PHASE-FIELD SIMULATIONS OF MICORSTRUCTURES INVOLVING LONG-RANGE ELASTIC, MAGNETOSTATIC AND ELECTROSTATIC INTERACTIONS
Open Access
- Author:
- Zhang, Jingxian
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 21, 2007
- Committee Members:
- Long Qing Chen, Committee Chair/Co-Chair
Zi Kui Liu, Committee Chair/Co-Chair
Darrell G Scholm, Committee Member
Qiang Du, Committee Member - Keywords:
- multiferroic
ferromagnetic
ferroelectric
phase-field - Abstract:
- The phase-field approach is one of the most powerful methods for modeling microstructure evolution processes. In this thesis, phase-field models were developed to study the domain structures and domain structure evolution in ferroelectric materials, ferromagnetic materials (giant magnetostrictive materials), and ferroelectric-ferromagnetic composite materials, i.e. the materials involving long-range elastic, magnetostatic and electrostatic interactions, which provide us a powerful tool for understanding the properties and behaviors of these materials. The effects of substrate constraint for the epitaxial BiFeO3 thin films were systemically studied by using thermodynamic calculations and phase-field simulations. It was demonstrated that the spontaneous polarizations, domain structures, and domain wall stabilities of BiFeO3 thin films could be effectively controlled by choosing the substrates with appropriate lattice parameters and orientations. These results provide guidance to modify ferroelectric properties of BiFeO3 by heteroepitaxy and strain engineering experimentally. It was also found that the domain structures of ferroelectric islands could be significantly different from those of continuous ferroelectric films due to the relief of substrate constraint. The dependence of the stress distribution on the aspect ratio of the islands gives us a new tool to control the domain structures and piezoelectric properties of the ferroelectrics. In this thesis, phase-field simulations were conducted to clarify the role of magnetostatic energy and elastic energy in determining the domain structures of giant magnetostrictive materials. It was also demonstrated that a compressive pre-stress could efficiently increase the overall magnetostrictive effect. The results agreed well with existing experiment measurements and observations. We also studied the ferroelectric-ferromagnetic nanocomposite films by using the phase-field model. It was shown that the magnetoelectric coupling effect was highly dependent of the thickness and morphology of the nanocomposite, and substrate constraint.