Strictions and new stripe phases of epitaxial erbium (0001) on sapphire (112̄0)
Durfee, Curtis Scott
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https://hdl.handle.net/2142/31314
Description
Title
Strictions and new stripe phases of epitaxial erbium (0001) on sapphire (112̄0)
Author(s)
Durfee, Curtis Scott
Issue Date
2001
Doctoral Committee Chair(s)
Flynn, C.P.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
erbium thin films
elastic energy
epitaxial misfit
magnetostatic energy
Language
en
Abstract
Erbium thin films provide a valuable model system with which to explore two phenomena
that have made the magnetic phase diagram of thin films hard to predict. These are the
elastic energy of epitaxial misfit and the magneto static energy of the equilibrium
configuration. In both cases, Er is particularly well suited to the fundamental investigation of
equilibrium magnetic phases. First, Er exhibits large strictions of~ 0.3% at its bulk
transition from the antiferromagnetic (AF) to the ferromagnetic (FM) phase. The elastic free
energy created by the substrate-induced strains is almost sufficient to suppress
ferromagnetism of the film entirely. Second, erbium can be grown epitaxially on its basal
plane. In this orientation, the magnetization is aligned perpendicular to the film, and greatly
enhances the magnetostatic energy. This is also sufficiently large to overcome the free
energy difference between the bulk FM and AF phases. In this thesis, thin epitaxial films of
Er were synthesized and their magnetic phase diagrams explored by magnetic and x-ray
measurements. The research reveals new fundamental behavior of magnetic thin films,
including the discovery of two new magnetic stripe phases.
The newly observed stripe phases separate the AF and FM phases in the phase
diagrams of the films and significantly reduce the large demagnetizing energy from that of
the uniform FM film. The low-temperature stripe phase is comprised of periodic FM
domains that alternate in direction along the hcp c axis. The high-temperature stripe phase is
formed by alternating AF and FM domains. These domains adjust their relative widths in an
increasing field in such a way that the net internal field is nearly zero throughout the
magnetization process. Thus, the demagnetizing energy nearly vanishes. The stripe phases
then mediate the AF to FM transition that occurs directly in the bulk.
The experimentally-determined phase diagrams were predicted from the Gibbs free
energies ofthe phases. The phase diagrams ofthe 1900 A and 9500 A films have distinct
111
differences. The experimental data points lie close to the predicted phase boundaries for all
the films.
Metastable phases were observed both above and below Tc. The transitions to the
stable phases, which occurred by nucleation, were accompanied by hysteresis.
Thermodynamics also allowed an explanation of the observed anelastic processes by
including the energy contributions of the dislocations.
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