Xiaoyan Lu

Phenomenological theory of 1–3 type multiferroic composite thin film: thickness effect

The effect of thickness on the para–ferro-phase transition temperatures, the spontaneous polarization and magnetization and hysteresis loops of 1–3 type multiferroic composite thin films was studied in the framework of Landau phenomenological theory. We took into account the electrostrictive and magnetostrictive effects, misfit strains induced from the interfaces of ferroelectric/ferromagnetic portions and film/substrate. Butterfly loops under external fields were also simulated.

Coupling interaction in 1-3-type multiferroic composite thin films

Using the time-dependent Ginzburg-Landau equation, the coupling interaction of the ferroelectric (FE) and ferromagnetic (FM) phases in epitaxial 1-3-type multiferroic thin films was investigated considering the effect of elastic stress arising from the FE/FM and film/substrate interfaces. The result of the authors shows that the maximum polarization and magnetization appear with the FM fractions of 70% and 30%, respectively. The significant changes of the FE and FM properties are caused by the special structure in which the induced misfit strain greatly affects the anisotropy of the crystals and the properties of the materials.

Phase coexistence in ferroelectric solid solutions: Formation of monoclinic phase with enhanced piezoelectricity

Phase morphology and corresponding piezoelectricity in ferroelectric solid solutions were studied by using a phenomenological theory with the consideration of phase coexistence. Results have shown that phases with similar energy potentials can coexist, thus induce interfacial stresses which lead to the formation of adaptive monoclinic phases. A new tetragonal-like monoclinic to rhombohedral-like monoclinic phase transition was predicted in a shear stress state. Enhanced piezoelectricity can be achieved by manipulating the stress state close to a critical stress field. Phase coexistence is universal in ferroelectric solid solutions and may provide a way to optimize ultra-fine structures and proper stress states to achieve ultrahigh piezoelectricity.

Domain evolution with electric field and delineation of extrinsic contributions in (K, Na, Li)(Nb, Ta, Sb)O3 single crystal

Extrinsic contributions play an important role in the functionalities of ferroelectric materials, while domain structure evolution is crucial for understanding the extrinsic dielectric and piezoelectric responses. In this work, domain configuration changes with an electric field applied along [001] C in the tetragonal (K, Na, Li)(Nb, Sb, Ta)O3 single crystal were studied by means of polarizing light microscopy. Results show that parts of the spontaneous polarizations in the (001) C plane are switched to [001] C direction, while others still stay in the (001) C plane due to high induced internal stresses. Single domain state cannot be achieved even under a high electric field. After being poled along [001] C , the volume fraction of domains with polarzations in the (001) C plane is still about 25.2%. The extrinsic contributions to the dielectric constant are 15.7% and 27.2% under the E field of 1 kV/cm and under 2 kV/cm, respectively, estimated by the Rayleigh analysis.

Adjustable ferroelectric properties in paraelectric/ferroelectric/paraelectric trilayers

A trilayer paraelectric/ferroelectric/paraelectric system is studied within the framework of the Ginzburg–Landau–Devonshire theory in consideration of the elastic interactions between each layer. An analytic expression of the ferroelectric phase transition temperature for the ferroelectric layer is derived. The polarization, dielectric properties and the response to the external field are studied numerically. Our results show that there are two types of thickness effects on the properties of the film considering the effect of dislocation. By changing the thickness of each layer, the ferroelectric layer can be highly adjusted and may have very good potential applications such as transducers, sensors and actuators.

Critical phase transition temperatures of 1-3 type multiferroic composite thin films

The critical phase transition temperatures of the ferroelectric (FE) phase and the ferromagnetic (FM) phase in epitaxial 1–3 type multiferroic thin films were obtained based on the thermodynamic model. Analytic expressions of the para–ferro transition temperatures were derived as functions of the volume fraction of the FM phase by considering the effect of the coupled elastic stresses arising from the FE/FM and the film/substrate interfaces. Our results show that the critical temperatures are significantly affected by the induced stresses and can be controlled by adjusting the volume fractions of the different phases within the thin film.

