Piezoresponse imaging and local characterization of ferroelectric domains in Pb(Zn1/3Nb2/3)O3–7%PbTiO3 single crystals

Ferroelectric domain structures of (001)‐oriented Pb(Zn_1/3Nb_2/3)O₃–7%PbTiO₃ (PZN‐7%PT) single crystals were visualized and characterized by piezoresponse force microscopy (PFM). Locally regular domain configurations are found to be possibly related to the stable macroscopic properties in the PZN‐7%PT single crystals. Nanoscale piezoresponse hysteresis loops measured by PFM tip revealed no evidence of local domain switching behavior in the PZN‐7%PT single crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

YMnO_3薄膜的铁电行为及其纳米尺度铁电畴的研究

六方YMnO_3是一种特殊的多铁性材料,因其具有介电常数低、单一极化轴、无挥发性元素等特点,在磁电领域具有独特的优势,但目前关于YMnO_3薄膜的铁电性特别是畴结构的研究相对较少.本文采用溶胶-凝胶法在Si(100)基片上制备了多铁性YMnO_3薄膜,利用掠入射X-射线衍射、原子力显微镜对薄膜的结构及表面形貌进行了分析,用压力显微镜(PFM)技术研究了纳米尺度畴结构及微区电滞行为,并通过I-V,P-E曲线进一步研究了薄膜的漏电流和宏观电滞行为.结果表明,该薄膜为六方钙钛矿结构,YMnO_3晶粒大小均匀并且结晶性较好,薄膜表面粗糙度为7.209 nm.PFM图显示出清晰的电畴结构,结合典型的微区振幅蝴蝶曲线和相位电滞回线,证实该YMnO_3薄膜具有较好的铁电性.由于受内建电场的作用,振幅曲线和相位曲线都向正向偏移,表现出非对称特征.该薄膜的漏电流密度低于10~(-6)A·cm~(-2),因而其电滞回线基本能够达到饱和.     

Distinct ferroelectric domain switching dynamics in epitaxial BiFeO3 (001) capacitors depending on the bias polarity

Understanding ferroelectric domain switching dynamics at the nanoscale is a great of importance in the viewpoints of fundamental physics and technological applications. Here, we investigated the intriguing polarity-dependent switching dynamics of ferroelectric domains in epitaxial BiFeO₃ (001) capacitors using transient switching current measurement and piezoresponse force microscopy. We observed the distinct behavior of nucleation and domain wall motion depending on the polarity of external electric bias. When applying the negative bias to the top electrode, the sideways domain wall motion initiated by only few nuclei was dominant to polarization switching. However, when applying the positive bias, most of domains started to grow from the pre-existed pinned domains and their growth velocity was much smaller. We suggest that the observed two distinct domain switching behavior is ascribed to the interfacial defect layer.     

Etch Damage Evaluation in Integrated Ferroelectric Capacitor Side Wall by Piezoresponse Force Microscopy

The etch damage in integrated ferroelectric capacitors side wall fabricated by the typical integrated process (TIP-FeCAP) and the innovated integrated process (IIP-FeCAP) are investigated by piezoresponse force microscopy (PFM). The IIP-FeCAP side wall exhibits fine and clear nanoscale domain images and the same piezoresponse signal as the thin film, and the domains can also be easily switched by an external voltage. In the TIP-FeCAP side wall, owing to the effect of etch damage, the very weak piezoresponse signal and some discrete domains can be observed, and the discrete domains cannot be switched by the applied 9V and -9V dc voltage. The PFM results reflect the etch damage in the integrated ferroelectric capacitors and also suggest that the PFM can be used as an efficacious tools to evaluate the etch damage at nanoscale and spatial variations.     

