Strain control of domain-wall stability in epitaxial BiFeO3 (110) films

We have studied the stability of domains and domain walls in multiferroic BiFeO3 thin films using a combination of piezoelectric force microscopy and phase-field simulations. We have discovered that a film-substrate misfit strain may result in a drastically different thermodynamic stability of two parallel domain walls with the same orientation. A fundamental understanding of the underlying physics, the stress distribution in a domain structure, leads to a novel approach to control the ferroelastic domain stability in the multiferroic BiFeO3 system.     

Investigation of domain behaviour in 0.7Pb(Mg1/3Nb2/3) O3-0.3PbTiO3 single crystals at different temperatures in the transmission electron microscope

Using transmission electron microscopy, in-situ changes in ferroelastic domains in 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ single crystals were observed at 60 to -163°C. At -163°C, the microscopic tweed morphology of the ferroelastic domains rotated by 90°, and certain orientation changes in the mesoscopic sawtooth domains took place. At this temperature, the ferroelastic domains became coarsened and certain S-shaped mesoscopic domains were reshaped. The disappearance and/or changes in the orientations of both the sawtooth and the ferroelastic domains were reversible upon return to room temperature. On heating to 60°C, both the mesoscopic sawtooth and the microscopic tweed domains were stable below 53°C. Above 53°C, the mesoscopic walls disappeared and the contrast of the tweed domains became blurred. Upon return to room temperature from 60°C, the mesoscopic domain patterns could not be retrieved, indicating that the transformation was irreversible. The morphology of the tweeds at this temperature indicated a structural transition from a two-variant domain state to a multivariant state, eliminating mesoscopic boundaries.     

Ferroelastic domain freezing and low-frequency elastic behaviour of [Formula: see text] (ALHS)

In this work elastic measurements on [Formula: see text] (ALHS) which were carried out in the low-frequency range between 1 and 50 Hz are presented. The temperature dependence of the inverse elastic compliance [Formula: see text] has been determined between 90 K and 420 K. Distinct anomalies have been found in the temperature dependence of [Formula: see text], which are connected to the motion of domain walls in the ferroelastic phase below [Formula: see text]. Around [Formula: see text] a (partial) ferroelastic `domain freezing' phenomenon has been observed. To the knowledge of the authors this is the first time that pure ferroelastic domain freezing has been reported. However, below [Formula: see text] the domain walls seem to retain a certain vibrational degree of freedom which could be responsible for an additional anomaly of the loss modulus which was observed. The elastic behaviour of a crystal of ALHS is dependent on the `history' of the given sample. During temperature cycling [Formula: see text] shows differences between the first run of heating and cooling and later runs. Finally, some basic insights concerning the domain wall motion were obtained; it was found that the domain wall mobility decreases by three orders of magnitude in the temperature region 170 - 230 K.     

Ferrimagnetic/ferroelastic domain interactions in magnetite below the Verwey transition: Part II. Micromagnetic and image simulations

Micromagnetic simulations have been used to explore the interaction between ferrimagnetic domain walls (DWs) and ferroelastic twin walls (TWs) below the Verwey transition in magnetite (Fe₃O₄). Simulations were performed using a thin-foil geometry in order to replicate the domain patterns observed experimentally using transmission electron microscopy. The magnetic microstructure is shown to be highly sensitive to the physical dimensions and crystallographic orientation of the foil, the spatial distribution and crystallographic classification of the TWs and the temperature/field history of the sample. A method to calculate the phase shift of a beam of electrons passing through the micromagnetic simulations is applied. The resulting phase maps provide a robust interpretation of experimental images obtained using Fresnel-mode Lorentz microscopy and off-axis electron holography. The interaction between ferrimagnetic and ferroelastic DWs during field cycling provides an explanation for the low-temperature ‘field-memory effect’ in magnetite.     

五磷酸钕晶体中铁弹畴界的X射线形貌研究

利用X射线衍射形貌法,研究了五磷酸钕(NdP₅O₁₄)晶体中铁弹畴界的衍射衬度。发现在μt=0.6—7范围内,畴界呈现黑的或白的衬度。畴界衍射衬度的特征可归结为(c₂-c₁=)△c∥g畴界衬度明显△c⊥g畴界衬度消失△c·g>0畴界呈黑衬度△c·g<0畴界呈白衬度利用Penuing-Polder波点迁移原理,对畴界的衬度形成原理做了定性解释。根据畴界的衍射衬度特征,定性讨论了畴界的结构特征。 关键词：     

Functional twin boundaries

Functional interfaces are at the core of research in the emerging field of ‘domain boundary engineering’ where polar, conducting, chiral, and other interfaces and twin boundaries have been discovered. Ferroelectricity was found in twin walls of paraelectric CaTiO₃. We show that the effect of functional interfaces can be optimized if the number of twin boundaries is increased in densely twinned materials. Such materials can be produced by shear in the ferroelastic phase rather than by rapid quench from the paraelastic phase.     

