| Summary: | 博士 === 國立清華大學 === 物理系 === 102 === Ultrathin metal films supported on metal substrates have extraordinary physical and chemical properties useful in applications such as material science, catalysis, and
magnetic technology. These properties count on factors such as the inter-diffusion between overlayer-substrate, the overlayer growth mode, and the substrate morphology. In recent years, some metal overlayers (Pd, Pt, Au, Ir, and Rh) are found to induce some substrates such as W and Mo to facet after annealing to temperature above 700 K. Moreover, the faceting of W(111) is used to produce a thermodynamically stable pyramidal structure with a single atom at the apex, applied to generate a highly coherent electron beams. How to obtain a well-defined magnetic tip is of great interest in recent years.
Fe films grown on faceted Pd/W{112} surface is studied to assess its potential to serve as a magnetic single atom tip. Structure and magnetic properties of Fe/faceted Pd/W{112} were characterized as functions of temperature and Fe coverage. For Fe films of various thickness deposited at 105 K, spin reorientation transition (SRT)
(from perpendicular to surface to in-plane direction) were found at Fe coverage of 2 thermal desorption monolayer (TML). We found that the adsorption of hydrogen greatly enhanced the perpendicular magnetic anisotropy and thus coercivity of the Fe films, e.g. 7 times for 1.9 TML thick Fe film. As a result, the critical thickness of the Fe film at which the SRT occurs was shifted from 2 to 2.9 TML, and the temperature dependence of the remnant magnetization was significantly influenced by
a hydrogen partial pressure of 1E-7 torr. Furthermore, we found the 3-dimensional(3D) islands formed by surplus Pd had an adverse effect to the magnetization of the Fe film. When their density was too high, the Fe film lost its perpendicular magnetization, with or without hydrogen.
Morphology, element composition and magnetic property of 3-11 TML Co and Fe films on faceted Pd/W{112} surface was systematically investigated upon thermal annealing. Co films aggregated and formed 3D islands at 400-450 K, while the
{112} facet still sustained after further annealing to 900 K. Fe films aggregated and formed 3D islands at a lower temperature, 300-350 K, and there was no observable
faceted structure after further annealing to 900 K. The magnetic coercivity (HC) of as grown Co and Fe films increased and decreased respectively with the increasing
thickness from 3 to 11 TML. The HC of Co films was significantly enhanced by 2-4 times after annealing to 400-450 K, but the HC of Fe films sustained invariant
upon thermal annealing. These comparative studies on n TML Co and Fe films on faceted Pd/W{112} surface clearly revealed the remarkable difference between the two systems and provides valuable information for future applications.
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