Novel magnetic behavior in heavy rare earth superlattices
Tsui, Tien Fang
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https://hdl.handle.net/2142/19816
Description
Title
Novel magnetic behavior in heavy rare earth superlattices
Author(s)
Tsui, Tien Fang
Issue Date
1992
Doctoral Committee Chair(s)
Flynn, C.P.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
Structural and magnetic properties of heavy rare earth films and superlattices synthesized by molecular beam epitaxy are investigated. Epitaxial films of rare earth metals, and superlattices of rare earths with Y interlayers, are grown along different hcp orientations using novel techniques developed specifically for this thesis research. Interesting growth patterns and surface reconstructions are observed, particularly on the a and b faces of the hcp material. Magnetic behavior is explored by neutron diffraction and SQUID magnetometry experiments.
The spatial form of the exchange interaction is explored by probing the dependence of magnetic coherence on crystal growth orientation. The results reveal that Dy/Y and Gd/Y superlattices grown along the a and b axis exhibit weak magnetic coupling between successive magnetic layers, even for nonmagnetic spacers as thin as $\sim$20 A. Previous studies of the c-axis materials established that long range magnetic order occurs even through Y spacer layers as thick as $\sim$120 A. This measured anisotropy between behavior along the c-axis and perpendicular to it, is over a factor ten in both the coupling strength and the range. The strongly anisotropic coupling behavior is shown to have its origin in nearly 2-dimensional nesting features of the rare earth Fermi surfaces. Nesting along the c-axis gives rise to long range exchange interactions along the c-axis, and the lack of nesting perpendicular to the c-axis leaves a rapid exponential decay of the exchange with spin separation.
Magnetic phases of epitaxial Dy films are examined as a function of epitaxial strain, growth orientation, temperature and field. The analysis of the observed anisotropic magnetoelastic behavior reveals the detailed energy balance in the Dy system.
The observed spin configurations in thin Gd films show that the magnetocrystalline anisotropy of bulk Gd is substantially modified in the films. This in turn gives insight into the origin of the unusual spin reorientation transition in Gd.
Magnetic order in buried monolayers of Gd-Y alloys is studied in order to explore the effects of both the reduced physical dimensions and the modified effective exchange interaction on phase transitions. No long range order occurs in the monolayer system even for nearly full Gd monolayer coverage. A spin-glass transition is observed instead. The zero field susceptibility exhibits a divergence at low temperatures. The analysis of the magnetic states reveals unusual scaling behavior and a dimensionality cross-over in the monolayer system.
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