Growth of multiferroic oxides by molecular-beam epitaxy.

摘要

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 o rocking curve full width at half maximum values as narrow as 11 arc sec (0.003"a) 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+/-0.1 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 +/- 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°). As the first-principles calculations predict, the strained EuTiO 3 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) DyScO 3 is ferromagnetic with TC = 4.6 K.