Time-resolved X-ray Microdiffraction Research Articles

Photostrictive Actuators Based on Freestanding Ferroelectric Membranes.

Complex oxides offer a wide range of functional properties, and recent advances in the fabrication of freestanding membranes of these oxides are adding new mechanical degrees of freedom to this already rich functional ecosystem. Here, photoactuation is demonstrated in freestanding thin film resonators of ferroelectric Barium Titanate (BaTiO3) and paraelectric Strontium Titanate (SrTiO3). The free-standing films, transferred onto perforated supports, act as nano-drums, oscillating at their natural resonance frequency when illuminated by a frequency-modulated laser. The light-induced deflections in the ferroelectric BaTiO3 membranes are two orders of magnitude larger than in the paraelectric SrTiO3 ones. Time-resolved X-ray micro-diffraction under illumination and temperature-dependent holographic interferometry provide combined evidence for the photostrictive strain in BaTiO3 originating from a partial screening of ferroelectric polarization by photo-excited carriers, which decreases the tetragonality of the unit cell. These findings showcase the potential of photostrictive freestanding ferroelectric films as wireless actuators operated bylight.

The role of intermediate S-polymorph towards high piezoelectricity in La-doped BiFeO3 epitaxial thin films

Mixed phases with stripe patterns are considered as a prerequisite for remarkable piezoelectric properties in epitaxial BiFeO3 (BFO) thin films. Among the mixed phases in BFO epitaxial thin films, the vital role in achieving high piezoelectric response is attributed to the intermediate S-polymorph that bridges the transition between rhombohedral and tetragonal phases. However, the correlation between the S-phase and the piezoelectric strain in mixed-phase BFO has not been understood well due to the lack of a probe technique that can investigate the structural and electromechanical variations simultaneously. Here, using the time-resolved X-ray microdiffraction technique, we investigate the electric field induced phase transformation dynamics of a mixed-phase epitaxial La-doped BFO (BLFO) thin film. The BLFO thin film exhibits phase transformations between monoclinic MC and tilted MC, S/tilted S (Stilt) phases under an applied electric field. Among the competing phases, we found that the S- or tilted S-phases exhibit the highest piezoelectric strain variation up to 0.7% at an electric field pulse of strength 1.3 MV/cm and width 5 μs. The present study correlates the high piezoelectric response in mixed-phase BLFO thin films with the presence of S/Stilt -phases, which offers a useful pathway into designing strain-engineered ultra-high piezoelectric BFO thin films.

Electric-Field-Driven Nanosecond Ferroelastic-Domain Switching Dynamics in Epitaxial Pb(Zr,Ti)O_{3} Film.

Epitaxial oxide ferroelectric films exhibit emerging phenomena arising from complex domain configurations even at pseudoequilibrium, including the creation of domain states unfavored in nature and abrupt piezoelectric coefficients around morphotropic phase boundaries. The nanometer-sized domain configurations and their domain switching dynamics under external stimuli are directly linked to the ultrafast manipulation of ferroelectric thin films; however, complex domain switching dynamics under homogeneous electric fields has not been fully explored, especially at the nanosecond timescale. This Letter reports the nanosecond dynamics of ferroelastic-domain switching from the 90° to 180° direction using time-resolved x-ray microdiffraction under homogeneous electric fields onto an epitaxial Pb(Zr_{0.35},Ti_{0.65})O_{3} film capacitor. It is found that the application of electric fields induces spatially heterogeneous domain switching processes via intermediate domain structures with rotated polarization vectors. In addition, the domain switching time is shown to be inversely proportional to the magnitude of the applied electric field, and electric fields higher than 480 kV/cm are found to complete the ferroelastic switching within nanoseconds.

Piezoelectricity in La0.85Ce0.15MnO3 layer of BiFeO3/ La0.85Ce0.15MnO3 based ferroelectric/semiconductor oxide superlattice

Superlattice comprising of ferroelectric (FE) and non-ferroelectric (non-FE) material has been developed to induce a ferroelectric response in the non-ferroelectric layer. In this study, a superlattice thin film comprising of BiFeO3 (BFO) as FE oxide and La0.85Ce0.15MnO3 (LCMO) as semiconducting non-FE oxide is fabricated and investigated the FE and piezoelectric responses in LCMO. We observed the piezoelectric response of individual layers using time-resolved X-ray microdiffraction experiment under an external electric field. Piezoelectric response of individual layers was resolved by comparing it with the values obtained from kinematic diffraction calculation. Piezoelectric coefficient (d33) of LCMO monolayer is found to be approximately 27 pm/V, which is a similar value as that of the BFO layer.

