鐵超薄膜以及鐵氮薄膜結構與磁性研究

Magnetic and structural properties of bcc-Fe ultra thin film and iron nitride (FeNx) thin film

林盈志(Yin-Chih Lin)

指導教授 ： 林敏聰

國立臺灣大學/理學院/物理研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.10045

摘要

垂直磁易軸以及高飽和磁矩是相當重要的討論議題，同時深具有工業上的應用潛力，鐵超薄膜以及鐵氮薄膜分別是最重要的候選材料之一，因此本論文先行討論薄膜本身的物理性質，然後更進一步，去研究鐵磁材料與反鐵磁材料之間的耦合機制，此項研究不僅研究薄膜本身的物理性質跟結構性質，同時也深入的研究鐵磁/反鐵磁層之間的耦合作用。

在本論文的第一部分，我們在主要在研究體心超薄膜鐵的磁性行為，隨著薄膜厚度的減少，在大約四個原子層時觀測到自旋軸翻轉的現象(磁易軸從平行於膜面轉變成垂直於膜面)，同時也展現出明顯小於其他系統的超小矯頑磁場(約1 Oe) ，依據低能量電子繞射的結果，從2.4個原子層到15個原子層的區間內，鐵超薄膜維持著體心結構，並未觀測到任何結構轉變或是壓縮伸長，因此我們忽略了結構磁異向能以及磁彈性異向能，認定此自旋軸翻轉現象是表面異向能跟形狀異向能競爭的結果，透過現象學模型所推出表面異向能明顯小於其他相關案例，這可能是鎳鋁基板本身的特殊表面重構所造成，而超小的繳頑磁場強度可能一個特殊表面重構以及鋁表面的影響。
第二部份主要是研究反鐵磁/鐵磁薄膜之間的耦合機制，體心錳是反鐵磁材料(互相彼此抵銷的磁矩)，因此柯爾磁光量測無法量測任何訊息，但是反鐵磁層會對鐵磁層的磁滯曲線產生劇烈變化，而鐵/錳雙層結構，當中間錳層超過九個原子層，矯頑磁場出現劇烈的提升，而這樣的現象顯示，當鐵薄膜層進行翻轉時，反鐵磁錳層也會隨之一起翻轉，實驗結果提供數據去研究鐵/錳之間耦合翻轉機制。
第三部分主要是研究特殊結構的鐵氮超薄膜的電子與磁性結構，α-鐵氮薄膜展現了明顯大於鐵的飽和磁矩，而經過精細分析的光電子能譜中，部分鐵譜線(代表位於4e以及8h幾何位置的原子)出現了束縛能偏移的現象，同時氮1s譜線也出現相反方向但類似的束縛能偏移現象，這代表了部分鐵跟氮之間具有交互混成以及電荷交換，導致了強的庫倫能在鐵氮八面體內，導致了鐵在鐵氮八面體有明顯大於其他鐵的磁矩(4個波爾磁元/鐵原子)，實驗結果為目前相關解釋理論假設提供了重要佐證，而另外一方面，反鐵磁鉻薄膜(尼爾溫度接近室溫)被鍍在鐵氮薄膜，實驗結果證明，鐵氮以及鉻會產生耦合現象一起翻轉，而一起翻轉也導致矯頑磁場有劇烈變化，以及跟鉻厚度成正比，提供了一個方法控制鐵氮的矯頑磁場。

關鍵字

體心鐵 ； 體心錳 ； 自旋軸翻轉 ； 高飽和磁矩 ； 鐵氮化合物 ； 鐵磁反鐵磁耦合現象 ； X光電子能譜 ； 電子結構混合 ； 矯頑磁場操控

並列摘要

Perpendicular magnetic anisotropy and giant saturation magnetization are both critical physical properties in fundamental science and contemporary industries.
Therefore they attract much attention in magnetic investigation. Iron(Fe) and its nitride are the candidates for the materials of perpendicular magnetic anisotropy and giant saturation magnetization. We devote to investigate these topics:

(1)The magnetic properties and crystalline structure of bcc-Fe on NiAl(001)
(2) The exchange coupling between Fe/bct-Mn systems
(3) The electronic structure,magnetic properties and AFM-induced coercivity enhancement of FeN thin films.

Ultrathin Fe films were deposited on NiAl(001) for the studies of the magnetic properties and crystalline structures. Spin reorientation transition (SRT) was observed at approximately 4 monolayers (MLs) at room temperature. At less than this critical thickness, the Fe _lms's easy axis is perpendicular to the surface plane with a ultra small coercive field (Hc ~ 1 Oe) which is much smaller than other related Fe systems. Such soft perpendicular magnetization may cause to the special surface reconstruction and termination of NiAl(001). During the transition region, the Fe films sustained body-centered-cubic (bcc) structure without any obvious phase transformation and variation. Thus, through a phenomenological model, the competition between the surface anisotropy and shape anistropy determine the SRT behavior. The surface anisotropy is 267.3 μeV/atom, which smaller than the cases of bct-Fe on Ag(001) and free standing bcc-Fe. The weaker surface anisotropy might be attributed the reconstructed NiAl(001) surface.

Expanded bcc-Mn films ( c/a = 1.15) were deposited on NiAl(001) for the studies in the magnetic property. In the Fe/Mn bilayer system, the coercive field enhancement is observed when the Mn thickness is greater than 9 ML. Through AFM/FM exchange theory, the coercive field enhancement without exchange bias implies the coherent rotation between Mn and Fe layers during the magnetization process.

Iron nitride, which exhibits a higher saturation magnetization than a bcc-Fe thin film, was grown on Au(001) texture on a GaAs(001) substrate for studies of crystalline structure, electronic structure and magnetic properties. Fe 2p3/2 and Fe 2p1/2 X-ray photoelectron spectra (XPS) reveal the electronic hybridization between the Fe atoms and the adjacent N atoms. Through the multipeak analysis, except main peak of Fe(4d), the remaining two peaks in the Fe 2p spectrum correspond to Fe(8h) and Fe(4e) geometrical sites and that they simultaneously show an energy shift toward higher energy which suggests the charge-transfer-induced electronic rearrangement of electronic configuration in Fe(8h) and Fe(4e) geometrical sites. These observations in electronic structure and hybridization between the N atoms and the adjacent Fe atoms could help to explain the saturation magnetization enhancement in the FeN system. On the other hand, the Cr thin film also was deposited on the FeN sample. AFM-induced coercive field enhancement is observed as the Cr thickness increases. These results provide a guide for the coercivity manipulation in the future.

並列關鍵字

bcc-Fe ； spin reorientation transition ； bct-Mn ； exchange coupling ； Fe16N2 ； high saturation magnetization ； XPS ； electronic hybridization ； coercivity manipulation

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