Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO3) films. Electric field-dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.

A new orthorhombic phase of the multiferroic BiFeO3 has been created via strain engineering by growing it on a NdScO(3)(110)(o) substrate. The tensile-strained orthorhombic BiFeO3 phase is ferroelectric and antiferromagnetic at room temperature. A combination of nonlinear optical second harmonic generation and piezoresponse force microscopy revealed that the ferroelectric polarization in the orthorhombic phase is along the in-plane {110}(pc) directions. In addition, the corresponding rotation of the antiferromagnetic axis in this new phase was observed using x-ray linear dichroism.

First-principles elastic constants c ij 's of BiFeO 3 with cubic nonmagnetic ͑NM͒/ferromagnetic ͑FM͒ structures and rhombohedral antiferromagnetic ͑AFM͒ structure have been calculated within the generalized gradient approximation ͑GGA͒ and the GGA+ U approach. In addition, the elastic properties of polycrystalline aggregates including bulk modulus and shear modulus are also determined and compared with experiments. It is found that the predicted c ij 's decrease with increasing volume ͑or decreasing pressure͒ except for the c 14 of the rhombohedral AFM phase. The cubic NM and FM phases are predicted to be harder than the rhombohedral AFM one, indicated by their smaller equilibrium volumes and larger bulk moduli. Additionally, the cubic FM phase is found nearly isotropy ͑by GGA and GGA+ U with U eff =6 eV͒, and the cubic NM phase is mechanical unstable at high temperatures. The presently predicted c ij 's of BiFeO 3 provide helpful guidance for future measurements, and make the stress estimation and elastic energy calculation in BiFeO 3 thin films possible.

The coarsening kinetics of a two-phase mixture with a large diffusional mobility disparity between the two phases is studied using a variable-mobility Cahn-Hilliard equation. The semi-implicit spectral numerical technique was employed, and a number of interpolation functions are considered for describing the change in diffusion mobility across the interface boundary from one phase to another. The coarsening rate of domain size was measured using both structure and pair correlation functions as well as the direct computation of particle sizes in real space for the case that the coarsening phase consists of dispersed particles. We discovered that the average size (R) versus time (t) follows the R 10/3 ∝t law, in contrast to the conventional LSW theory, R 3 ∝ t, and the interface-diffusion dominated two-phase coarsening, R 4 ∝ t.

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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.

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