickness is around 7 ML. Due to the existence of inverse contrast of domains in Co layer and Mn layer, anti-parallel coupling between Co layer and Mn layer can be confirmed. Due to the emergence of parallel strip patterns with one monolayer period in Mn layer, the likelihood is extremely that spin configurations in Mn layer are layered AFM structures. Furthermore, in regimes where orientations of magnetizations in Co layer swing, 160° non-collinearly coupling in between Co spins and Mn spins has been found.

並列關鍵字

magnetic ultrathin film ； perpendicular magnetic anisotropy ； exchange coupling ； spin reorientation transition ； magnetic domain

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