X-ray photoelectron diffraction studies of structural and magnetic disordering transitions near surfaces

Abstract

This thesis deals with order/disorder transitions near solid surfaces as studied by x-ray photoelectron diffraction and photoelectron holography. Transitions involving both atomic positional order and magnetic order have been studied. Further evidence for a reversible high-temperature surface-disordering phase transition on Ge(111) has been found using Ge 3p x-ray photoelectron diffraction (a short-range- order probe of surface structure that is sensitive to the first 3-5 spheres of neighbors) and photoelectron holography (a recently suggested direct 3-d imaging technique). Azimuthal diffraction data at takeoff angles with respect to the surface of ϴ=19° (including nearest-neighbor forward scattering directions and yielding high surface sensitivity) and ϴ=55° (for which second-nearest-neighbor forward scattering directions and more bulk sensitivity are involved) show abrupt drops in intensity of ~30%-40% over the temperature interval of 900-1200 K. Photoelectron holographic near-neighbor images at temperatures both below and above the transition region furthermore indicate an identical near-neighbor structure for all atoms present in ordered sites. These combined diffraction and holography data show that by 1200 K, the Ge(111) surface is covered by a completely disordered overlayer of about 2 Ge monolayers in thickness. The rate of growth of this overlayer with increasing temperature is also in excellent agreement with recent medium-energy ion scattering results, although the thickness we find for the overlayer is 1.5-2.0x larger than that derived from ion scattering. Based on these data, a disordering model for the Ge(111) surface phase transition occurring at 1050 K is discussed. Spin-polarized photoelectron diffraction is a recently developed and promising application of photoelectron diffraction to the study of the magnetic structure near surfaces. This technique is based on an internal source of spin-polarized electrons as produced in core-level multiplet splittings and it is thus sensitive to the short-range magnetic order around a given type of emitter in the crystal. In prior studies, it has been applied to two antiferromagnets, KMnF3 and MnO, and the effects seen at temperatures well above the Neel (or long-range-order) temperature have been attributed to an abrupt loss of short-range order. In this thesis, data on MnO have been extended. to include the use of synchrotron radiation at photon energies of 150, 170, and 190 eV. Attempts to measure similar effects on two ferromagnets, Fe and Gd, were also made; these yielded preliminary results that should be useful for future studies.