鎳/鈷/鉑(111)及鈷/鎳/鉑(111)系統其結構與磁性性質之研究 Comparative studies in structural and magnetic properties between Ni/Co/Pt(111) and Co/Ni/Pt(111)

Abstract
本研究論文主要是利用歐傑電子能譜儀(Auger electron spectroscopy; AES),低能量電子繞射儀(low-energy electron diffraction; LEED),紫外光能譜術(ultra-violet photoemission spectroscopy; UPS)、以及磁光柯爾效應儀(magneto-optical Kerr effect; MOKE) 來研究Ni/Co/Pt(111) 及Co/Ni/Pt(111) 鏡像系統其成長模式、合金形成及表面磁性的關係。 根據LEED(0,0)光束強度及AES訊號強度隨蒸鍍時間變化的關係,我們得知在室溫條件下,無論是Ni超薄膜在1 ML Co/Pt(111) 上成長(ML: monolayer),或者是Co超薄膜在1 ML Ni/Pt(111)上成長時,都會先形成2層的層狀成長之後才開始3維的島狀成長。對此二系統而言,其升溫形成合金的過程都可被分成2階段,首先是升溫過程中,Co和Ni會先混合,然後Ni-Co混合層在更高溫時會擴散進入Pt基底,形成Ni-Co-Pt合金。其中,1-3 ML Ni/1 ML Co/Pt(111)系統開始產生Ni與Co混合的溫度皆為420 K,此溫度與Ni覆蓋層的厚度無關;然而對1-3 ML Co/1 ML Ni/Pt(111) 系統而言,產生Ni與Co混合的溫度隨Co覆蓋層的厚度增加而升高。此二系統的Ni-Co混合層開始擴散進入Pt基底形成Ni-Co-Pt合金的溫度,皆隨著覆蓋層的厚度增加而升高。 我們同時也量測在室溫成長時,其磁性隨覆蓋層厚度變化的關係。1層至24層Ni超薄膜在1 ML Co/Pt(111) 成長時,其磁化易軸(the easy axis of the magnetization)會在垂直樣品表面的方向,具有很強的垂直磁異向性(perpendicular magnetic anisotropy; PMA);1至3層Co原子層蒸鍍在1 ML Ni/Pt(111)上,無論是垂直或者是平行樣品表面我們皆量測不到磁滯的訊號,此現象可能與Ni緩衝層阻隔了Co與Pt接觸有關。樣品經過升溫效應所產生的磁性變化其擴散過程一致。經過高溫處理過後的樣品形成了Ni-Co-Pt合金,合金的矯頑力(coercivity)大小可經由升溫時產生的合金濃度變化來控制。 根據比較1 ML Ni/1 ML Co/Pt(111)與1 ML Co/1 ML Ni/Pt(111)的實驗結果,我們發現當退火溫度(annealing temperature)介於750 K 和780 K之間時,表面合金結構會由NixCo1-xPt轉變成NixCo1-xPt3,藉由計算接近居禮溫度(Curie temperature)時的值(critical exponent),我們得知此時表面的磁性結構亦由2維磁性結構的轉變成3維磁性結構,並且,在表面合金結構由NixCo1-xPt轉變成NixCo1-xPt3之時,居禮溫度隨退火溫度升高而下降的現象變得更明顯。此外,在相同退火溫度條件下,1 ML Ni/1 ML Co/Pt(111)系統的居禮溫度一直比1 ML Co/1 ML Ni/Pt(111)系統的居禮溫度高,我們認為這種現象與Ni、Co的成分比有關。我們也經由研究2 ML Ni/1 ML Co/Pt(111)、2 ML Co/1 ML Ni/Pt(111)、12 ML Ni/1 ML Co/Pt(111)、以及24 ML Ni/1 ML Co/Pt(111)等系統來探討Ni、Co的成分比對居禮溫度的影響。 另一組鏡像系統,2 ML Ni/2 ML Co/Pt(111)和2 ML Co/2 ML Ni/Pt(111),經過退火之後,我們意外地發現樣品產生了spin reorientation transition (SRT),這種現象在以1層Co及1層Ni當作緩衝層的系統中,完全沒有被發現過。我們認為Ni、Co的成分比及其分佈的均勻度應是造成此現象的重要因素,在本論文中我們會加以討論。
The comparative studies in structural and magnetic properties between Ni/Co/Pt(111) and Co/Ni/Pt(111) were investigated by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), ultra-violet photoemission spectroscopy (UPS), and magneto-optical Kerr effect (MOKE). The oscillation of the specular beam of LEED and the Auger uptake curve were used to calibrate the thicknesses of Ni films and Co films, especially to study the growth modes at room temperature. The growth modes at room temperature of both dNi Ni/1 ML Co/Pt(111) and dCo Co/ 1 ML Ni/Pt(111) (d: thickness) are at least 2 ML in layer-by-layer growth before the 3-dimensional island growth begins. For the both systems, the Co and Ni atoms intermix to each other at low temperature annealing, after which the Co and Ni atoms diffuse into the Pt substrate together. The starting temperatures of the intermixing process for 1-3 ML Ni/1 ML Co/Pt(111) are independent on the thickness of Ni overlayer. But for the dCo Co/ 1 ML Ni/Pt(111) system, the starting temperatures of the intermixing process is thickness