Antiparallel Ferroelectric Domains Research Articles

To Be or Not To Be Polar: The Ferroelectric and Antiferroelectric Nematic Phases.

We report two new series of compounds that show the ferroelectric nematic, NF, phase in which the terminal chain length is varied. The longer the terminal chain, the weaker the dipole-dipole interactions of the molecules are along the director and thus the lower the temperature at which the axially polar NF phase is formed. For homologues of intermediate chain lengths, between the non-polar and ferroelectric nematic phases, a wide temperature range nematic phase emerges with antiferroelectric character. The size of the antiparallel ferroelectric domains critically increases upon transition to the NF phase. In dielectric studies, both collective ("ferroelectric") and non-collective fluctuations are present, and the "ferroelectric" mode softens weakly at the N-NX phase transition because the polar order in this phase is weak. The transition to the NF phase is characterized by a much stronger lowering of the mode relaxation frequency and an increase in its strength, and a typical critical behavior is observed.

Two new high-temperature molecular ferroelectrics [1,5-3.2.2-Hdabcn]X (X = ClO4−, ReO4−)

Molecular ferroelectrics have attracted much attention because of their excellent piezoelectricity, mechanical workability, and second harmonic effect. Here, we successfully prepared two molecular ferroelectrics [1,5–3.2.2-Hdabcn]X (X = ClO4−, 1; ReO4−, 2) by reactions of a quasi-spherical amine 1,5-diazabicycle[3.2.2]nonane (1.5–3.2.2-dabcn) with HX aqueous solution. Compounds 1 and 2 undergo high-temperature phase transitions at 381 K (1) and 396 K (2). Before and after the phase transition, they crystallize in the polar point group mm2, and the centrosymmetric point groups mmm and 4/mmm, respectively. According to Aizu rules, these two compounds experience mmmFmm2 and 4/mmmFmm2 type ferroelectric phase transitions, respectively. The ferroelectricity of both compounds is well expressed in their polycrystalline film at room temperature with low coercive voltages of 13 V for 1 and 25 V for 2. Using piezoelectric force microscopy (PFM), the 180° anti-parallel ferroelectric domains and the reversible polarization switching can be clearly observed in 1 and 2. This high-temperature molecular ferroelectric material has great application potential in flexible materials, biomechanics, intelligent wearables and other fields.

Compact lensless Fizeau holographic interferometry for imaging domain patterns in ferroelectric single crystals.

Domain patterns in ferroelectric single crystals are physical systems that are fascinating from a theoretical point of view and essential for many applications. A compact lensless method for imaging domain patterns in ferroelectric single crystals based on a digital holographic Fizeau interferometer has been developed. This approach provides a large field-of-view image while maintaining a high spatial resolution. Furthermore, the double-pass approach increases the sensitivity of the measurement. The performance of the lensless digital holographic Fizeau interferometer is demonstrated by imaging the domain pattern in periodically poled lithium niobate. To display the domain patterns in the crystal, we have used an electro-optic phenomenon, which, when an external uniform electric field is applied to the sample, produces a difference in refractive index values in domains with different polarization states of the crystal lattice. Finally, the constructed digital holographic Fizeau interferometer is used to measure the difference in the index of refraction in the antiparallel ferroelectric domains in the external electric field. The lateral resolution of the developed method for ferroelectric domain imaging is discussed.

Axial crystals – macroscopic symmetry and tensor properties

ABSTRACTAxial crystals have axial symmetry which keeps invariant straight line with a fixed point. Axial symmetry groups include 27 non-cubic crystallographic point groups and 5 limit groups describing symmetry of textures and liquid crystals. We show that, except for four cases, each axial symmetry belongs to one of five axial types: polar, chiral, pseudopolar (three basic axial types), directional (possessing none of characteristic properties of basic types) or rotational (exhibiting characteristic properties of all basic types). Each basic type can appear in two structurally different variants with the same symmetry. These variants can coexist and form a mesoscopic structure (antiparallel ferroelectric or chiral domain structure, mixture of enantiomers). We examine macroscopic properties of axial types and variants, and experimental accessivity of their characteristic features.