Electronic absorption spectrum of Cs<Subscript>2</Subscript>ZnI<Subscript>4</Subscript> thin films

The absorption spectrum of Cs₂ZnI₄ thin films in the energy range 3–6 eV at temperatures from 90 to 340 K has been investigated. It is established that this compound belongs to direct-gap insulators. Low-frequency exciton excitations are localized in ZnI₄ structural elements of the lattice. Phase transitions at 280 K (paraelectric phase ? incommensurate phase), 135 K (incommensurate phase ? monoclinic ferroelastic phase), and 96 K (monoclinic phase ? triclinic ferroelastic phase) have been found from the temperature dependences of the spectral position and halfwidth of the low-frequency exciton band. Additional broadening of the exciton band is observed for ferroelastic phases; it is likely to be due to exciton scattering from strain fluctuations near domain walls.     

Equations for domain walls in a coordinate system of the ferroelectric phase

Two methods are suggested for writing equations for domain walls in a coordinate system of the ferroelectric phase in ferroelastics and multiaxial ferroelectrics. The equations for domain walls in ferroelectric barium titanate and ferroelastic lead orthophosphate are derived. It is shown that suborientation states are possible in these crystals. The suggested methods make it possible to find the matrices of the transformation from the coordinate system of the paraelectric phase to a coordinate system of the ferroelectric phase for each orientation state.     

Interaction between free boundaries and domain walls in ferroelastics

We study the relaxation of ferroelastic domain walls in the vicinity of a free surface, by means of a nonlinear continuum elastic model treated with finite elements on an adaptive grid. Domain walls bend towards the free surface, as a consequence of the interplay of the energy per unit length of the domain wall and the long-range elastic strains which are generated by deviations from the prescribed compatible orientation. We also analyze the order parameter on the free surface. For walls orthogonal to the free surface we find, in accordance with previous studies, a double-peak structure. For different angles the picture is more complex, and in some cases only one small peak survives.Received: 14 July 2004, Published online: 21 October 2004PACS: 68.35.Gy Mechanical properties; surface strains - 61.72.Mm Grain and twin boundaries - 62.20.Dc Elasticity, elastic constants     

Changes in the optical properties at phase transitions in TEA2MeCl4 (Me=Zn, Mn, Hg, Cu) crystals

Optical properties of TEA₂MeCl₄ crystals, where Me is a divalent metal ion, have been studied under polarization microscope in temperatures ranging from 200 to 300 K. Abrupt changes in the interference colours and strong cracking of the sample have been observed for all crystals except the TEA₂CuCl₄ one, for which only brightening of the plate image has been noted. The cracks are approximately parallel to the cleavage planes. These changes occur in two stages. The brightening of the images of the crystal samples may suggest the ferroelastic deformation in microregions. No classical ferroelastic domain structure has been observed in the low-temperature phase.     

Shape-memory effect in PMN-PT(65/35) ceramic

We report the observation of fairly large and fully recoverable shape-memory strain in thin bars of lead magnesium niobate lead titanate ceramic for a composition (65/35) near its morphotropic phase-boundary. The recoverable shape strain produced by bending and thermal-cycling experiments is ∼0.3%, similar in magnitude to that reported for the shape-memory effect (SME) observed in some other dielectric ceramics. An explanation of the observed SME and the recoverable shape-strain is given in terms of the ferroelastic phase transitions, and the availability of a large number of competing phases and domain states near the morphotropic phase boundary in this material. The stress-induced shifting of the temperatures at which the ferroelastic phase transitions occur also plays an important role in ensuring good crystallographic reversibility around the thermal-cycling experiments.     

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.     

Optical studies of the ferroelastic phase transitions in KFe(MoO4)2

ABSTRACT

The ferroelastic phase transitions in KFe(MoO₄)₂ have been studied by means of polarized light microscopy. The crystal undergoes a sequence of ferroelastic phase transitions. It has been found that the second transition consists of two transitions separated by the temperature interval of about 0.4 K. Both these transitions are of the first order and are evidenced through a phase front passing, without the domain structure rebuilding. The disposition of optical indicatrix axes n_g, n_m has been established, and the birefringence has been measured in the plane (0001) in the temperature range covering all ferroelastic phases. From temperature studies of the morphic birefringence, a critical exponent of the order parameter has been estimated.     

