電漿在鐵鈀和鈷鈀磁性合金薄膜上誘發磁性圖騰和其氫氣效應之研究

Abstract

這篇論文主要著重在兩種以鈀為基底的磁性薄膜合金;即鐵鈀和鈷鈀磁性合金薄膜。並因種類將實驗分成以鐵鈀合金薄膜為主和鈷鈀合金薄膜為主的兩部分並在其中細分有曝氫以及沒曝氫氣的小節。在第一部分以鐵鈀合金薄膜為主的實驗中，一開始我們使用氧氣和氬氣的電漿結合了電子束微影在30 nm鐵鈀合金薄膜上去製作週期性排列的正方形磁性圖騰，用了原子力顯微鏡(Atomic force microscope)分析了表面形貌發現被電漿處理過的區域表面形貌會變得平整，以及在X光電子能譜(The X-ray photoelectron spectroscopy)發現到被電漿處理過的區域發現會有較完整的三氧化二鐵(α-Fe2O3)的訊號，並在深度的X光電子能譜()量測中觀察到在表面氧化鐵以及其下層鐵鈀合金薄膜之間介面合金組成也受到影響，這代表著電漿不但會改變鐵鈀合金薄膜的表面形貌而且也會影響其表面及介面間的元素組成。此外，我們利用磁光柯爾顯微鏡(Magneto-optic Kerr effect microscopy)在去觀察到了磁性圖騰的結果，被電漿處理過的區域會產生磁疇壁釘紮(Domain wall pinning)。藉由鐵鈀合金薄膜本身擁有的交換偏置場(Exchangebias)去進一步的操控磁性圖騰的翻轉模式。在鐵鈀合金薄膜的曝氫實驗中，我們發現到了鐵鈀合金薄膜在曝氫前的磁性狀態會影響曝氫後的結果，也就是交換偏至場的有無會影響氫氣在鐵鈀合金薄膜中擴散的速率，其結果是藉由曝氫後磁滯曲線隨時間變化的速率來判斷氫氣擴散速率的快慢，並在氫氣擴散的過程中發現到了非共線性磁異向性的磁區而且磁滯曲線會由原本的對稱(真空下)變成不對稱(曝氫後第一次量測)，在氫氣壓力處於0.1 bar時發現到了自發性交換偏至場。在第二部分以鈷鈀合金薄膜為主的實驗中，相似的樣品製程也同樣用在8 nm鈷鈀合金薄膜上，與前者不同的是這邊電漿扮演的是一個加速氧化的誘發因子，即被電漿處理過後會催化該區氧化的速率，其氧化的結果會造成晶格異向性下降並反映在矯頑場的縮減，前者我們使用了在X光電子能譜和顯微X光吸收光譜(micro-area X-ray absorption spectroscopy)去分析薄膜深度的氧化以及表面侷域氧化的元素分析，並證實了電漿處理過後確實會催化該區氧化的速率，後者則是透過柯爾顯微鏡去進行空間中的解析的功能去分析該區磁滯曲線和磁區的變化，基於上述的結果，我們可以藉由電漿調控不同的氧化速率去操控侷域的氧化去改變該區的磁性進而在鈷鈀合金薄膜上產生了磁性圖騰。在此實驗中我們分別量測了0、10、42、217天氧化對於磁性的影響，並發現到在42天時磁性行為發生了不尋常的改變，此時被電漿處理過的區域矯頑場大於沒被處理的區域，我們推測是由於邊界深入氧化產生了一個捕捉效應導致被電漿處理過的區域矯頑場變大。在217天後被電漿處理過的區域則是完全沒有磁性的反應。在鈷鈀合金薄膜的曝氫實驗中，我們延續第二部分的製程在鈷鈀合金薄產生磁性圖騰，在曝氫後緊接著抽真空後利用磁光柯爾顯微鏡進行磁性量測，我們發現到被電漿處理過的區域有著令人驚豔的結果，該區域會同時存在著反鐵磁耦合(Antiferromagnetic coupling)以及Dzyaloshinskii-Moriya interaction，其結果反應在訊號的轉向以及磁區在正負磁場下翻轉時其翻轉方向不對稱。