Multiphoton-induced luminescence contrast between antiparallel ferroelectric domains in Mg-doped LiNbO3

We report on differentiating antiparallel ferroelectric domains in congruent Mg-doped LiNbO3 (Mg:LNO) single crystals through a multiphoton photoluminescence technique. Sample illumination with femtosecond laser pulses at λ = 790 nm results in a broad multiphoton emission spectrum revealing a domain contrast of >3% between virgin and inverted domains. The contrast decreases via annealing and shows an exponential decay in the temperature range from 80 to 150 °C. Our findings give clear ground of a thermally induced structural change by surpassing a specific activation energy. Hence, the reported contrast dynamics must be closely connected to the thermal activation of charged defects, which dramatically alters the internal bias field of these defects. This explanation is also supported when using single crystal LNO of different Mg doping levels showing much lower multiphoton effects for a < 5% Mg concentration. Based on this effect of multiphoton luminescence, it becomes easy to microscopically monitor and quantify virgin and switched domains in LNO and other samples.

Three-dimensional ferroelectric domain visualization by Čerenkov-type second harmonic generation

We show that focusing a laser light onto the boundary between antiparallel ferroelectric domains leads to the non-collinear generation of two second harmonic (SH) beams. The beams are emitted in a plane normal to the domain boundaries at the angles that satisfy the Cerenkov-type phase matching condition. Moreover, these beam disappear when the laser light is focused on a homogenous part of a single domain. We utilize this effect for 3-dimensional visualization of fine details of the ferroelectric domain pattern with a submicron accuracy.

Influence of feedback parameters on ferroelectric domain imaging with piezoresponse force microscopy

The piezoresponse (PR) amplitude and phase signals for antiparallel ferroelectric domains were investigated with different feedback parameters. It is found that the drive frequency and setpoint can strongly alternate the PR image contrast, while the drive phase can be considered as a constant adding to the PR phase signal. The PR amplitude is proportional to the drive amplitude while the PR phase is drive amplitude independent. The larger piezoelectric vibration amplitude and fitting piezoelectric constants (PCs) obtained by vectorial analysis compared with the known values are originated from the sample resonance, and the local electrostatic force can lead to a nonlinear shift in the measured PCs from the theoretical expectations.

Second-harmonic near-field imaging of ferroelectric domain structure ofYMnO3

Near-field tip-enhanced and symmetry-selective nonlinear light scattering is demonstrated for imaging nanoscopic ferroelectric order. The surface topology of antiparallel ferroelectric domains in multiferroic ${\text{YMnO}}_{3}$ is spatially resolved by second-harmonic generation (SHG). Optical phase contrast is obtained by interference between the local near-field SHG and a self-homodyne far-field reference SHG field. Ferroelectric domains anisotropically elongated along the hexagonal crystallographic axis were found, with typical dimensions of $\ensuremath{\sim}1000\ifmmode\times\else\texttimes\fi{}200\text{ }{\text{nm}}^{2}$. The irregular domain shapes seem to contrast the behavior of ordinary ferroelectrics. Relying on symmetry properties only, SHG scattering-type scanning near-field optical microscopy can be extended to probe any ferroic order.

The influence of 180° ferroelectric domain wall width on the threshold field for wall motion

Unlike ideal 180° ferroelectric walls that are a unit cell wide (∼0.5 nm), real walls in ferroelectrics have been reported to be many nanometers wide (1–10 nm). Using scanning nonlinear dielectric microscopy of lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) ferroelectrics, we show that the wall width at surfaces can vary considerably and even reach ∼100 nm in places where polar defects adjoin a wall. The consequence of such variable wall widths is investigated on the specific property of threshold field required for wall motion. Using microscopic phase-field modeling, we show that the threshold field for moving an antiparallel ferroelectric domain wall dramatically drops by two to three orders of magnitude if the wall was diffuse by only ∼1–2 nm, which agrees with experimental wall widths and threshold fields for these materials. Modeling also shows that wall broadening due to its intersection with a surface will influence the threshold field for wall motion only for very thin films (1–10 nm) where the surface broadening influences the bulk wall width. Such pre-existing and slightly diffuse domain walls with low threshold fields for wall motion may offer a general mechanism to explain significantly lower experimental coercive fields for domain reversal in ferroelectrics as compared to the thermodynamic predictions.

