Formation Of Domain Structures Research Articles

Screening and retardation effects on180°-domain wall motion in ferroelectrics: Wall velocity and nonlinear dynamics due to polarization-screening charge interactions

The effect of the domain wall intrinsic width, relaxation time of the screening charges, and the dead layer thickness on the velocity of the planar $180\ifmmode^\circ\else\textdegree\fi{}$-domain wall moving under homogeneous external electric field in ferroelectric capacitor is analyzed. The limiting cases of domain wall motion, including (i) the motion induced by the external and local internal field originated at the wall-surface junction for nonzero dead layer thickness and (ii) the motion induced by the effective electric field averaged over the domain wall surface, are considered. We demonstrate the crossover between two screening regimes: the first one corresponds to the low domain wall velocity, when the wall drags the sluggish screening charges, while the second regime appears for high domain wall velocity, when the delay of sluggish screening charges are essential and the wall depolarization field is screened by the instant free charges located at the electrode. The integral and approximate analytical expressions for electric field and algebraic equation for the domain wall velocity are derived. It is shown that in the local-field limit the motion can be unstable, since the internal field at the wall-surface junction decreases for larger domain wall velocities, making possible self-acceleration of the wall near the top surface. The instability may lead to the domain wall-surface bending and actual broadening in thick samples, as well as formation of periodic domain structures in the direction of wall motion. The motion in the limit of the averaged effective field is always stable.

Formation of phase domain structures in thin films under conditions of a first-order magnetic phase transition

This paper reports on the results of a theoretical investigation into the magnetic and resonance properties of thin films in the range of the transition from a paramagnetic state to a ferromagnetic state in the case where the magnetic transition is a first-order phase transformation. It is demonstrated that, in an external magnetic field directed perpendicular to the film plane, the formation of a specific domain structure consisting of domains of the coexisting paramagnetic and ferromagnetic phases can appear to be energetically favorable. The parameters of the equilibrium system of stripe phase domains and their dependences on the temperature, the magnetic field, and the characteristics of the material are calculated. The specific features of the magnetic resonance spectra under the conditions of formed stripe phase domains are considered. A relationship is derived for the dependence of the resonance field of the system of ferromagnetic domains on the magnetization and temperature. It is shown that the alternating external field can fulfill an orientation function in the formation of stripe phase domains.

Magneto-microstructural coupling during stress-induced phase transformation in Co49Ni21Ga30 ferromagnetic shape memory alloy single crystals

The present study reports on direct magneto-microstructural observations made during the stress-induced martensitic transformation in Co49Ni21Ga30 alloy single crystals with optical, scanning electron, and magnetic force microscopy (MFM). The evolution of the microstructure and the associated magnetic domain morphology as a function of applied strain were investigated in the as-grown condition and after thermo-mechanical training. The results demonstrated that the stress-induced martensite (SIM) evolves quite differently in the two conditions and depending on the martensite formation mechanisms, the magnetic domain configuration was dissimilar. In the as-grown crystals two twin-related martensite variants were formed and the growth of these twin variants resulted in large strain. After thermo-mechanical training a morphology similar to a self-accommodating martensite structure was present at the initial stages of the transformation and thereafter martensite reorientation (MR) was the main transformation mechanism. The magnetic domains were found to be superimposed on the nano-scaled martensite twins in the as-grown condition, whereas training brought about the formation of domains on the order of a few microns without showing the one-to-one correspondence between domains and the twin structure. After the thermo-mechanical training detwinning at high-strain levels led to the formation of stripe-like domain structures. The ramifications of the results with respect to the magneto-microstructural coupling that may cause the magnetic shape memory effect (MSME) in Co–Ni–Ga alloys under constant external stress is addressed.

Local Concentration of Gel Phase Domains in Supported Lipid Bilayers under Light Irradiation in Binary Mixture of Phospholipids Doped with Dyes for Photoinduced Activation

Recently, lipid bilayers supported on solid substrates are considered to offer potential as biological devices utilizing biological membranes and membrane proteins. In particular, artificially patterned supported bilayers hold great promise for the development of biological devices. In this study, we show control of the formation and location of phase-separated domain structures by light irradiation for gel phase and liquid-crystalline phase separation structures in a DMPC-DOPC binary lipid bilayer tagged with dye molecules on SiO2/Si substrates. Upon light irradiation, the gel phase domain structures disappeared from the phase-separated bilayers. This disappearance indicates that the light irradiation causes a local increase in the temperature of the lipid bilayer. In this disappearance phenomenon, the photoinduced activation of dye lipids, e.g. fluorescent lipids, is considered to play an important role, since the same phenomenon does not occur in lipid bilayers that have a low concentration of dye lipids. Thus, the local increase in temperature is propagated by light absorption of the dye lipid and subsequent photoinduced activation of nonradiative molecular vibrations. Subsequent interruption of the photoinduced activation for molecular motion allowed the gel phase domain structures to precipitate and grow again. Moreover, the domain area fraction remaining after the photoinduced activation was higher than that before the photoinduced activation. This result indicates that the local increase in temperature propagated by dye-excitation enhances formation of the gel phase domains. By utilizing this phenomenon, we could preferentially induce formation of domain structures within the light-irradiated regions. This technique could be the basis for a new patterning technique based on domain structures. Moreover, these domain structure patterns can be eliminated by increasing the temperature, allowing rewritable patterning.