Direct characterization of nanoscale domain switching and local piezoelectric loops of (Pb,La)TiO3 thin films by piezoresponse force microscopy

〈111〉 and 〈001〉, 〈100〉 preferentially oriented lanthanum-modified lead titanate thin films have been studied at the nanometre scale by means of piezoresponse force microscopy. The nanoscale domain structures, domain switching, and local piezoelectric loops of the films have been analysed. The imaging of the domain structures after the application of a dc field suggests the existence of 90° and 180° domains within the regions with intermediate contrast. The variation of piezoresponse under an electric field in domains of two types has been discussed. Significant differences have been found between the local piezoelectric loops measured in the films deposited on different substrates. These differences are related to the different textures present in the films. PACS 61.16.Ch; 77.84.Dy; 77.80.Dj     

Observation of mechanical fracture and corresponding domain structure changes of polycrystalline PbTiO3 nanotubes

PbTiO₃ (PTO) nanotubes (NTs) were synthesized at various temperatures by gas phase reaction between PbO gas and anatase TiO₂ NTs and characterized by piezoresponse force microscopy (PFM). PTO ferroelectric phase was synthesized at as low as 400 °C as evidenced by PFM domain images and piezoresponse hysteresis loop measurement. Furthermore, the PTO NTs fabricated at above 500 °C underwent mechanical fracture through development of nanocracks due to grain growth, leading to ferroelectric domains with larger size and lower aspect ratio. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Domain structure of triglycine sulfate family crystals according to the data of piezoresponse force microscopy and their macroscopic dielectric properties

The domain structure of triglycine sulfate (TGS) crystals, specifically nominally pure crystals and those with impurities (LADTGS+ADP, DTSG, and TGS:Cr), is studied by piezoresponse force microscopy (PFM). The measured macroscopic dielectric characteristics are compared with microscopic data on the domain structure of these crystals. The values of the spontaneous polarization, bias voltage, and dielectric permeability as a function of temperature ?(T) are shown to be in agreement with the PFM data. The anomalous behavior of the dependence ?(T) was observed for LADTGS+ADP crystals.     

Domain configuration and dielectric properties of Ba0.6Sr0.4TiO3 thin films

The microstructure and electrical properties of Ba_0.6Sr_0.4TiO₃ thin films have been investigated. Nanometer-sized domains, ranging from 8 to about 30 nm, were observed by piezoresponse force microscopy (PFM). The critical size, below which only single domains exist, is found to be about 31 nm. The film exhibits ferroelectric behavior characterized by polarization hysteresis loop and capacitance-voltage curve.     

Influence of the inhomogeneous field at the tip on quantitative piezoresponse force microscopy

Ferroelectric domain imaging with piezoresponse force microscopy (PFM) relies on the converse piezoelectric effect: a voltage applied to the sample leads to electromechanical deformations. In the case of PFM one electrode is realized by a tip, therefore generating a strongly inhomogeneous electric field distribution inside the sample which reaches values up to 10⁸ V/m directly underneath the apex of the tip. Although often assumed, this high electric field does not lead to an enhancement of the electromechanical deformation of the sample. On the contrary, internal clamping of the material reduces the deformation as compared to the theoretically expected value which depends only on the voltage applied to the tip, thus being independent of the exact field distribution. PACS 77.80.Dj; 68.37.Ps; 77.84.-s     

Characterization of 90° domain structure and polarization switching in Pb(Zr0.4Ti0.6)O3 film by piezoresponse force microscope

A lead zirconate titanate Pb(Zr_0.4Ti_0.6)O₃ (PZT40/60) film was deposited on a Pt(111)/Ti/SiO₂/Si(100) substrate by a sol–gel process followed by thermal annealing at 650 °C for 5 min. Piezoresponse force microscope observation revealed a lamellar domain structure in the PZT40/60 grains and we attribute the lamellar domains as 90° ferroelectric domains. The polarization-switching mechanism of the 90° domains in the PZT40/60 film under external electric fields has also been studied and it was revealed that a large-area polarization switching is usually accompanied by the appearance of a new direction of 90° domains in order to reduce the stress in the grains. By contrast, a nanometer-sized polarization switching is believed to be accomplished by generating 180° domains within a single lamellar domain. PACS 77.80.-e; 77.84.-s; 68.55.-a; 68.37.-d     

