| Summary: | 博士 === 國立臺灣師範大學 === 物理學系 === 94 === 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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