Strictions and new stripe phases of epitaxial erbium (0001) on sapphire (112̄0)

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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2012

Author dc:creator

• Durfee, Curtis Scott

Contributors dc:contributor

• Flynn, C.P.

• erbium thin films
• elastic energy
• epitaxial misfit
• magnetostatic energy

Statement dc:rights

• ©2001 Durfee

Language dc:language

en

Identifier

4552749

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/31314