dependent. The starting temperatures of the Ni-Co intermixing layer diffusing into the Pt substrate for both dNi Ni/1 ML Co/Pt(111) and dCo Co/ 1 ML Ni/Pt(111) increase with the thickness of the overlayers. The polar and longitudinal hysteresis loops were detected during the initial growths. The easy axis of the magnetization of 1-24 ML Ni/1 ML Co/Pt(111) is in the out-of-plane direction. But for the dCo Co/ 1 ML Ni/Pt(111) system, no Kerr signal is observed at room temperature when the thickness of Co film is below 3 ML. The disappearance of the polar Kerr rotation for dCo< 3 ML in Co/1 ML Ni/Pt(111) system at room temperature may due to the Ni buffer layer preventing the Co atoms in contact with Pt substrate. The evolution of the magnetization versus annealing temperature for dCo Co/ 1 ML Ni/Pt(111) was consistent with diffusion process to form Ni-Co-Pt surface alloy after annealing at high temperatures. The coercivity of the Ni-Co-Pt system can be adjusted by changing the annealing temperature, due to the variety in concentrations of the alloy formation. The comparative study in structural properties for the mirror systems, 1 ML Ni/1 ML Co/Pt(111) and 1 ML Co/1 ML Ni/Pt(111), reveal an information of a structural phase transition from NixCo1-xPt to NixCo1-xPt3 when the annealing temperature is between 750 K and 780 K, while the value of critical exponent  near the Curie point exists a crossover from a 2D-like magnetic phase to a 3D-like one. The Curie temperature depresses rapidly when the subsurface structure changes from NixCo1-xPt to NixCo1-xPt3. The Curie temperature of 1 ML Ni/1 ML Co/Pt(111) are always higher than that of 1 ML Co/1 ML Ni/Pt(111). We found that this phenomenon is corresponding to the ratio of Ni% to Co% in the subsurface region. The influence of the concentration ratio in Curie temperature is also confirmed by the studies of 2 ML Ni/1 ML Co/Pt(111), 2 ML Co/1 ML Ni/Pt(111), 12 ML Ni/1 ML Co/Pt(111), and 24 ML Ni/1 ML Co/Pt(111). Another mirror systems, 2 ML Ni/2 ML Co/Pt(111) and 2 ML Co/2 ML Ni/Pt(111), were performed to compare with the mirror system, 1 ML Ni/1 ML Co/Pt(111) and 1 M L Co/1 ML Ni/Pt(111). A spin reorientation transition (SRT) occurred after high-temperature annealing. It is interesting that no SRT was observed in the systems with one-ML buffer layer. The temperature dependence of the ratio of Ni% to Co% for 2 ML Ni/2 ML Co/Pt(111) and 2 ML Co/2 ML Ni/Pt(111) causes the SRT is discussed.
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Keywords
歐傑電子能譜儀, 低能量電子繞射儀, 紫外光能譜術, 磁光柯爾效應儀, , , 鉑(111), 居禮溫度, 垂直磁異向性, 鐵磁性合金, 矯頑力, Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), ultra-violet photoemission spectroscopy (UPS), magneto-optical Kerr effect (MOKE), Cobalt, Co, Nickel, Ni, Platinum, Pt(111), Curie temperature, perpendicular magnetic anisotropy, PMA, ferromagnetic alloy, coercivity
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