Secondary ferroic properties of ferroelectric crystals

Twenty five types of structural phase transitions that are accompanied by the appearance of a spontaneous optical activity in the low-temperature phase are analyzed. A technique is proposed for describing the enantiomorphic domain structure (orientation states) in these crystals.     

Polymorphism in ferroic functional elements

The present study describes an approach for the scale-bridging modeling of ferroic materials as functional elements in micro- and nanoelectronic devices. Ferroic materials are characterized by temperature-dependent complex ordering phenomena of the internal magnetic, electronic, and structural degrees of freedom with several involved length and time scales. Hence, the modelling of such compounds is not straightforward, but relies on a combination of electronic-structure-based methods like ab-initio and density-functional schemes with classical particle-based approaches given by Monte-Carlo simulations with Ising, lattice-gas, or Heisenberg Hamiltonians, which incorporate material-specific parameters both from theory and experiment. The interplay of those methods is demonstrated for device concepts based on electroceramic materials like ferroelectrics and multiferroics, whose functionality is closely related with their propensity towards structural and magnetic polymorphism. In the present case, such scale-bridging techniques are employed to aid the development of an organic field effect transistor on a ferroelectric substrate generated by the self-assembly of field-sensitive molecules on the surfaces of ferroic oxides. Electronic-structure-based methods yield the microscopic properties of the oxide, the surface, the molecules, and the respective interactions. They are combined with classical particle-based methods on a scale-hopping basis. This combination allows to study the morphology evolution during the self-assembly of larger adsorbate arrays on the (defective) oxide surface and to investigate the interplay of low-temperature magnetic ordering phenomena with the ferroelectric functionality at higher temperatures in multiferroic oxides like the hexagonal manganites. The combination of density-functional data with classical continuum modelling also yielded a model Hamiltonian for the quick determination of the properties of a gate structure based on bio-functionalized carbon nanotubes.     

Tensor tomography of residual stresses of hexagonal single crystals

A method for determining residual stresses in elongated transparent blanks of hexagonal single crystals the optical axis of which is directed along the crystal is proposed. It is assumed that the residual deformation tensor is of thermal character and is characterized by a fictitious temperature. Characteristic parameters of polarized light have been measured by the tomographic method in the plane orthogonal to the single crystal axis.     

Surface polarity induced domains in liquid crystals

Observations that the free surface of liquid crystals sometimes break up into a pattern of regularly spaced domains are explained as originating from surface polarity. Linear coupling to the surface polarity demands some splay deformation and under appropriate conditions, this will require disclinations. Similar effects may occur with biological membranes.     

Above-threshold dynamics of acoustic domains in liquid crystals

Owing to a nonquadratic spectrum, the Landau levels in graphene in crossed magnetic and electric fields should be affected by an electric field. Oscillations of magnetization in gapped and gapless graphene in crossed magnetic and electric fields are studied. The contribution of the Coulomb interaction to these oscillations is considered.     

Visualization of ferroelectric domains in bulk single crystals

In recent years ferroelectric domain patterning has become a popular topic of physical research because it enables photonic applications as well as data storage. For generation of tailored domain structures and for further understanding of ferroelectricity, a visualization of the domain patterns is required. A large number of imaging techniques have therefore been developed. This review summarizes these techniques and highlights systematically their strengths and weaknesses. PACS 77.84.-s; 78.20.-e; 68.37.-d     

Optical properties of acoustically induced domains in liquid crystals

Optical properties of acoustically induced domains in planar liquid crystal layers are considered: the diffraction of light and the focusing of light by domains.     

Dynamic drift of the magnetic domains in ferrite-garnet crystals

The drift of stripe-domain walls in a (111) ferrite-garnet plate with orthorhombic anisotropy and quality factor Q < 1 in an alternating magnetic field of 30–1000 Hz was investigated. The obtained experimental dependences of the domain wall drift velocity on the magnetic field frequency and amplitude are compared to the corresponding theoretical dependences.     

Elastic matching of phases and domains in KCN-type crystals

The present work contains a new information on the role of elastic matching in formation and rearrangement of complex domain structures as well as at phase coexistence in KCN, (KBr)_1-x (KCN)_x and (KCl)_1-x (KCN)_x crystals.     

Origin of magnetic and magnetoelastic tweedlike precursor modulations in ferroic materials

Based on experimental observations of modulated magnetic patterns in a Co0.5Ni0.205Ga0.295 alloy, we propose a model to describe a (purely) magnetic tweed and a magnetoelastic tweed. The former arises above the Curie (or Néel) temperature due to magnetic disorder. The latter results from compositional fluctuations coupling to strain and then to magnetism through the magnetoelastic interaction above the structural transition temperature. We discuss the origin of purely magnetic and magnetoelastic precursor modulations and their experimental thermodynamic signatures.     

Mechanism of drift of stripe magnetic domains in ferrite-garnet single crystals

The drift of stripe magnetic domains in (111) single-crystal ferrite-garnet plates in an alternating magnetic field (25–1000 Hz) has been investigated. The dependences of the drift velocity of stripe domains on the field frequency and amplitude have been obtained for magnetic fields of different orientations. A dislocation drift mechanism is proposed. Enhanced motion of magnetic dislocations in a system of stripe domains is established.     

