Effect of elastic boundary conditions on morphotropic Pb(Zr,Ti)O3 piezoelectrics

Spin-coupling-induced Improper Polarizations and Latent Magnetization in Multiferroic BiFeO3

Multiferroic BiFeO₃ (BFO) that exhibits a gigantic off-centering polarization (OCP) is the most extensively studied material among all multiferroics. In addition to this gigantic OCP, the BFO having R3c structural symmetry is expected to exhibit a couple of parasitic improper polarizations owing to coexisting spin-polarization coupling mechanisms. However, these improper polarizations are not yet theoretically quantified. Herein, we show that there exist two distinct spin-coupling-induced improper polarizations in the R3c BFO on the basis of the Landau-Lifshitz-Ginzburg theory: ΔP_LF arising from the Lifshitz gradient coupling in a cycloidal spin-density wave, and ΔP_ms originating from the biquadratic magnetostrictive interaction. With the help of ab initio calculations, we have numerically evaluated magnitudes of these improper polarizations, in addition to the estimate of all three relevant coupling constants. We further predict that the magnetic susceptibility increases substantially upon the transition from the bulk R3c BFO to the homogeneous canted spin state in a constrained epitaxial film, which satisfactorily accounts for the experimental observation. The present study will help us understand the magnetoelectric coupling and shed light on design of BFO-based materials with improved multiferroic properties.

Simultaneous Stress and Field Control of Sustainable Switching of Ferroelectric Phases

In ferroelectrics, manifestation of a strong electromechanical coupling is attributed to both engineered domain morphology and phase transformations. However, realization of large sustainable and reversible strains and polarization rotation has been limited by fatigue, nonlinearity and hysteresis losses. Here, we demonstrate that large strain and polarization rotation can be generated for over 40 × 10⁶ cycles with little fatigue by realization of a reversible ferroelectric-ferroelectric phase transition in [011] cut Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) relaxor ferroelectric single crystal. Direct tuning of this effect through combination of stress and applied electric field, confirmed both macroscopically and microscopically with x-ray and Raman scattering, reveals the local symmetry while sweeping through the transition with a low applied electric field (<0.2 MV/m) under mechanical stress. The observed change in local symmetry as determined by x-ray scattering confirms a proposed polarization rotation mechanism corresponding to a transition between rhombohedral and orthorhombic phases. These results shed more light onto the nature of this reversible transformation between two ferroelectric phases and advance towards the development of a wide range of ferroic and multiferroic devices.

Residual Stress in Pzt Thin Films and its Effect on Ferroelectric Properties

The residual stress in solution derived Pb(Zr.53Ti.47)o3, PZT 53:47, films was determined by measuring the bending of the substrate due to the stress. The substrate consisted of an oxidized (100) silicon wafer with 300 nm coating of platinum. In all cases the stress was tensile. Films fired at a temperature in the range where pyrochlore formation occurs (500° to 575°C) had the highest residual stresses, 200 to 350 MPa, whereas those fired at higher temperatures, 600° to 650°C, where the perovskite phase forms had stresses of 100 to 200 MPa. Stress measurements made during film firing indicate that the pyrochlore containing films had higher residual stress because their coefficient of thermal expansion was much larger than that of predominantly perovskite films. The effect of the amount of stress on ferroelectric properties was studied by making measurements on a film with and without the application of an external stress. The external stress was applied by bending a circular section of the substrate, which effectively lowered the amount of tensile stress in the film by ∼30%. Decreasing the stress in this manner was found to increase the remanent polarization by ∼11% and the dielectric constant by ∼2%.

Electromechanical properties of high- coupling (1-xi)Pb(Zn1/3Nb2/3)O3-xiPbTiO3 single crystals for sound projectors

Isothermal weak field frequency sweeps were conducted over the temperature range 30 to 140 degrees C on multidomain, oriented, and poled (1-x)Pb(Zn(1/3)Nb(2/3))O(3)-xPbTiO(3) (PZN-PT) single crystals with x = 0.045, 0.06, 0.07, and 0.08 near the ferroelectric rhombohedral (FR)-ferroelectric tetragonal (F(T)) morphotropic phase boundary (MPB). The temperature dependencies of the small-signal 33-mode electromechanical properties were derived from the isothermal frequency sweeps. Morphotropic compositions with x = 0.06 and 0.07 PT were found to exhibit the largest electromechanical coupling and piezoelectric coefficient, with k(33) > 0.90 and d(33) approximately 2300 pC/N, and the lowest short-circuit Young's modulus of approximately 9 GPa. Reversible zero-field elastic instability attributable to a F(R)- F(O) transition was observed under isothermal compression-decompression of a 0.94PZN-0.06PT single crystal. The application of a dc bias field enhanced the F(R) stability under compression. The large-signal, dc biased, 33-mode electromechanical response was measured under mechanical compressions similar to those used in sound projectors. The remarkable small-signal electromechanical properties are in a good agreement with the large-signal response. The crystal response to thermal, electrical, and mechanical variable is discussed in terms of high-coupling single crystal models.

Mechanical and thermal transitions in morphotropic PZN-pT and PMN-PT single crystals and their implication for sound projectors

Isothermal compression experiments on multidomain [001] oriented and poled ferroelectric rhombohedral PZN-0.07PT and PMN-0.30PT single crystals revealed elastic instabilities corresponding to zero field ferroelectric-ferroelectric phase transition under mechanical compression. The application of an appropriate dc bias field doubled the stability range of the ferroelectric rhombohedral state under uniaxial compression for both crystals and maintained a linear elastic response. Young's modulus as derived from the quasistatic, zero field stress-strain linear response agreed well with that derived from small signal resonance for the ferroelectric rhombohedral FR state of both PZN-PT and PMN-PT. Elastic compliances s(E)33 as determined from high temperature resonance revealed a monotonically decreasing Young's modulus as a function of temperature in the ferroelectric rhombohedral state with a sudden stiffening near the ferroelectric rhombohedral (FR)-ferroelectric tetragonal (FT) transition. The reversible ferroelectric-ferroelectric transition of morphotropic PZN-PT and PMN-PT single crystals as accessed by mechanical compression is discussed in terms of strain calculations from Devonshire's theory, domain unfolding, and morphotropic phase boundary shift with mechanical stress. The mechanically-induced and thermally-induced ferroelectric-ferroelectric transition trajectories are discussed in terms of the Devonshire theory. Implications of these observations for sound projectors are discussed. A single crystal tonpilz projector fabricated into a 16-element array and a segmented cylinder transducer demonstrated the outstanding capabilities of single crystals to achieve compact, broadband, and high-source level projectors when compared to conventional lead zirconate-titanate PZT8 projectors.