Self-organized and high-density filamentous nanodomain patterns fabricated in lithium niobate by discharge poling

We report on spontaneous formation of nanoscale surface domains obtained in lithium niobate substrates by electric field discharge poling. A constant high voltage was applied at room temperature to virgin and unpatterned crystals in air, by using metal tip-shaped electrodes. An enhancement of the electric field in correspondence of the electrode tips was expected due to the fringing field effect, and electric discharge was observed during the process. The standard wet etching in hydrofluoric acid solution revealed the formation of nanoscale domain structures that exhibit filamentous self-organized geometries. Topographic measurements of the fabricated structures were performed by an atomic force microscope, and the corresponding results are presented and discussed. The investigation provides remarkable information about the shortest domain size possibly obtainable by electric field poling.

Kinetics of B2- and D03-type ordering and formation of domain structures in Fe–Al alloys

Time-dependent Ginzburg–Landau (TDGL) formulation has been developed for the ordering processes ofB2 and D03 types in binary alloy systems. In the formulation, three order parameters aredefined in order to describe the state of order. Equivalent variants of B2 andD03 structures are distinguished using these order parameters. The mean-field free energyis defined in the form of a Landau-type expansion using the order parametersand a composition parameter. Interface energies due to local variations in thedegrees of order and concentration are given with a gradient square approximation.Kinetic equations are derived from the Ginzburg–Landau-type potential in orderto describe the time-evolutions of the order parameters and the concentration.Numerical simulations of the kinetic equations have been performed for B2- andD03-type ordering as well as concurrent ordering and phase separation to disorderedA2+D03. The simulated results provide a good reproduction of the formation processes of B2 andD03 ordereddomains in an Fe3Al alloy.

Controlling the formation of gold nanoparticle domains onto inorganic substrates

The ability to control the spatial arrangement of molecular nanocrystals is of fundamental importance for the fabrication of nanoscale devices. In this work, we report on experimental procedures to control the formation of 50 nm, organically capped, gold nanoparticles by tuning the deposition conditions. We found the parameters which lead to the formation of different domain structures, from three-dimensional aggregates to two-dimensional structures, to even isolated particles.

Nanoscale Domain Effects in Ferroelectrics. Formation and Evolution of Self-Assembled Structures in LiNbO3 and LiTaO3

We discuss the most interesting nanoscale domain effects resulting in formation and evolution of self-assembled domain structures in various representatives of LiNbO 3 and LiTaO 3 family. Several exotic domain kinetic scenarios for complete suppression of the classical wall motion have been revealed. It has been shown that the modification of the surface layer allows to obtain: (1) the wall motion controlled by merging with nanoscale domains generated in front of the moving wall and (2) the discrete switching by enlargement of the web-like self-assembled ensemble of isolated domains. The formation of nanoscale quasi-periodic and self-similar structures during cooling after pulse heating without any application of external electric field was studied. It has been shown that all experimental results can be explained, if the retardation of the bulk screening of the depolarization field is taken into account.

Effect of dimensions and anisotropy on the formation of domain structures of uniaxial single crystals

Distribution of magnetization in infinitely long single-crystal prisms of Co, Nd2Fe14B, and Ni80Fe20 with different cross sections has been calculated using the method of minimization of the free-energy functional and by solving the Landau-Lifshitz equation. It has been shown that in cobalt single crystals, domain structures of the Landau type arise. In Nd2Fe14B single crystals the domain structures have a stripe type; in Ni80Fe20 single crystals, the domain structure is in a single-domain state.