Nanoscale polarization profile across a 180° ferroelectric domain wall extracted by quantitative piezoelectric force microscopy

The structure of a single antiparallel ferroelectric domain wall in LiNbO3 is quantitatively mapped by piezoelectric force microscopy (PFM) with calibrated probe geometry. The PFM measurements are performed for 49 probes with the radius varying from 10 to 300 nm. The magnitude and variation of the experimental piezoelectric coefficient across a domain wall match the profiles calculated from a comprehensive analytical theory, as well as three-dimensional finite element method simulations. Quantitative agreement between experimental and theoretical profile widths is obtained only when a finite disk-type tip radius that is in true contact with the sample surface is considered, which is in agreement with scanning electron microscopy images of the actual tips after imaging. The magnitude of the piezoelectric coefficient is shown to be independent of the tip radius, and the PFM profile width is linearly proportional to the tip radius. Finally we demonstrate a method to extract any intrinsic material broadening of the ferroelectric wall width. Surprisingly wide wall widths of up to 100 nm are observed in the limit of zero tip radius.

Transverse second-harmonic generation from disordered nonlinear crystals

Abstract We investigate the transverse second-harmonic generation in as grown strontium barium niobate (SBN) crystals with a random structure of anti-parallel ferroelectric domains. We consider both, single and counter-propagating pulse geometries. We investigate polarization properties of the second harmonic signal and discuss applications of this process for short pulses characterization.

Nanoscale piezoelectric response across a single antiparallel ferroelectric domain wall

Surprising asymmetry in the local electromechanical response across a single antiparallel ferroelectric domain wall is reported. Piezoelectric force microscopy is used to investigate both the in-plane and out-of- plane electromechanical signals around domain walls in congruent and near-stoichiometric lithium niobate. The observed asymmetry is shown to have a strong correlation to crystal stoichiometry, suggesting defect-domain wall interactions. A defect-dipole model is proposed. Finite element method is used to simulate the electromechanical processes at the wall and reconstruct the images. For the near-stoichiometric composition, good agreement is found in both form and magnitude. Some discrepancy remains between the experimental and modeling widths of the imaged effects across a wall. This is analyzed from the perspective of possible electrostatic contributions to the imaging process, as well as local changes in the material properties in the vicinity of the wall.

Evaluation of the internal field in lithium niobate ferroelectric domains by an interferometric method

We report on the evaluation of internal electric field of a ferroelectric engineered-domain in a LiNbO3 wafer crystal by detecting optical path length variation with a noninvasive interferometric inspection method. The lithium niobate wafer has been patterned and subjected to electric field poling to obtain two antiparallel ferroelectric domains separated by a single domain wall. The crystal has been mounted into one arm of a Mach–Zehnder-type interferometer to study the phase map and evaluate the effects of domain reversion by a digital holographic technique. Evaluation of the internal field and consequent variation of the electro-optical properties of the different domains is analyzed.

Nonlinear optical properties and domain microstructure of epitaxial SrBi 2Nb 2O 9 thin films on SrTiO 3 and on MgO substrates studied by second-harmonic generation

The nonlinear optical properties and domain microstructure of ferroelectric thin films of SrBi 2Nb 2O 9 epitaxially grown by pulsed laser deposition on SrTiO 3 and MgO substrates are studied using second-harmonic generation (SHG). The input light polarization dependence of the measured SHG intensities along the principal crystallographic directions of the film is fully interpreted on the basis of a simple nonlinear model considering four possible domain variants. Ratios of the nonlinear optical coefficients and of the net populations of antiparallel ferroelectric domains of SrBi 2Nb 2O 9 are determined for the first time to our knowledge.

Creation of Periodical Antiparallel Domain Structure in Lithium Niobate Crystals During Growth Process

The advantages of producing a periodically poled lithium niobate (PPLN) structure during crystal growth process are the possibility of obtaining thicker and wider structures that leads to greater useful surfaces in addition to the elimination of the subsequent poling process. We report in this paper a new technology of creating bulk periodically poled LiNbO 3 single crystals with antiparallel ferroelectric domains, by direct electric field poling during growth processes. Growth system configuration, crystal composition and geometry selection are explored to allow successful control of the direction of spontaneous polarization using external electric field. PPLN crystal samples with periodicity 20-100μm were grown using this electric field modulated growth technique.

