Bipolar-homogeneous structural phase transition in nematic droplets formed in the polymer matrix in a magnetic field

The phase transition from the bipolar structure to the homogeneous structure in droplets formed from the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl in poly(vinyl butyral) in the presence of a magnetic field has been investigated. The phase transition is associated with the expansion of the regions with a tilted orientation of the director in the vicinity of defects (the disappearance of boojums) and with the alignment of the nematic director lines in the bulk of the droplet in the direction of the magnetic field. In the temperature range T = 24−34°C, cyclic mutual transformations between the bipolar configuration of the nematic director and the homogeneous structure, which have a period of ∼0.5–3.5 s and result from thermal fluctuations of the order parameter, are observed in droplets 3–15 μm in size after the forming field is switched off.     

Magnetic Field Dependence of Laser Light Scattering through CBOOA in Nematic Phase

In the nematic phase of homeotropically aligned liquid crystals which have the smectic A phase at lower temperature, interference rings have been observed above some threshold magnetic field H_c which nearly coincides with the ocurrence of Freedericksz transition, only for the light polarized parallel to the direction of magnetic field. This interference ring disappears with increasing magnetic field above second threshold H_s . The origin of these interference rings is explained tentatively by a periodic deformation of bulk directors.

The bend elastic constant of CBOOA estimated from the measurements of the interference ring as a function of temperature, diverges with the critical exponent (0.5 ± 0.02) near the smectic A-nematic transition temperature and agrees well with a recent suggestion of McMillan based on the mean field theory.     

On the problem of the “Colossal” magnetoelectric effect

In connection with the recent experiments on manganites, it is shown in the framework of the Landau theory that the magnetoelectric effect appears under a critical magnetic field. The signs of the spinorbit interaction constant and the helix wave number are interrelated. A helical structure and weak ferromagnetism occur simultaneously due to the first-order phase transition at a critical temperature, which depends on the magnetic field. The temperature range of the effect increases with the field. In this range, the magnetic moment increases and the correction to the electric polarization, being negative, also increases in magnitude with the magnetic field above its threshold. These changes are directly related to the increase in the helix wave number with a decrease in temperature below the phase transition point.     

Field and Frequency Effects in a Nematic Mixture of Negative and Positive Dielectric Anisotropy

The molecular alignment in the nematic liquid crystal mixture of p-ethoxy benzylidene p′-n-butyl aniline (EBBA) and p-ethoxy benzylidene p′-cyano aniline (EBCA) has been investigated by measuring the dielectric constant at 1 MHz. Detailed investigation has been carried out for a composition of 2.74%, of EBCA in EBBA for different electric and magnetic field strengths and at different frequencies of the applied electric field. Both parallel and perpendicular electric and magnetic field configurations have been employed. Temperature and frequency dependence of the dielectric anisotropy have been studied.

Two interesting features have been observed. I) In the crossed field configuration complete molecular alignment could be observed by suitably changing the electric field strength. The range of electric field strength required to change the molecular ordering, is frequency dependent. This range shows a minimum in the neighbourhood of 2 kHz. 2) It has also been observed that in the parallel field configuration minimum dielectric constant change is observed apparently independent of frequency and electric field strength. Detailed results of the field and frequency effects and of optical studies in this mixture are reported.     

Dependence of Electrical Conductivity (σ) and Activation Energy of Lithium Ferrite on Sintering Temperature and Porosity

The electrical conductivity (σ) of lithium ferrite sintered at 950, 1000, 1050 and 1100 °C was investigated in the temperature range of 300 to 1000 K. Three distinct regions have been observed in log (σT) vs 10³/T curves for four samples of lithium ferrite sintered at different temperatures. The conduction in the first region is due to impurities. In the second and third region is due to ordered and disordered state of the material. The transition from the first region to the second region is due to lowering of symmetry. The transition from second to third region is due to magnetic transition, i.e. ferrimagnetic to paramagnetic state. The transition temperatures are nearly equal to the Curie temperatures of the materials. The porosity and activation energy were calculated. It was found that the electrical conductivity is progressively increasing with increase of sintering temperature while the porosity and activation energy decrease continuously. The Seebeck coefficient (Q), carrier concentration (n) and mobility (μ) of charge carriers have been discussed as a function of sintering temperature and temperature.     