In situ observation of atomic movement in a ferroelectric film under an external electric field and stress

Atomic movement under application of external stimuli (i.e., electric field or mechanical stress) in oxide materials has not been observed due to a lack of experimental methods but has been well known to determine the electric polarization. Here, we investigated atomic movement arising from the ferroelectric response of BiFeO3 thin films under the effect of an electric field and stress in real time using a combination of switching spectroscopy, time-resolved X-ray microdiffraction, and in situ stress engineering. Under an electric field applied to a BiFeO3 film, the hysteresis loop of the reflected X-ray intensity was found to result from the opposing directions of displaced atoms between the up and down polarization states. An additional shift of atoms arising from the linearly increased dielectric component of the polarization in BiFeO3 was confirmed through gradual reduction of the diffracted X-ray intensity. The electric-field-induced displacement of oxygen atoms was found to be larger than that of Fe atom for both ferroelectric switching and increase of the polarization. The effect of external stress on the BiFeO3 thin film, which was controlled by applying an electric field to the highly piezoelectric substrate, showed smaller atomic shifts than for the case of applying an electric field to the film, despite the similar tetragonality.

PA21 SmA相電傾効果における局所層構造の動的解析(物理・物性)

PA21 SmA相電傾効果における局所層構造の動的解析(物理・物性)

Depth resolved lattice-charge coupling in epitaxial BiFeO3 thin film.

For epitaxial films, a critical thickness (tc) can create a phenomenological interface between a strained bottom layer and a relaxed top layer. Here, we present an experimental report of how the tc in BiFeO3 thin films acts as a boundary to determine the crystalline phase, ferroelectricity, and piezoelectricity in 60 nm thick BiFeO3/SrRuO3/SrTiO3 substrate. We found larger Fe cation displacement of the relaxed layer than that of strained layer. In the time-resolved X-ray microdiffraction analyses, the piezoelectric response of the BiFeO3 film was resolved into a strained layer with an extremely low piezoelectric coefficient of 2.4 pm/V and a relaxed layer with a piezoelectric coefficient of 32 pm/V. The difference in the Fe displacements between the strained and relaxed layers is in good agreement with the differences in the piezoelectric coefficient due to the electromechanical coupling.

Time-resolved X-ray microdiffraction for ferroelectric heterostructures

Time-resolved X-ray microdiffraction for ferroelectric heterostructures

Domain- and symmetry-transition origins of reduced nanosecond piezoelectricity in ferroelectric/dielectric superlattices

Complex-oxide superlattices (SLs) with atomic-scale periodicity have dynamical properties that are distinct from thin films of uniform composition. The origins of these properties are closely related to the dynamics of polarization domains and to field-driven changes in the symmetries resulting from interfacial coupling between different components. These dynamics are apparent at timescales from a few nanoseconds to several milliseconds in experiments probing the piezoelectricity of a ferroelectric/dielectric BaTiO3(BTO)/CaTiO3 (CTO) SL using time-resolved x-ray microdiffraction. At the 100 ns timescale, the piezoelectric distortion is approximately ten times smaller than in the millisecond regime. This reduced piezoelectricity at short timescales is not observed in previously studied PbTiO3/SrTiO3 SLs or compositionally uniform ferroelectrics such as tetragonal compositions of Pb(Zr,Ti)O3. The unusual behavior of the BTO/CTO SL can be linked to the switching of a nanodomain state into a uniform polarization state or to a field-induced crystallographic symmetry transition. A comparison of the results with the characteristic timescales of these two dynamical phenomena in other complex oxides with different compositions suggests that the phase transition is a more likely possibility.

Nanosecond Dynamics of Ferroelectric/Dielectric Superlattices

The nanosecond response of a PbTiO(3)/SrTiO(3) ferroelectric/dielectric superlattice to applied electric fields is closely linked to the dynamics of striped domains of the remnant polarization. The intensity of domain satellite reflections observed with time-resolved x-ray microdiffraction decays in 5-100 ns depending on the magnitude of the electric field. The piezoelectric response of the superlattice within stripe domains is strongly suppressed due to electromechanical clamping between adjacent regions of opposite polarization. Regions of the superlattice that have been switched into a uniform polarization state by the applied electric field, however, exhibit piezoelectricity during the course of the switching process. We propose a switching model different from previous models of the switching of superlattices, based instead on a spatially heterogeneous transformation between striped and uniform polarization states.

