Study of exchange bias couping through spin-polarized quantum well

為了了解透過非磁性金屬層中的量子干涉效應對於磁性交換偏耦合所產生 的影響，本實驗先在銅(001)上鍍上鐵錳合金反鐵磁層，再鍍上鈷的鐵磁層，而後在樣品上面隨著 Z 軸來做銅的階梯式薄膜(Cu;Co;FeMn;Cu(001)) 。在超高真空腔中，利用分子磊晶 (Molecular Beam Evaporation, MBE) 技術來穩定且均勻鍍膜，在此同時利用中能電子繞射 (Medium Election Energy Diffraction, MEED) 來即時監控膜的層數與生長模式，並使用歐傑電子能譜儀 (Auger Election Spectroscopy, AES)來確定薄膜的厚度以及其成分比例﹔至於薄膜結構方面，我們利用低能電子繞射 (Low Election Energy Diffraction, LEED) 來分析薄膜的表面結構，並利用磁光柯爾效應(Magnetic-Optic Kerr Effect, MOKE) 來觀察超薄膜的磁性特性。經由一系列有系統的研究後，我們發現誚b最上層的銅對於16ML Co;20ML FeMn;Cu(001) 的系統中的磁性特性有很劇烈的影響。隨著銅厚度(t_[Cu[) 的改變，系統中的交換偏耦合(Exchange Bias) ，矯頑力(Covercivity) ，磁化量 (Magnetization)都有很明顯的改變。依據不同的磁性性質，我們將銅的影響可分成五區，第一區為 t_[Cu]=0ML ，其主要是觀察鐵磁與反鐵磁層界面上磁性偏耦合現象，第二區為t_[Cu]<1ML 表面上銅開始引起磁性性質變化，第三區為1ML<t_[Cu]<3ML，矯頑力急速變大，偏耦合場漸漸變小，飽和磁化量變少，第四區為 3ML<t_[Cu]<5ML，磁性性質在這個範圍中成穩定無變化狀態，第五區為 5ML<t_[Cu]<7.1ML，可以發現矯頑力又繼續變大，且偏耦合場的值由小漸漸變大，可發現其偏耦合場的週期震盪可能有4-5ML 的長距離。To realize the relation of quantum interference in the space layers between vacuum and Co/FeMn bilayers, it was observed by magneto-optic kerr effect (MOKE). FeMn alloy was first deposited on Cu(001) to serve as the antiferromagnetic substrate. Co layers were used as the ferromagnet to study applications of exchange bias. Cu wedge was deposited on the top of Co/FeMn/Cu(001) by moving the substrate behind a knife-edge shutter. The sample was prepared by molecular beam evaporation (MBE) method, deposited on Cu(001). Composition and film thickness were calibrated by oscillations of the medium energy electron diffraction (MEED), and cross checked by Auger electron spectroscopy (AES). All grown modes were accompanied by clear intensity oscillations in medium energy electron diffraction , indicating Frank der Merve growth in a layer by layer fashion. On the other hand, low energy electron diffraction (LEED) and I/V LEED curves were used to measure film structure. The focus laser was used to measure magneto-optics Kerr effect (MOKE), which gives us excellent resolution of the thickness dependence. We presented a systematic study of the exchange bias and coercivity as a function of the Cu layers thickness in the Cu(wedge)/Co/FeMn/Cu(001). The coercive field displayed five times than original magnitude, when Cu thickness overcomes approximate 3ML. When Cu thickness over 3ML, coercive field stayed about 280 Oe. Until Cu thickness over 5ML, coercive field increased continuously. In the wide range, the oscillation of exchange bias in Cu/Co/FeMn/Cu(001) was 4-5ML.1.簡介---1 2.基礎概念---2 2.1量子井---2 2.2磁性性質---4 2.2.1磁滯曲線---4 2.2.2居禮溫度---5 2.2.3尼爾溫度---6 2.3交換偏耦合---6 2.3.1磁滯偏移---6 2.3.2凍結溫度---8 2.3.3直覺理論模型---8 2.3.4反磁層磁區理論---10 2.3.5隨機場理論---11 2.3.6微磁性計算理論---12 3實驗方法與原理---13 3.1超高真空腔---13 3.2實驗流程---15 3.3奈米尺度下的超薄膜---15 3.3.1電子蒸鍍鎗---15 3.3.2薄膜生長模式---15 3.3.3階梯式薄膜---20 3.4薄膜結構---20 3.4.1平面結構---20 3.4.2垂直結構---21 3.5中能量電子繞射---23 3.6歐傑電子能譜---24 3.6.1原理---24 3.6.2應用---27 3.6.3合金比例校正與厚度校正---31 3.7磁性性質---31 3.8雷射聚焦---36 3.8.1原理---36 3.8.2儀器---36 4結果與討論---38 4.1合金比例校正與厚度校正---38 4.2聚焦雷射實驗結果---39 4.3磁性超薄膜生長---40 4.3.1不同生長溫度對鐵錳層影響---43 4.3.2不同生長溫度下對鈷的影響---43 4.4鈷與鐵錳所產生磁性---45 4.5階梯式薄膜---46 4.6交換偏耦合在不同厚度的銅影響---47 4.6.1磁易軸---47 4.6.2Training effect---52 4.6.3外加場的大小對磁性影響---53 4.7階梯式銅對交換偏耦合系統的一系列變化---55 4.7.1無銅對磁性的影響---55 4.7.2低厚度的銅對磁性的影響---55 4.7.3中厚度的銅對磁性的影響---58 4.7.4次高厚度的銅對磁性的影響---59 4.7.5高厚度的銅對磁性的影響---59 5結論---6