Effect of shear stress in ferroelectric solid solutions with coexisting phases

One common feature of ferroelectric solid solutions with large piezoelectricity is the coexistence of two or more phases. Due to the strain mismatch among coexisting phases, adaptive structures near the interfaces or domain walls develop to maintain the atomic coherency. Shear stresses commonly exist, especially when the domain size is small. The effect of shear stresses on phase morphology in Pb(Zr1-xTix)O3 solid solutions with compositions within the morphotropic phase boundary region was studied within the framework of Landau phenomenological theory. Our results show that the coexisting rhombohedral (R) and tetragonal (T) phases can be modified to form stable or metastable R-like and/or T-like monoclinic phases under shear stresses. Large stresses may also induce first order or second order phase transitions.One common feature of ferroelectric solid solutions with large piezoelectricity is the coexistence of two or more phases. Due to the strain mismatch among coexisting phases, adaptive structures near the interfaces or domain walls develop to maintain the atomic coherency. Shear stresses commonly exist, especially when the domain size is small. The effect of shear stresses on phase morphology in Pb(Zr1-xTix)O3 solid solutions with compositions within the morphotropic phase boundary region was studied within the framework of Landau phenomenological theory. Our results show that the coexisting rhombohedral (R) and tetragonal (T) phases can be modified to form stable or metastable R-like and/or T-like monoclinic phases under shear stresses. Large stresses may also induce first order or second order phase transitions.

Tetragonal (K, Na)NbO3 based lead-free single crystal: Growth, full tensor properties, and their orientation dependence

A Li and Ta modified (K, Na)NbO3 lead-free single crystal with a large size (13 × 10 × 20 mm3) has been grown by using the top-seeded solution growth method. The large size allows us to carry out an extensive study on this tetragonal crystal. We have measured a complete set of elastic, dielectric, and piezoelectric constants for the [001]C poled crystal with the single domain state. The crystal exhibits high shear piezoelectricity with d15 = 518 pC/N and k15 = 0.733, showing excellent potential in shear electro-sonic energy transformation devices. It is found that the high shear piezoelectricity originates from the vicinity of orthorhombic-tetragonal phase transition, which favors polarization rotation greatly. The orientation dependence of longitudinal dielectric, piezoelectric, and elastic constants and electromechanical coupling factor in the 3-dimentional space were calculated based on the single domain dataset. We believe that this work is of great importance for both fundamental studies and device d...

Temperature dependent piezoelectric anisotropy in tetragonal 0.63Pb(Mg1/3Nb2/3)-0.37PbTiO3 single crystal

In this work, we studied the temperature-dependent effective piezoelectric coefficient d 33 * along the arbitrary direction of a tetragonal 0.63Pb(Mg1/3Nb2/3)-0.37PbTiO3 single crystal. Results show that the crystal changes from a rotator ferroelectrics with the maximum d 33 * occurring along the nonpolar direction to a extender type with maximum piezoelectricity along [001]C. Two polymorphic phase transitions, orthorhombic-tetragonal and tetragonal-cubic, greatly influence the PS dynamics, leading to the change of d 33 * anisotropy with temperature. The [011]C oriented crystals possess both improved piezoelectricity and high thermal stability, hence are the best choice for practical applications.In this work, we studied the temperature-dependent effective piezoelectric coefficient d 33 * along the arbitrary direction of a tetragonal 0.63Pb(Mg1/3Nb2/3)-0.37PbTiO3 single crystal. Results show that the crystal changes from a rotator ferroelectrics with the maximum d 33 * occurring along the nonpolar direction to a extender type with maximum piezoelectricity along [001]C. Two polymorphic phase transitions, orthorhombic-tetragonal and tetragonal-cubic, greatly influence the PS dynamics, leading to the change of d 33 * anisotropy with temperature. The [011]C oriented crystals possess both improved piezoelectricity and high thermal stability, hence are the best choice for practical applications.

Bending response of terfenol-D/BaTiO3 bilayer to external magnetic and electric fields

The interplay of ferroelectrics and ferromagnetics gives an additional freedom for device design. Based on thermodynamic theory, the bending response of a bilayer terfenol-D/BaTiO3 system to external magnetic and electric fields was investigated. The vertical displacement induced by an asymmetry stress in the bilayer system can be controlled by the thicknesses of each layer, external electric field and magnetic field. A high relative static displacement of 55% can be obtained under a saturated magnetic field. The large bending response controlled by external electric and magnetic fields simultaneously may have potential applications for multifunctional devices.