This thesis is divided into the experiment of generating magnetic patterning in FePd andCoPd thin films and subsections with and without hydrogen exposure.In the first part of the FePd-based experiment, we used oxygen and nitrogen plasma combined with e-beam lithography to make periodic square magnetic patterning on 30 nmFePd thin films and analyzed the surface morphology with an atomic force microscope. It isfound that the surface morphology of the plasma-treated area will become smooth. The X-ray photoelectron spectroscopy measurement also demonstrates a pronounced enhancementof theα-Fe2O3signal. Furthermore, Depth-dependent X-ray photoelectron spectroscopyspectra show that the alloy composition of the interface between the oxidized Fe on thesurface and the underlying FePd layer was also affected, which means that the plasma willnot only change the surface morphology of FePd but also affect the element compositionbetween the surface and the interface. In addition, we observed the results of magneticpatterning and domain wall pinning using a magneto-optic Kerr effect microscope. Besides,an intriguing competition between the uniaxial anisotropy and the exchange bias is observedin our system, and can be microscopically controlled by altering the magnetic anisotropy vialithographed plasma treatment.In the experiment of hydrogen exposure in FePd thin films, hydrogenation-induced non-collinear magnetic anisotropy is observed from the evolution of the magnetic domains inFePd alloy thin films using magneto-optic Kerr effect microscopy. Kerr images reveal com-plicated competition between different magnetic anisotropies during hydrogen diffusion intothe film. An intriguing enhancement of the hydrogen diffusion rate due to the presence ofan initial exchange bias induced by a high magnetic field is thereby discovered. Further-more, the hysteresis curve will be symmetrical from the original (Under vacuum) becameasymmetric (first measurement after hydrogen exposure), and spontaneous exchange biaswas present when the hydrogen pressure was at 0.1 bar.In the second part of the CoPd-based experiment, e-beam lithography and O2-plasmatreatment are used to generate magnetic patterns on the 8 nm CoPd thin films. After that,it will catalyze the oxidation rate in the plasma-treated areas, and the result of oxidation willcause a decrease in the magnetocrystalline anisotropy and reflect the reduction in the mag-netic coercivity. We used X-ray photoelectron spectroscopy and micro-area X-ray absorption spectroscopy to analyze the oxidation of the film depth and the local surface oxidation. The elemental analysis confirmed that the plasma treatment would indeed catalyze the oxidationrate in plasma-treated areas. For variation in magnetism in CoPd thin films, the evolutionof Co oxides on CoPd alloy thin films pretreated with patterned O2plasma treatment isinvestigated by exploring the behaviors of magnetic domains of the film via magneto-opticKerr effect microscopy. As oxidation evolves, intriguing different or even opposite behaviorsof the patterned magnetic domains are observed upon magnetization reversal, indicatingvarious forms of Co oxides during the oxidation. Based on the above, we can manipulatethe local oxidation to change the magnetic properties of the region by adjusting the differ-ent oxidation rates of the plasma to generate magnet patterns on the CoPd thin films. Inthis experiment, we measured the effects of oxidation on magnetic properties at 0, 10, 42,and 217 days and an intriguing opposite effect of oxidation on the magnetic coercivity isobserved on Day 42; As magnetic coercivity decreases with time, one can see that magneticcoercivity inside the squares generally stays lower than that outside, except for the Day-42case, where magnetic coercivity inside is higher, exhibiting an opposite effect of evolvingoxidation on magnetic coercivity, which leads to a reversed outcome of the domain patternson Day 42, contrary to 10-day oxidation. We speculate that the magnetic coercivity of theplasma-treated area becomes more extensive due to a trapping effect caused by the deepoxidation of the boundary. On Day 217, the magnetism inside the squares disappears afterextensive oxidation.In the experiment of hydrogen exposure in CoPd thin films, We continued generating magnet patterns on CoPd thin films. After the first dehydrogenation, the plasma treatmentareas presence antiferromagnetic coupling and interlayer Dzyaloshinskii-Moriya interactionin CoPd thin films, resulting in the polarity of the polar signal reversed and asymmetricbehavior in switching fields.