Principle of ferroelectric domain imaging using atomic force microscope

The contrast mechanisms of domain imaging experiments assisted by atomic force microscope (AFM) have been investigated by model experiments on nonpiezoelectric (silicon oxide) and piezoelectric [Pb(Zr,Ti)O3] thin films. The first step was to identify the electrostatic charge effects between the tip, the cantilever, and the sample surface. The second step was to explore the tip–sample piezoelectric force interaction. The static deflection of the cantilever was measured as a function of dc bias voltage (Vdc) applied to the bottom electrode (n-type Si wafers) for noncontact and contact modes. In addition, a small ac voltage (Vac sin ωt) was applied to the tip to measure the amplitude (Aω) and phase (Φω) of the first harmonic (ω) signal as a function of Vdc. By changing from the noncontact to the contact mode, a repulsive contribution to the static deflection was found in addition to the attractive one and a 180° phase shift in Φω was observed. These results imply that in the contact mode the cantilever buckling is induced by the capacitive force between the cantilever and the sample surface. This interaction adds to the tip–sample piezoelectric interaction thereby overlapping the obtained tip vibration signal. Therefore, the antiparallel ferroelectric domain images obtained at zero dc bias voltage will show a variation in Aω but a negligible one in Φω. The capacitive force contribution to the tip vibration signal was further verified in piezoelectric hysteresis loop measurement assisted by the AFM. The observed vertical offset of the loops was explained by the contact potential difference between the cantilever and the bottom electrode. The shape of the curve could be explained by the capacitive force interaction combined with the tip–sample piezoelectric interaction. The experimental results obtained in this study support the interpretation of the cantilever–sample capacitive force contribution to the tip vibration signal in ferroelectric domain imaging experiments using AFM as a probing tool. The use of a large area top electrode between the tip and the sample resulted in the elimination of the electrostatic cantilever–sample interaction with negligible degradation of the domain contrast. This method proved to be successful because the cantilever–sample interaction was hardly detected and only the tip–sample interaction was observed.

Periodically poled potassium niobate for second-harmonic generation at 463 nm

We report on the fabrication and characterization of quasi-phase-matched potassium niobate crystals for second-harmonic generation. Periodic 30-mum -pitch antiparallel ferroelectric domains are fabricated by means of poling in an electrical field. Both birefrigence and periodic phase shift of the generated second harmonic contribute to phase matching when the d(31) nonlinear optical tensor element is used. 3.8 mW of second-harmonic radiation at 463 nm is generated by frequency doubling of the output of master-oscillator power-amplifier diode laser in a 5-mm-long crystal. The measured effective nonlinear coefficient is 3.7pm/V. The measured spectral acceptance bandwidth of 0.25 nm corresponds to the theoretical value.

Imaging of ferroelectric domains in LiTaO3 by enviromental scanning electron microscopy

Antiparallel ferroelectric domains in LiTaO3 single crystal can be directly imaged by using environmental scanning electron microscopy in secondary electron emission mode. Because the observation was performed at a water vapor atmosphere (wet mode), the sample surface was free from charging, therefore, no surface treatment was required. The image of domains can be seen for several hours. The technique may be extended to study domain structures in other ferroelectric crystals and ceramics.

Photorefractive detection of antiparallel ferroelectric domains in BaTiO 3 and BaTiO 3 :Co crystals

An all-optical method involving one coherent beam of light and based on photorefractive wave mixing is used to reveal antiparallel ferroelectric domains in one pure, and two cobalt-doped, barium titanate crystals (BaTiO 3 ). Rod-shaped domains with square cross sections are revealed.

X-ray topographic investigation of tungstate flux-grown KTiOAsO4 crystals

White-beam synchrotron radiation and Lang topographic investigations of non-linear optical crystal KTiOAsO4 have been carried out. It is shown that domains and growth striations are primary planar defects in tungstate flux-grown single crystals. By selecting the wavelength to activate anomalous scattering effect near the absorption edge of the As atom, antiparallel ferroelectric domains are revealed, and in reflection pairshkl and\(\overline {hkl} \), the domains demonstrate contrast reversal. The differences in integrated intensity from a pair of opposite domains are also calculated and the harmonics in a Laue spot are given. Finally, the domain wall configurations are discussed in terms of the structural characteristics of KTiOAsO4.