Fast X-ray microdiffraction techniques for studying irreversible transformations in materials

A pair of techniques have been developed for performing time-resolved X-ray microdiffraction on irreversible phase transformations. In one technique capillary optics are used to focus a high-flux broad-spectrum X-ray beam to a 60 µm spot size and a fast pixel array detector is used to achieve temporal resolution of 55 µs. In the second technique the X-rays are focused with Kirkpatrick-Baez mirrors to achieve a spatial resolution better than 10 µm and a fast shutter is used to provide temporal resolution better than 20 µs while recording the diffraction pattern on a (relatively slow) X-ray CCD camera. Example data from experiments are presented where these techniques are used to study self-propagating high-temperature synthesis reactions in metal laminate foils.

Component-specific electromechanical response in a ferroelectric/dielectric superlattice

The electronic and electromechanical properties of complex oxide superlattices are closely linked to the evolution of the structure and electrical polarization of the component layers in applied electric fields. Efforts to deduce the responses of the individual components of the superlattice to applied fields have focused on theoretical approaches because of the limitations of available experimental techniques. Time-resolved x-ray microdiffraction provides a precise crystallographic probe of each component using the shift in wave vector and change in intensity of superlattice satellite reflections. We report in detail the methods to measure and analyze the x-ray diffraction patterns in applied electric field and their application to a 2-unit-cell ${\text{BaTiO}}_{3}/4$-unit-cell ${\text{CaTiO}}_{3}$ superlattice. We find that the overall piezoelectric distortion is shared between the two components. Theoretical predictions of the electromechanical properties of a superlattice with the same composition constrained to tetragonal symmetry are in excellent agreement with the experiments. Lattice instability analysis, however, suggests that the low-temperature ground state could exhibit antiferrodistortive rotations of ${\text{TiO}}_{6}$ octahedra within and/or at the interfaces of the ${\text{CaTiO}}_{3}$ component.

Time-resolved x-ray microdiffraction studies of phase transformations during rapidly propagating reactions in Al/Ni and Zr/Ni multilayer foils

We showed how intermetallic formation reactions can be studied under rapid heating (106–107 K s−1) using x-ray microdiffraction with temporal resolution on microsecond time scales. Rapid heating was achieved by initiating an exothermic reaction in multilayer foils comprising alternating nanoscale layers of elemental metals. The reaction occurred in a front ∼100 μm wide which propagated across the foil at ∼1–10 m s−1. By using synchrotron x-rays focused to a small spot (60 μm diameter) and a fast pixel-array detector, we were able to track the evolution of phases in the reaction front during the initial heating transient, which occurred in approximately 1 ms, through cooling over a period of hundreds of milliseconds. In Al/Ni multilayer foils, the first phases to form were an Al-rich liquid and the cubic intermetallic AlNi (which likely formed by nucleation from the liquid). In foils of overall composition AlNi, this is the stable intermetallic and the only phase to form. In foils of composition Al3Ni2, during cooling we observed a peritectic reaction between AlNi and the remaining liquid to form Al3Ni2, which is the stable phase at room temperature and the final product of the reaction. This is in contrast to the sequence of phases under slow heating, where we observed formation of nonequilibrium Al9N2 first and do not observe formation of a liquid phase or the AlNi intermetallic. We also observed formation of an amorphous phase (along with crystalline ZrNi) during rapid heating of Zr/Ni multilayers, but in this system the temperature of the reaction front never reached the lowest liquidus temperature on the Zr–Ni phase diagram. This implies that the amorphous phase we observed was not a liquid arising from melting of a crystalline phase. We suggest instead that a Zr-rich amorphous solid formed due to solid-state interdiffusion, which then transformed to a supercooled liquid when the temperature exceeded the glass transition temperature. Formation of the supercooled liquid presumably facilitated continued rapid intermixing, which may be necessary to sustain a self-propagating reaction front in this system.

