以電化學方法製備之Ni/Cu(100)薄膜的磁性研究

Abstract

本實驗利用電化學電鍍方式在單晶銅(100)電極上成長鎳薄膜，同時使用循環伏安法(Cyclic Voltammetry)、電化學磁光柯爾效應系統(EC-MOKE)、電化學掃描式電子穿隧顯微鏡(EC-STM)來研究單晶銅(100)上所成長鎳薄膜的表面特性與磁特性。 實驗使用銀當作電化學參考電極，此電極屬於pseudo-reference electrode，其電位利用能士特方程式計算與文獻參考比較結果，與標準氫電極電位差大約是+87~130 mV 之間。經過多次實驗測試，在本實驗系統中數據呈現高再現性。以循環伏安法檢測，單晶銅(100)電極在1 mM HCl電解液中電化學過程，發現電流成對峰值：銅溶解與銅沉積，往陰極方向加大範圍掃描，-800 mV(vs Ag)開始有氫氣產生反應(質子還原：H++e-→1/2H2)出現。加入鎳的電解液1 mM HCl+1 mM NiCl2則出現另外一個成對峰值分別在-1200 mV與-400 mV，實驗數據顯現此對峰之間相關性甚大，推測是鎳的吸附(Ni2++2e-→Ni)與退吸附(Ni→Ni2++2e-)反應。選擇在-1200 mV電位下電鍍鎳，控制電鍍時間以製造不同鎳膜厚度，透過積分CV圖的鎳退吸附峰算出電荷量和已知電鍍面積(0.292 cm2)可分析沉積鎳的膜厚。電解液裡的氯離子會修飾銅(100)電極表面，透過STM掃描圖像可以觀察到銅(100)表面直角台階的特徵。 電鍍鎳/銅(100)磁性行為主要分成四部分結論：(1)在1.52 ML以下沒有磁性原因是電鍍鎳量很少又加上氫氣產生的效應。 (2)在2.47~7.05 ML認為是磁異向能的轉換，易軸變成Polar方向，表示有垂直磁異向能出現，與UHV系統有相同的現象發生。 (3)在13.4 ML~29.0 ML之間的磁化易軸變成平行樣品表面，趨向塊材現象以形狀異向性為主要因素。隨著厚度增加殘磁逐漸變大，因此越厚的鎳層需要更大能量才能磁化。 (4)鎳退吸附後In-situ量測L-MOKE還有磁性的現象是在13.4 ML以上才有，推測是電鍍厚度越厚，水溶液離子數變少，使得水溶液導電度不夠無法將鎳退吸附掉。The electrodeposition of Ni on a Cu(100) electrode in diluted hydrochloric acid was investigated by means of cyclic voltammetry (CV), electrochemical scanning tunnelling microscopy (EC-STM) and electrochemical magneto-optic Kerr effect (EC-MOKE). Silver was used as the electrochemical reference electrode which belongs to one kind of pseudo-reference electrodes. With Ag reference electrode, the potential converted to normal hydrogen electrode (NHE) is about +80~130 mV. In pure supporting electrolyte (1 mM HCl), the typical current peak pair in the CV measurements corresponds to copper dissolution and re-deposition. Hydrogen evolution reaction (HER) starts at E=-800 mV (vs Ag). STM images show well right-angle steps for the adsorption of chloride anions from hydrochloric acid solutions. After adding 1 mM NiCl2, new peak pair appears at -1200 mV and -400 mV which correspond to nickel deposition and dissolution. Systematic investigation for different nickel thickness has been performed by depositing at -1200 mV for different period. The thickness has been calculated by integrating the area under the anodic dissolution peak which gives the total amount of charge passed on a surface area of the deposit. Magnetic properties of Ni/Cu(100) was observed at the potential smaller than -700 mV. As the Ni thickness increases, it can be distributed three regions. (1) Under 1.52 ML: No magnetic hysteresis appears. This is due to too little Ni deposit and hydrogen evolution reaction. (2) 2.47~7.05 ML: Magnetic easy axis is out-of-plane. Perpendicular magnetic anisotropy appears. This is comparable with UHV system. (3) 13.4~29.0 ML: Magnetic easy axis changes to be in-plane due to the shape anisotropy. After sweeping to the potential for nickel dissolution, in-situ L-MOKE measurement shows that the magnetization remains for films thicker than 13.4 ML. This may be due to the lowered conductivity of the electrolyte.