Guang Sheng

Phase-field simulation of strain-induced domain switching in magnetic thin films

The strain-induced magnetic domain switching in epitaxial CoFe2O4 (CFO) thin films was studied using phase-field method. In particular, we investigated the domain switching from an initial in-plane direction to out-of-plane under the action of in-plane elastic strains. An abrupt switching feature is observed for a single-domain film while the switching of a multidomain CFO thin film is gradual. Typical magnetic domain structures as a result of the biaxial isotropic in-plane strains are presented.

Misfit strain–misfit strain diagram of epitaxial BaTiO3 thin films: Thermodynamic calculations and phase-field simulations

The effect of anisotropic strains on the phase transitions and domains structures of BaTi03 thin films was studied using both thermodynamic calculations and phase-field simulations. The misfit strain -misfit strain domain stability diagrams, i.e. the graphical representations of stable ferroelectric phases and domain structures as a function of strains, were predicted. The similarity and significant differences between the diagrams from thermodynamic calculations assuming single domains and from phase-field simulations were analyzed. Typical domain structures as a result of anisotropic misfit strains are presented.

Piezoelectric response of single-crystal PbZr1−xTixO3 near morphotropic phase boundary predicted by phase-field simulation

The piezoelectric property of hypothetic single-crystal PbZr1−xTixO3 (PZT) is studied using phase-field simulations. The dependence of piezoelectric coefficient (d33) on PbTiO3 compositions (x) near the morphotropic phase boundary of PZT was obtained. Using the existing thermodynamic description of PZT, it is shown that d33 reaches a peak value of 720 pC/N at x=0.47 which is more than three times of that for the ceramic counterpart. The relation between the domain structure of the poled PZT single crystals and the enhancement of d33 near the MPB composition is discussed.

Domain stability of PbTiO3 thin films under anisotropic misfit strains: Phase-field simulations

The domain stability and domain structures of (001)-oriented PbTiO3 ferroelectric thin films subject to anisotropic in-plane strains were studied using phase-field method. Based on the simulation results, a room temperature domain/phase stability diagram was constructed for PbTiO3 thin films with the in-plane strains ranging from −5% to 5%. The predicted diagram is both quantitatively and qualitatively different from those obtained using thermodynamic calculations based on a single-domain assumption.

A modified Landau-Devonshire thermodynamic potential for strontium titanate

The range of reported values of the Landau energy coefficients of bulk SrTiO3 leads to uncertainty in not only the magnitude but also the direction of the calculated spontaneous polarization in SrTiO3 thin films in a state of biaxial tension. In this study, we use experimental results from strained SrTiO3 films together with phase-field simulations to refine the values of the Landau energy coefficients and report a modified thermodynamic potential for bulk strontium titanate. The transition temperatures and ferroelectric/antiferrodistortive domain stabilities predicted from this modified potential agree well with measurements on biaxially strained SrTiO3 thin films.

Phase-field simulation of electric-field-induced in-plane magnetic domain switching in magnetic/ferroelectric layered heterostructures

The electric-field-induced in-plane magnetic domain switching in magnetic/ferroelectric (FE) layered heterostructures was studied using phase-field simulations. In particular, we chose the CoFe2O4 (CFO) magnetic film and the Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) FE layer as a representative example due to their strong respective magnetoelastic and piezoelectric couplings. In-plane 90° magnetic domain switching in the CFO film was observed when a transverse electric field was applied to the PZN-PT layer. The detailed switching behaviors as well as the corresponding magnetic domain structures are presented for CFO films with different geometric sizes and initial magnetization configurations. The effect of a dynamic electric field on the switching process, i.e., a time-dependent electric-field-induced magnetic domain switching, is also discussed.

Large electric field induced strains in ferroelectric islands

An effective mechanism that generates large recoverable electric field induced strains in ferroelectric islands is studied by phase-field modeling. The large strains originate from the reversible 90° domain switching between a1 (a2) domains and c domains, driven by an applied electric field and an internal stress field. The electric field induced strains could be effectively controlled by the magnitude of island-substrate misfit and the aspect ratio of islands.

Phase transitions and domain stabilities in biaxially strained (001) SrTiO3 epitaxial thin films

We applied phase-field approach to investigate both ferroelectric and antiferrodistortive transitions in (001) SrTiO3 epitaxial thin films that are strained biaxially. A domain/phase stability diagram of “misfit strain-temperature” was constructed for equibiaxially strained (001) SrTiO3 thin films, which exhibits significant differences from previous diagrams obtained using thermodynamic analysis of a single domain. For unequibiaxially strained (001) SrTiO3 thin films, “misfit strain-misfit strain” domain stability diagrams at several representative temperatures were obtained. The predicted phase transitions, domain stabilities, and domain structures in three different SrTiO3 thin films under either equibiaxial or unequibiaxial strains agree well with experimental observations.