Nanopowders of ferroic oxides for magnetoelectric composites

Piezoelectric oxides are currently being considered in combination with magnetic materials for the development of magnetoelectric composites, in which stress transfer across the interface is a key issue. In this context, we report here a detailed study of the mechanochemical activation processes of the ferroelectric perovskite BiScO₃–PbTiO₃ and of the ferrimagnetic spinel NiFe₂O₄. Highly sinterable, single-phase nanopowders of both ferroic oxides are synthesised, and used for the preparation of high-density materials and two-phase composites by hot pressing. Emphasis is put on studying chemical reactions at and interdiffusion across the interface between the two phases using high spatial resolution techniques such as micro X-ray diffraction and piezoresponse force microscopy. The feasibility of preparing magnetoelectric composites by this approach is demonstrated, for which the necessity of controlling physicochemical processes at the interface is key to obtain functionality.     

Role of nanosecond processes in the formation of domains in ferroelectric crystals

The contribution from pyroelectric and photovoltaic effects to the formation of light-induced domains in crystals of oxide ferroelectrics of lithium niobate type upon irradiation by nanosecond focused laser beams is considered. It is established that inverted domains are formed in irradiated regions under the combined effect of generated pyroelectric and photovoltaic fields with a sharp increase in temperature. It is shown that the most stable domains are formed in lithium niobate samples containing Fe²⁺ and Fe³⁺ impurity ions.     

Optically induced anisotropy and electrooptics in ferroic organic nanocomposites

The occurrence of linear electrooptics and optical anisotropy is observed in the ferroic crystals $[\hbox {NH}_{2}(\hbox {C}_{2}\hbox {H}_{5})_{2}]_{2} \hbox {CuCl}_{4 }$ [ NH 2 ( C 2 H 5 ) 2 ] 2 CuCl 4 embedded into polymethylmethacrylate polymer matrix. The studies were performed for the optically treated bulk crystals and the large-size nanocrystals (with sizes higher than 50 nm) embedded into the polymer matrix. The specimens were both in a form of bulk as well as in the form of the composite films. To understand the mechanisms giving the principal role to the effect we have performed polarized absorption measurements for the samples treated in the external dc-electric field and bicolour 532 nm/1,064 nm laser treatments. The behaviour of the birefringence and of the linear electrooptics coefficients were studied within a temperature range covering the incommensurate phase existence.     

Study of domains in globular photonic crystals by bragg reflection of light

The spatial distribution of the intensity of light reflected from the surface of a globular photonic crystal prepared on the basis of artificial opal was studied. Using a CCD array as a selective radiation receiver and a special computer program, reflection spectra were measured for photonic crystal regions with different colors, referred to as color domains. An analysis of the obtained spectra allows estimation of the homogeneity of the irradiated surface of the photonic crystal, relative areas of color domains, differences in crystal structures of individual color domains, and crystal structure defects.     

Linear domains induced in nematic liquid crystals by an acoustohydrodynamic action

Linear domains formed in a planar nematic liquid crystal layer under the effect of an oscillating hydrodynamic flow induced by audio-frequency shear oscillations are studied for an arbitrary angle between the flow velocity and the director describing the orientation of mesophase molecules in the layer. It is demonstrated that, in addition to the previously observed domains with a spatial period comparable to or greater than the layer thickness, spatially modulated structures with a period much smaller than the layer thickness are formed. The existence of the region of critical angles characterized by the most pronounced influence of the angle value on the distortion pattern of the planar layer is revealed.     

Secondary ferroic properties of partial mixed ferroelectric ferroelastics

Nineteen types of phase transitions in nonmagnetic crystals that exhibit only partial mixed ferroelectric-ferroelastic properties in the low-temperature phase are analyzed. A crystal-physical method is used to establish that all partial mixed ferroelectric ferroelastics are full ferroelastoelectrics and partial ferrobielastics except for the crystals undergoing the $\overline 4 3m \to 3$ phase transition, which results in the appearance of both full ferroelastoelectric and full ferrobielastic properties. The possible appearance of partial mixed ferroelectric-ferroelastic properties in perovskite-like crystals is discussed.     

The interactions between dislocations and ferroelectric domains in KNbO3 crystals

It is found that it is rather easy to introduce dislocations into KNbO₃. With transmission electron microscopy we have determined the slip plane to be (110) and the Burgers vector of dislocations to be [110]. Using hot stage in a JEM-200CX electron microscope, we have made in situ observations during phase transitions at 435°C (cubic-tetragonal) and 225°C (tetragonal-orthorhombic). We have found evidence indicating the interactions between dislocations and ferroelectric domains. Especially during phase transitions, the new ferroelectric phase first appear in the vicinity of dislocations showing that the stress field of dislocations may raise Curie point of the crystals.     

Optical retardation and structure of developable domains in tubular discotic liquid crystals

The simplest distorted texture of tubular discotic materials are “developable domains”. For small curvature, the tubes are expected to bend circularly around a straight line core. We have observed the birefringence fringes from such a texture in the hexapentoxytriphenylene, close to the isotropic transition. The optical retardation shows a cusp-like singularity above the core. It can be quantitatively described by the assumed circular texture for the tubes. The birefringence is Δn = 0.14 ± 0.01 for λ = 0.65 μm.