| Summary: | 博士 === 國立臺灣師範大學 === 物理學系 === 96 === Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and surface magneto-optical Kerr effect (SMOKE) are used to investigate the growth mode, alloy formation, and magnetic properties of ultrathin Fe films deposited on Pt(111) surface. The correlation between structural and magnetic properties is explored for Fe films grown at two different temperatures, 180 K (low temperature: LT) and 300 K (room temperature: RT). The growth mode of Fe films on Pt(111) at RT is at least 3 ML layer-by-layer growth before three-dimensional (3-D) island growth begins. When the thickness of Fe films (dFe) is 1 ML, no Kerr signal is observed at RT. Both the polar and longitudinal Kerr signals can be detected when dFe is 1.5 ML. When dFe ≥ 2 ML, only the longitudinal Kerr signal can be observed, the easy axis of the magnetization is in the in-plane direction. It is corresponded with a fcc to bcc structural transformation at dFe = 2 ML. For LT-growth, the growth mode of Fe deposited on Pt(111) is 3-D island growth. A polar Kerr signal can be detected when dFe is 0.75 ML then the easy axis of the LT-grown films changes from out-of-plane to in-plane at 1.3 ML Fe. The results show that the Fe films on Pt(111) has a structural change, starting from a critical thickness, which depends on the deposition temperature. The critical thickness is estimated as 1.3 ML and 2 ML for Fe films deposited at LT and RT, respectively.
The formation of Fe–Pt surface alloy for 1.2 ML and 5 ML ultrathin Fe films on a flat Pt(111) surface after high temperature annealing, the longitudinal Kerr signals enhance to 160% and 150%, in the meantime the coercivities also enhance to 30% and 1100%, respectively. A polar Kerr hysteresis loop can be observed for 1.2 ML Fe/Pt(111) when the annealing temperature is between 600 K and 700 K. The formation of Fe–Pt surface alloy is confirmed by AES and LEED. The evolutions of the magnetic properties are strongly related to the diffusion process of Fe thin films into Pt substrate.
The spin-reorientation transition (SRT) is observed when Ag overlayers deposited on Fe/Pt(111) by magneto-optical Kerr effect measurements. The polar Kerr intensity and its coercivity as a function of Ag coverage are investigated through SRT. The easy axis of the magnetization changes completely from the in-plane to out-of-plane direction after 1 ML of Ag coverage. Both the polar Kerr intensity and perpendicular coercivity have their maximal value at dAg = 1 ML. The easy axis of magnetization can be reversed back to the in-plane direction after the Ag overlayers are removed by sputtering. The chemical compositions of the interfaces are measured by Auger electron spectroscopy. The hybridization of electron states at the Ag/Fe interface causing the change in interface magnetic anisotropy may be the mechanisms of the spin-reorientation transition induced by Ag.
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