Cold-field switching in PVDF-TrFE ferroelectric polymer nanomesas

Polarization reversal in ferroelectric nanomesas of polyvinylidene fluoride with trifluoroethylene has been probed by ultrahigh vacuum piezoresponse force microscopy in a wide temperature range from 89 to 326 K. In dramatic contrast to the macroscopic data, the piezoresponse force microscopy local switching was nonthermally activated and, at the same time, occurring at electric fields significantly lower than the intrinsic switching threshold. A "cold-field" defect-mediated extrinsic switching is shown to be an adequate scenario describing this peculiar switching behavior. The extrinsic character of the observed polarization reversal suggests that there is no fundamental bar for lowering the coercive field in ferroelectric polymer nanostructures, which is of importance for their applications in functional electronics.     

Ferroelectric domain phenomena and microdomain engineering in BaMgF4 single crystal

Ferroelectric domain phenomena and periodic domain patterning of orthorhombic as-grown BaMgF₄ single crystal were investigated in the present study. An isolated ferroelectric domain shows a hexagonal form and exhibits a high domain-wall anisotropy. Periodic domain patterning was demonstrated on the polar surface by a periodic macropoling technique. In-plane polarization switching-induced microdomain patterning was tailored by the lateral component of the inhomogeneous electric field through a biased atomic force microscope tip. Such in-plane domain switching behavior exhibits potential promise for periodic domain engineered phonon devices. PACS 77.80.Dj; 77.80.Fm; 07.79.Lh     

Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates

We investigated ferroelectric characteristics of BiFeO₃ (BFO) thin films on SrRuO₃ (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates grown by pulsed laser deposition. YSZ buffer layers were employed to grow highly crystallized BFO thin films as well as SRO bottom electrodes on glass substrates. The BFO thin films exhibited good ferroelectric properties with a remanent polarization of 2P_r = 59.6 μC/cm² and fast switching behavior within about 125 ns. Piezoelectric force microscopy (PFM) study revealed that the BFO thin films have much smaller mosaic ferroelectric domain patterns than epitaxial BFO thin films on Nb:SrTiO₃ substrates. Presumably these small domain widths which originated from smaller domain energy give rise to the faster electrical switching behavior in comparison with the epitaxial BFO thin films on Nb:SrTiO₃ substrates.     

Combined piezoresponse force microscopy and Raman scattering investigation of domain boundaries in BiFeO3 ceramics

Domain structure of BiFeO₃ ceramics has been investigated using a combination of piezoresponse force microscopy (PFM) and Raman scattering techniques. Both methods demonstrate the presence of ferroelastic domains, separated by almost parallel planar domain walls, as well as the presence of large homogeneous single ferroelastic domain regions. In addition to highly resolved domain pattern obtained by PFM, small frequency shifts of the Raman-active modes give us complementary information about the angle φ between the surface normal and the rhombohedral axis of the BiFeO₃ crystal for any measured position at the surface of the sample.     

Nanoscale imaging of ferroelectric domain and resistance switching in hybrid improper ferroelectric Ca3Ti2O7 thin films

Hybrid improper ferroelectrics have their electric polarization generated by two or more combined non-ferroelectric structural distortions such as the rotation and tilting of Ti-O octahedral in Ca₃Ti₂O₇ (CTO) family. In this work, we prepared different thickness CTO thin films on Pt substrates by pulsed laser deposition, and investigated their ferroelectric polarization reversal and the current transport properties by using the piezoresponse force microscopy and conducting atomic force microscopy, respectively. It is found that the CTO films exhibit clear ferroelectric domain switching and ferroelectric resistance switching behaviors, and the maximum resistive ratios of CTO film reaches ∼1750. These results demonstrate that hybrid improper ferroelectrics CTO films are promising materials for being employed in non-volatile memory and logic devices.     