Stability of the unswitched polarization state of ultrathin epitaxialPb(Zr,Ti)O3in large electric fields

The initial stage of polarization switching in ferroelectric thin films depends on phenomena that occur at characteristic time scales of tens to hundreds of nanoseconds, including the nucleation polarization domains and the propagation of domain walls. These long intrinsic times allow short-duration electric fields with magnitudes far above the low-frequency coercive electric field to be applied across capacitor devices without inducing switching. Using time-resolved x-ray microdiffraction, we have found that a series of 50 ns duration electric field pulses switches the polarization of a 35-nm-thick ferroelectric $\text{Pb}(\text{Zr},\text{Ti}){\text{O}}_{3}$ film only at electric fields greater than 1.5 MV/cm, a factor of three higher than the low-frequency coercive field. There is no switching in response to a large number of short pulses with amplitudes lower than 1.5 MV/cm, even when the total duration reaches several milliseconds. In comparison, a series of microsecond-duration pulses causes cumulative changes in the area of switched polarization and eventually switches the entire capacitor. The difference between long- and short-duration electric field pulses arises from effects linked to domain nucleation and charge transport in the ferroelectric film. A phase-field model shows that the shrinking of the switched domain in the interval between pulses is a less important effect. This opportunity to apply large fields for short times without inducing switching by domain-wall motion raises the possibility that future experiments could reach the intrinsic coercive field of ferroelectric layers and provides a way to study the properties of materials under high electric fields.

Structural Response of BaTiO3/CaTiO3 Superlattice to Applied Electric Fields

AbstractThe structural response of a ferroelectric BaTiO3/dielectric CaTiO3 superlattice to the bipolar applied electric field was studied using time-resolved x-ray microdiffraction. Structural results were compared to the polarization-electric field hysteresis curve obtained from electrical measurements. The superlattice x-ray reflections were found to have a broad distribution of intensity in reciprocal space under applied electric fields exceeding the nominal coercive electric field. The broad distribution of the lattice constant at high electric fields is compared with a model in which the constituent layers of the superlattice have different coercive fields for the polarization switching.

NANOSECOND STRUCTURAL VISUALIZATION OF THE REPRODUCIBILITY OF POLARIZATION SWITCHING IN FERROELECTRICS

ABSTRACT As the polarization of a ferroelectric thin film reverses in response to an applied electric field, concomitant structural changes can be visualized using time-resolved x-ray microdiffraction. We report the details of this visualization approach and discuss the structural signature of polarization switching measured by time-resolved x-ray diffraction.

D043 Time-resolved X-ray microdiffraction imaging of nanosecond structural transformations in thin ferroelectric films

D043 Time-resolved X-ray microdiffraction imaging of nanosecond structural transformations in thin ferroelectric films

Nanosecond Domain Wall Dynamics in FerroelectricPb(Zr,Ti)O3Thin Films

Domain wall motion during polarization switching in ferroelectric thin films is fundamentally important and poses challenges for both experiments and modeling. We have visualized the switching of a Pb(Zr, Ti)O(3) capacitor using time-resolved x-ray microdiffraction. The structural signatures of switching include a reversal in the sign of the piezoelectric coefficient and a change in the intensity of x-ray reflections. The propagation of polarization domain walls is highly reproducible from cycle to cycle of the electric field. Domain wall velocities of 40 m s(-1) are consistent with the results of other methods, but are far less than saturation values expected at high electric fields.

Structural dynamics of PZT thin films at the nanoscale

AbstractWhen an electric field is applied to a ferroelectric the crystal lattice spacing changes as a result of the converse piezoelectric effect. Although the piezoelectric effect and polarization switching have been investigated for decades there has been no direct nanosecond-scale visualization of these phenomena in solid crystalline ferroelectrics. Synchrotron x-rays allow the polarization switching and the crystal lattice distortion to be visualized in space and time on scales of hundreds of nanometers and hundreds of picoseconds using ultrafast x-ray microdiffraction. Here we report the polarization switching visualization and polarization domain wall velocities for Pb(Zr0.45Ti0.55)O3 thin film ferroelectric capacitors studied by time-resolved x-ray microdiffraction.

Dynamic local-layer response of surface-stabilized ferroelectric liquid crystals to a high electric field by time-resolved x-ray microdiffraction.

Time-resolved synchrotron x-ray microdiffraction measurements have directly revealed the dynamic local-layer response to the high electric field in a surface-stabilized ferroelectric liquid crystal. The irreversible layer transformation under the increasing electric field is found to consist of two stages; the initial vertical chevron structure transforms to the alternate vertical and horizontal chevrons and, with increasing field, the chevron angle decreases and the horizontal chevron structure develops. The time-resolved microdiffraction measurement has clarified the detailed reversible layer transformation between the mostly horizontal chevron at the high field and the so-called quasibookshelf at the low field during the ac field application. The transient layer response time is about 0.1-0.2 and 0.3-0.4 ms for falling and rising edges, respectively, and is slightly longer than the optical response time. The layer transformation is discussed in terms of electric torque and surface anchoring. The local-layer response in the antiferroelectric liquid crystal is also discussed for comparison.