Tin Compounds Interaction with Membranes of Egg Lecithin Liposomes

This work is a continuation of earlier research concerning the influence of tin compounds on the dynamic properties of liposome membranes produced with lecithin hen egg yolks (EYL). The experiments were carried out at room temperature (about 25 degrees C). Four tin compounds were chosen, including three organic ones, (CH3)4Sn, (C2H5)4Sn and (C3H7)3SnCl, and one inorganic, SnCl2. The investigated compounds were admixed to water dispersions of liposomes. The content of the admixture changed within the range 0 mol-% to 11mol-% in proportion to EYL. Two spin probes were used in the experiment: 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 2-ethyl-2-(15-methoxy-15-oxopentadecyl)-4,4-dimethyl-3-oxazolidinyloxyl (16-DOXYL-stearic acid), which penetrated through different areas of the membrane. It was found that tin compounds containing chlorine were the most active in interaction with liposome membranes. In the case of (C3H7)3SnCl, after exceeding 4% admixture content, an additional line appeared in the spectrum of the TEMPO probe which can be a result of formation of domain structures in the membranes of the studied liposomes. Compounds containing chlorine are of ionized form in water solution. The obtained results can thus mean that the activity of admixtures can be seriously influenced by their ionic character. In case of an admixture of non-ionic compounds the compound with a longer hydrocarbon chain displayed a slightly stronger effect on the spectroscopic parameters of the probes.

The role of Jahn-Teller ions in the optical formation of domains in lithium niobate

Local repolarization processes were studied in initially single-domain iron-ion-containing LiNbO3 crystals exposed to focused laser radiation. The dependence of the formation of local domains and periodic domain structures on the temperature and the applied electric field strength was determined. It was shown that these effects are controlled by the total concentration of iron ions and the ratio of the Fe2+ and Fe3+ concentrations.

Resistance behavior and magnetization reversal analysis of individualConanowires

Since the direct observation of magnetization reversal processes in nanostructured systems is hard to achieve, we use magnetoresistance measurements as an indirect sensing tool. To avoid dipolar interactions present in nanowire gratings individual $\mathrm{Co}$ nanowires are prepared by electron beam lithography on $\mathrm{Si}$ substrates. Using a special two step process nonmagnetic gold contacts are attached to the $\mathrm{Co}$ nanowires to exclude an influence of the contact structures on the magnetization reversal process. To support the magnetoresistance analysis also the structure and morphology as well as the magnetic properties of the $\mathrm{Co}$ nanowires have been characterized using various microscopy and measurement techniques. Depending on the direction of the applied field three different magnetoresistance measurements can be distinguished. Based on the anisotropic magnetoresistance effect the transversal and the polar magnetoresistance reflect a coherent rotation of the magnetization independent of the wire width. In contrast, the longitudinal magnetoresistance shows a wire widths dependence of the magnetization reversal processes. In small wires nucleation processes occur whereas in wide wires the formation of complex domain structures has been observed. Nanowires with a width in between these groups show ``transition''-like reversal processes.

Cascading third-order Raman process and local structure formation in binary liquid mixtures of benzene and n-hexane

The cascading third-order Raman process in binary mixtures of benzene and n-hexane was studied by six-wave mixing coherent anti-Stokes Raman scattering spectroscopy. By examining the concentration dependence of the cascading third-order signal intensity, we investigated the formation of local structures of benzene in the binary mixtures. A significant deviation from the dependence expected for homogeneous mixtures was observed at benzene concentrations above 7 mol dm(-3). This deviation can be interpreted in terms of optical inhomogeneity caused by the formation of domain structures of benzene molecules. We discuss the feasibility of the cascading third-order process as a sensitive probe for the microscopic structures that are formed in liquids and solutions.

Transmission electron microscopy and structural phase transitions in anion-deficient perovskite-based oxides

This review deals with the structural transitions arising in anion-deficient Mn- and Cu-based perovskites when changing the anion content. Special attention is devoted to the local structure of such materials, studied with electron diffraction and high-resolution electron microscopy. Using examples of three-dimensional or layered manganites and cuprates, it is shown that oxidation/reduction reactions in such compounds are accompanied by the formation of domain structures. Fragmentation into domains can have intrinsic reasons, related to the loss of symmetry elements due to the ordering of anion vacancies, or because of the simultaneous realization of several different anion ordering patterns with close formation energies. When extra anions are inserted at low temperatures, limited diffusion ability also results in the appearance of domains of different structures arising from a local inhomogeneity in the anion distribution. The relevance of the investigation of the local structure for a correct structure interpretation is discussed.

Temperature dependence of magnetization reversal in Co and Fe3O4 nanowire arrays

In this paper, we investigate the magnetization reversal of cobalt and magnetite nanowires, 4nm in diameter, synthesized within the pores of mesoporous silica thin films. A SQUID magnetometer was used to study the magnetic properties of the nanowire arrays over a broad temperature interval, T=1.8–300K. The magnetization reversal process was found to be strongly temperature dependent. While a coherent rotation may occur at room temperature, a process involving the formation of domain structures takes place as the temperature decreases down to 1.8K.