Effect of built-in bias fields on the nanoscale switching in ferroelectric thin films

Polarization switching in Pb(Zr_0.3Ti_0.7)O₃ thin films has been studied in nanoscale level using piezoresponse force microscopy. It is found that the switching in ferroelectric thin films is significantly influenced by the built-in bias field. It has two different origins: one time-independent, and the other time-dependent, which can be reasonably attributed to the effect of the film/electrode interface and the space charges, respectively. The redistribution of the unstable field can be triggered by a long pulse, and is shown to follow an exponential law. PACS 77.80.-e; 68.37.-d; 77.55.+f     

Domain structures in tetragonal Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals studied by piezoresponse force microscopy

The domain structures in (001) surface of Pb(Mg_1/3Nb_2/3)O₃-40% PbTiO₃ single crystals were investigated by piezoresponse force microscopy. Both micron-sized fingerprint 180° and parallel 90° domains were observed in the sample. Different sets of favourable {110} oriented domain patterns were found to meet, intersect or grow through each other. In addition, the piezoelectricity decreases sharply at the domain walls in 180° structures, but does not in the 90° domain structures.     

Superdomain structure and high conductivity at the vertices in the (111)-oriented epitaxial tetragonal Pb(Zr,Ti)O3 thin film

Recently, in ferroelectric materials, there have been many experimental efforts to find out more intriguing topological objects and their functionalities, such as conduction property. Here we investigated ferroelectric domain structures and related topological defects in the (111)-oriented epitaxial tetragonal PbZr_0.35Ti_0.65O₃ thin film. Systematic piezoresponse force microscopy measurements revealed that the field-induced polarization switching can form thermodynamically stable superdomain structures composed of nano-sized stripe subdomains. Within such superdomain structures, we observed the exotic equilateral triangular in-plane flux-closure domains composed of three stripe domain bundles with 120/120/120 degrees of separation. The conductive-atomic force microscopy measurements under vacuum showed that some vertices have significantly higher conductivity compared to other surrounding regions. This work highlights electric field-driven polarization switching and unique crystallographic symmetry (here, three-fold rotational symmetry) can generate exotic ferroelectric domain structures and functional topological defects, such as conductive vertices.     

The effect of annealing temperature on the morphology and piezoelectric characteristics of BaTiO3 nanofibers and domain switching under different temperatures

Effects of annealing temperature (600–750 °C) on crystalline structure, the morphology and piezoresponse hysteresis loops of BaTiO₃ nanofibers prepared by electrospinning are characterized by X-ray diffraction, scanning electronic microscopy, transmission electron microscope and piezoresponse force microscope. When the annealing temperature is 700 °C, the nanofibers become smoother and have a diameter of 100–300 nm. Meanwhile the typical butterfly-shaped amplitude loop and 180°phase change represents the best ferroelectric and piezoelectric properties at 700 °C. So the 700 °C was found to be optimum for good piezoelectric characteristics at annealing temperature of 600 °C–750 °C. In order to give more clear evolution of domain states at different external fields, the three dimensional topographic and phase images of the nanofiber at different temperatures are observed by piezoresponse force microscope. The 90° domain switching is observed during heating from room temperature to 125 °C and the domain switching tends to be stable when the temperature exceeds a critical value. The thermal stress due to the high temperatures is responsible for switching mechanism from the perspective of equilibrium state free energy. This work suggests that the temperature variation should be considered while designing the ferroelectric devices based on one dimensional material.     

Temperature dynamics of triglycine sulfate domain structure according to atomic force microscopy and dielectric spectroscopy data

A hydrogen-containing ferroelectric triglycine sulfate (TGS) was comprehensively studied with an atomic force microscopy (AFM) and dielectric spectroscopy. The domain structure dynamics was in situ investigated with piezoresponse force microscopy (PFM) during heating and cooling the TGS crystal near phase transition. Relaxation dependencies of domain boundaries general perimeter and domain dimensions were obtained. TGS dielectric spectra measured at the frequency range from 10 to 10¹¹ Hz were analyzed on basis of significant contribution of conductivity into the dielectric response of ferroelectrics and a good agreement with the experimental data was received. It allows us to obtain more information about temperature dynamics of the domain structure.