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https://hdl.handle.net/2142/25209
Description
Title
Grazing angle neutron diffraction
Author(s)
Ankner, John Francis
Issue Date
1990
Doctoral Committee Chair(s)
Zabel, Hartmut
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
interface-sensitive diffraction
low-energy electron diffraction
atom diffraction
x-ray diffraction
neutron diffraction
neutron dynamical grazing-angle diffraction
Language
en
Abstract
We describe the conceptual development and practical implementation of grazing-angle neutron diffraction as a new method of non-destructive surface and interface analysis. Diffraction in the grazing-angle geometry with x rays and neutrons possesses two unique advantages over specifically surface-sensitive methods for structure determination. First, the interaction of these probe waves with matter is weak, so that the scattering may be described within the first Born approximation. Secondly, one can vary the depth probed to observe structure at different levels within the sample. Grazing-angle x-ray diffraction has emerged as an important tool in surface science over the past decade. The comparative flux disadvantage of neutron sources had prevented a similar development of neutron techniques until our experiments.
By relaxing the collimations of the incident diffraction and exit grazing angles, but keeping the incident grazing angle well defined, one can maximize the intensity on the sample yet still retain interface sensitivity. We demonstrated the feasibility of this approach by modifying a conventional neutron diffractometer and observing grazing-angle diffraction from a perfect silicon crystal. We then studied a Cr/Nb/Al$\sb2$O$\sb3$ film grown by molecular-beam epitaxy and observed grazing-angle diffracted beams from the Cr(112) and Nb(220) in-plane Bragg reflections. We measured the diffracted intensity of the 400 A niobium layer at a high enough count rate to show that much thinner layers ($<$80 A) should be observable. Finally, we have studied the magnetism of the Y-GD interface by polarized-neutron reflectometry and find that the interfacial region possesses a different ordered magnetic moment from the bulk and that both interfacial and surface ordered moments disappear at the bulk Curie temperature.
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