Anisotropy of exchange stiffness and its effect on the properties of magnets

Using the spin-spiral formulation of the tight-binding linear muffin-tin orbital method, the principal components of the exchange stiffness tensor are calculated for typical hard magnets including tetragonal CoPt-type and hexagonal YCo 5 alloys. The exchange stiffness is strongly anisotropic in all studied alloys. This anisotropy makes the domain wall surface tension anisotropic. Competition between this anisotropic surface tension and magnetostatic energy controls the formation and dynamics of nanoscale domain structures in hard magnets. Anisotropic domain wall bending is described in detail from the general point of view and with application to cellular Sm–Co magnets. It is shown that the repulsive cell-boundary pinning mechanism in these magnets is feasible only due to the anisotropic exchange stiffness if suitably oriented initial pinning centers are available. In polytwinned CoPt-type magnets the exchange stiffness anisotropy controls the orientation of macrodomain wall segments. These segments may reorient both statically during microstructural coarsening and dynamically during the macrodomain wall splitting in external field. Reorientation of segments may facilitate their pinning at antiphase boundaries.

Anisotropy of exchange stiffness and its effect on the properties of magnets

Using the spin-spiral formulation of the tight-binding linear muffin-tin orbital method, the principal components of the exchange stiffness tensor are calculated for typical hard magnets including tetragonal CoPt-type and hexagonal YCo5 alloys. The exchange stiffness is strongly anisotropic in all studied alloys. This anisotropy makes the domain wall surface tension anisotropic. Competition between this anisotropic surface tension and magnetostatic energy controls the formation and dynamics of nanoscale domain structures in hard magnets. Anisotropic domain wall bending is described in detail from the general point of view and with application to cellular Sm–Co magnets. It is shown that the repulsive cell-boundary pinning mechanism in these magnets is feasible only due to the anisotropic exchange stiffness if suitably oriented initial pinning centers are available. In polytwinned CoPt-type magnets the exchange stiffness anisotropy controls the orientation of macrodomain wall segments. These segments may reorient both statically during microstructural coarsening and dynamically during the macrodomain wall splitting in external field. Reorientation of segments may facilitate their pinning at antiphase boundaries.

Atomic force microscopy studies of lateral phase separation in mixed monolayers of dipalmitoylphosphatidylcholine and dilauroylphosphatidylcholine

Atomic force microscopy imaging of dipalmitoylphosphatidylcholine (DPPC)/dilauroylphosphatidylcholine (DLPC) monolayers deposited onto alkanethiol modified-gold surfaces by the Langmuir–Schaefer technique was used to investigate domain formation in a binary system where phase separation arises from a difference in the alkyl chain lengths of the lipids. We have established how the condensed domain structure (shape and size) in DPPC/DLPC monolayers depends on the surface pressure and lipid composition. The mixed monolayers exhibit a positive deviation from an ideal mixing behavior at surface pressures of ⩽32 mN/m. Lateral compression to pressures greater than the liquid-expanded-to-liquid-condensed (LE-to-LC) phase transition pressure of the mixed monolayer (∼8–16 mN/m) induces extensive separation into condensed DPPC-rich domains and a fluid DLPC matrix. The condensed structures observed at a few milliNeutons per meter above the LE-to-LC transition pressure resemble those reported for pure DPPC monolayers in the LE/LC co-existence region. At a bilayer equivalence pressure of 32 mN/m and 20 °C, condensed domains exist between x DPPC ∼0.25 and ∼0.80, analogous to aqueous DPPC/DLPC dispersions. Compression from 32 to 40 mN/m results in either a striking distortion of the DPPC domain shape or a break-up of the microscopic DPPC domains into a network of nanoscopic islands (at higher DPPC mol fractions), possibly reflecting a critical mixing behavior. The results of this study provide a fundamental framework for understanding and controlling the formation of lateral domain structures in mixed phospholipid monolayers.

Influence of weak pulsed magnetic fields on triglycine sulfate crystals

The influence of weak (≤0.02 T) pulsed magnetic fields on the ferroelectric and dielectric characteristics of nominally pure triglycine sulfate crystals was detected for the first time. A short-term (seconds) pulsed magnetic impact caused long-term (hundreds of hours) changes in the coercive field and the temperature dependences of the dielectric constant, the dissipation factor, and the relaxation time of the dielectric constant near the ferroelectric phase transition. It was assumed that the effects detected were caused by unpinning of domain walls and dislocations from stoppers, followed by the formation of new defect and domain structures.

Size dependence of magnetic domain patterns in exchange-biased Permalloy/NiO microstructures

The magnetic domain structure in Permalloy (Ni81Fe19) micropatterns (10–100 μm) on NiO has been investigated by means of soft x-ray photoemission electron microscopy. The exchange anisotropy between the Ni81Fe19 patterns and the NiO layer results in the formation of complex domain structures which markedly differ from the simple Landau–Lifshitz configurations. The domain structures reflect the competition between the exchange anisotropy and the dipole–dipole interaction in a weakly coupled system. The observed domain structures change with the feature size, as the domain patterns lose complexity in the smaller structures.