Thin film microscopy of lead on germanium(111) phase transitions and iron(x)/nickel(1-x) on copper(111) magnetic surface alloy.

摘要

The manner in which one phase develops is a manifestation of its physical properties at a particular coverage in the phase diagram. Thus, studying the growth of phases and phase transitions that occur between them can provide a better understanding of the surface. In the first part of this dissertation, a low energy electron microscope (LEEM) was used to investigate the growth, the phase diagram, and a novel mechanism through which the order-disorder phase transition can happen for the Pb/Ge(111).;LEEM data indicate that when Pb is initially deposited on the c(2x8) reconstructed Ge(111) surface, Pb atoms replace the Ge adatoms. Then the dilute 3x3 R30° structure (alpha phase) starts to nucleate over the surface and coalesces at 0.33ML. Above 0.33ML, the alpha phase transforms to the (1x1) phase in a step flow manner until the surface saturates. The (1x1) phase is a disordered phase and continues to accommodate more Pb atoms until it is dense enough to transform into the dense 3x3 R30° structure (beta phase) with a saturation coverage of 1.33ML.;The transition of the beta phase into the (1x1) phase at high temperature has remained controversial despite numerous studies. Within the coexistence region of the two phases, I have discovered a novel phase separation mechanism involving the phase fluctuations of small domains and that a subtle difference in the Pb atomic density influences the transformation significantly. This striking mechanism occurs because nanoscale-size domains can have density fluctuations comparable to the density difference between the two phases. The change in the transformation behavior with a change in the coverage is attributed to the change in the vacancy concentration and its effect on the occurrence of the phase fluctuation.;In the second part of the dissertation, using a photoemission electron microscope (PEEM), the dependence of the magnetic domains of a thin ferromagnetic alloy film on the film thickness and alloy composition was investigated for thin FexNi(1-x) alloy films grown on Cu(111). PEEM's element specificity and its sensitivity to magnetic ordering on a surface make it an ideal technique to study this system.