GROWTH OF MULTIFERROIC OXIDES BY MOLECULAR-BEAM EPITAXY

Open Access
Author:
Lee, June Hyuk
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 03, 2011
Committee Members:
  • L Q Chen, Dissertation Advisor
  • Long Qing Chen, Committee Chair
  • Venkatraman Gopalan, Committee Member
  • Zi Kui Liu, Committee Member
  • Peter E Schiffer, Committee Member
Keywords:
  • Molecular-beam epitaxy
  • Multiferroics
  • Ferroelectrics
  • Ferromagnet
Abstract:
Multiferroic materials, which simultaneously exhibit ferroelectricity and ferromagnetism, have attracted a great deal of attention due to their physical origin and potential application in novel electronic devices. In this dissertation, the synthesis and properties of multiferroic thin films grown by molecular-beam epitaxy have been studied to understand the nature of multiferroism and therefore to explore a new route to creating strong multiferroic materials. Epitaxial thin films of BiMnO3 have been grown by a reactive molecular-beam epitaxy. The pressure-temperature region for the adsorption-controlled growth region was calculated and experimentally established by reflection high energy electron diffraction and x-ray diffraction. Under the optimal growth region, phase-pure and epitaxial BiMnO3 films with ƒç rocking curve full width at half maximum values as narrow as 11 arc sec (0.003¢X) were synthesized. The structural and magnetic properties of stoichiometric films are found to depend on the oxygen activity used during growth. Optical absorption measurements reveal that BiMnO3 has a direct band gap of 1.1„b0.1 eV. Phase-pure, stoichiometric, unstrained, and epitaxial (001)-oriented EuTiO3 thin films have been grown on (001) SrTiO3 substrates by a reactive molecular-beam epitaxy. Magnetization measurements show antiferromagnetic behavior with TN = 5.5 K. Spectroscopic ellipsometry measurements reveal that unstrained EuTiO3 thin films have a direct optical band gap of 0.93 „b 0.07 eV. Strained, epitaxial (001)-oriented EuTiO3 films have been grown on (001) LSAT and (110) DyScO3 substrates in a reactive molecular-beam epitaxy to induce 0.9% and +1.1% of biaxial strain, respectively. A full width at half maximum of a rocking curve is as narrow as 8 arc sec (0.002¢X). As the first-principles calculations predict, the strained EuTiO3 film on (110) DyScO3 exhibits simultaneous ferroelectricity and ferromagnetism. Optical second harmonic generation reveals that this film experiences a phase transition at ~250 K to polar point group in agreement with theory, and in this state domain switching by electric fields is observed. Magneto-optic Kerr effect and superconducting quantum interference device measurements show that the strained EuTiO3 film on (110) DyScO3 is ferromagnetic with TC = 4.6 K.