Strain-gradient-induced polarization in SrTiO3 single crystals

Piezoelectricity is inherent only in noncentrosymmetric materials, but a piezoelectric response can also be obtained in centrosymmetric crystals if subjected to inhomogeneous deformation. This phenomenon, known as flexoelectricity, can significantly affect the functional properties of insulators, particularly thin films of high permittivity materials. We have measured strain-gradient-induced polarization in single crystals of paraelectric SrTiO3 as a function of temperature and orientation down to and below the 105 K phase transition. Estimates were obtained for all the components of the flexoelectric tensor, and calculations based on these indicate that local polarization around defects in SrTiO3 may exceed the largest ferroelectric polarizations. A sign reversal of the flexoelectric response detected below the phase transition suggests that the ferroelastic domain walls of SrTiO3 may be polar.     

一种新的C_(60)二维畴拓扑结构

利用低温扫描隧道显微镜对吸附在硫醇膜表面的二维C60岛进行研究,首次观察到化学键分辨的C60分子结构,并发现一种新型的C60二维取向畴界.这种畴界仅仅由于两边C60分子的取向不同而存在,附近没有结构缺陷,畴界附近C60阵列的位置平移序和键取向序都得到了保持.     

Polarized light microscopy of the ferroelastic domains of YBa2Cu3O7−x

A short overview is given on recent polarized light microscopy studies of the orthorhombic ferroelastic domains and domain structures of YBa₂Cu₃O_7-x . The optical characteristics of large domains and lamellar domains with a high density of twin walls are pointed out, both for reflected and transmitted light. The polarizing microscope has allowed to observe the ferroelastic detwinning in situ and to determine the activation energy of the ferroelastic wall movements. The question of a potential ferroelectric phase transition is also discussed.     

Proton dynamics in letovicite, (NH4)3H(SO4)2: a 1H and 14N NMR spectroscopic study

The improper ferroelastic phase letovicite (NH₄)₃H(SO₄)₂ has been studied by ¹H MAS NMR as well as by static ¹⁴N NMR experiments in the temperature range of 296–425 K. The ¹H MAS NMR resonance from ammonium protons can be well distinguished from that of acidic protons. A third resonance appears just below the phase transition temperature which is due to the acidic protons in the paraelastic phase. The lowering of the second moment M₂ for the ammonium protons takes place in the same temperature range as the formation of domain boundaries, while the signals of the acidic protons suffer a line narrowing in the area of T_c. The static ¹⁴N NMR spectra confirm the temperature of the motional changes of the ammonium tetrahedra. Two-dimensional ¹H NOESY spectra indicate a chemical exchange between ammonium protons and the acidic protons of the paraphase.     

High-temperature memory in multiferroic cobalt–bromine boracite

The observation of the domain dynamics of the ferroelectric–ferroelastic domains near the paraelectric–ferroelectric phase transition is reported. The study was performed by means of high-temperature polarized light microscopy. Close to the phase transition an increase in the number of domain walls was observed. After several heating–cooling cycles around the transition temperature, a memory domain microstructure at high temperature was observed. The possible physical mechanisms responsible for the high-temperature domain dynamics are discussed. Using a complex systems approach the complexity of the domain behavior is characterized.     

Magnetic domain structure of anisotropic Nd2Fe14B-based magnets produced via the hydrogenation,decomposition, desorption and recombination (HDDR) process

Lorentz transmission electron microscopy and optical Kerr microscopy studies of the hydrogenation, decomposition, desorption and recombination (HDDR)-processed anisotropic Nd₂Fe₁₄B-based magnets show that a magnetic domain structure develops in which a unit domain includes a number of submicron Nd₂Fe₁₄B grains coupled by the intergrain-exchange interaction. Grain boundaries and particles of an intergranular phase have been observed to pin the domain boundary walls, and their magnetic characteristics and distribution may determine the size of magnetic domains forming in a thermally demagnetized state. Measurements of differential susceptibilities indicate that the motions of the domain walls become sluggish after magnetization and dc-field demagnetization, compared to that in a thermally demagnetized state.     

Phase diagram and domain structure of the Rb2 x Tl2(1− x )Cd2(SO4)3 solid solutions

From measurements of temperature dependences of the birefringence and from observations of the domain structure the x, T–phase diagram for the Rb_{2 x} Tl_{2(1? x )}Cd₂(SO₄)₃ solid solutions have been obtained. Ferroelastic domain structure and movement of phase boundaries have been studied in the course of phase transitions in Rb_{2 x} Tl_{2(1? x )}Cd₂(SO₄)₃ crystals. Possible reasons for the appearance of mechanically non-compatible (‘forbidden’) ferroelastic domain walls are given.