Field-Induced Order in Nematic Liquid Crystals

The effect of an external magnetic field on the orientational order of a nematic liquid crystal has been examined using both Landau-de Gennes and Maier-Saupe theories. In the Maier-Saupe approach a rotationally invariant form of the pseudo-potential is introduced, which in the absence of an external field leads to three degenerate isomorphic solutions for the order parameter, corresponding to alignment along three principal axes; a similar result is obtainable from the Landau-de Gennes theory. Application of a magnetic field lifts the degeneracy of these solutions, and for materials having a positive diamagnetic susceptibility anisotropy, the uniaxial solution with alignment along the field direction is always energetically favorable. For materials with a negative susceptibility anisotropy, a biaxial solution minimizes the free energy at low temperatures, but on increasing the temperature there is a transition from a biaxial phase to an uniaxial phase. The field dependence of the transition temperatures is evaluated, and for positive materials there is a critical field, corresponding to a second order transition above which the nematic and isotropic phases are indistinguishable. A contrasting behavior is predicted for negative materials, and above a certain critical field the biaxial/uniaxial transition changes from first order to second order. For weakly ordered systems it is shown that the Landau-de Gennes expression for the free energy is identical to that obtained from the Maier-Saupe theory. However, for more ordered systems, the results of the two approaches differ, and in particular the Maier-Saupe theory predicts a susceptibility divergence temperature T* which increases with applied field, in agreement with recent experiments.     

Modification of Fluid Flow and Heat Transport in Vertical Bridgman Configurations by Rotating Magnetic Fields

Applying a rotating magnetic field to an electrically conducting liquid, a Lorentz force is induced which generates a melt rotation of a certain angular velocity. A cylindrical gallium melt (aspect ratio 2.5) has been used as a model liquid. The melt has been heated from the bottom (Ra = 10⁶) or from the top (Ra = −10⁶) and the resulting temperature fluctuations in the melt have been measured in dependence on the rotating field strength (B_max = 30 mT). In the case of the unstable gradient 0.8 mT are sufficient to dominate the buoyancy driven convection and to reduce the amplitude of the buoyancy caused temperature oscillations for more than one order of magnitude. At the same time, the fluctuation frequency increases with the field strength. In the case of the stabilizing temperature gradient, low amplitude/high frequency temperature fluctuations are generated by the rotating magnetic field, indicating the transition to a time-dependent flow. In both cases we see an increase of the convective heat transport for magnetic inductions higher than approximately 5 m T. Applying the rotating magnetic field to the Bridgman growth of gallium doped germanium, the same behavior can be seen: Growing with a top-seeded arrangement, the intensity of the dopant striations is decreased and their frequency is increased. Growing with a bottom-seeded arrangement, the interface curvature changes from concave to convex and the flow becomes time-dependent.     

Convective Temperature Fluctuations in Liquid Gallium in Dependence on Static and Rotating Magnetic Fields

The influence of static axial, static transversal, and rotating magnetic fields on convective temperature fluctuations in liquid gallium (temperature range around 800 °C, Prandtl number 2.5 · 10⁻³) has been investigated. For a Rayleigh number of 6.3 · 10⁻⁵ (ΔT = 10°C, h = 50 mm), convective temperature fluctuations with peak to peak values of 3 °C have been measured. Depending on the strength, the frequency, and the configuration of the field, they could be eliminated to ΔT < 0.01 °C (static axial field of 182 mT), damped to a nearly periodic state with 0.1 °C amplitudes (static transversal field of 45 mT), or reduced to 0.03 °C oscillations by applying a rotating field.     

Orientation of Thermotropic Liquid-Crystalline Polyesters in High Magnetic Fields

Strongly oriented poly-crystalline samples of polyesters containing mesogenic moieties and flexible spacers in the main chain have been produced by slowly cooling the molten samples in magnetic fields of up to 16 Tesla. This simple method appears to be successful for those polymers forming a “true” nematic phase over an extended range of temperatures. Cholesteric polymers are not oriented. Mesogenic polymers (such as SEB-5, SUC-5, and SEB-7) which pass only through a state of partial parallel ordering of chain segments before cystallizing cannot be oriented macroscopically. In these samples crystallization may interfere with the chain alignment in the magnetic field. In contrast to low molecular weight liquid crystals, the isotropic-liquid crystalline transition of the polyesters investigated has a strong thermal hysteresis.     

Effect of structure on low temperature plasticity and magnetic properties of ageing Pb?Sb alloys below Tc

It was carried out comparative structural, magnetic and mechanical studies of single crystals of Pb—Sb alloys with 0.5–5 at.% Sb, which are II type superconductors. The structure was studied by X-ray diffractometry, small-angle X-ray scattering, microhardness and resistometry methods. It was determined the total volume fraction and the precipitate size. The second critical field H_c2 and the magnetic flux trapping measured at 4.2 K are determined by the solid matrix concentration and the precipitate volume fraction respectively. Plasticity of Pb—Sb crystals was investigated by tension tests at 4.2 K. The dependence of the change of the flow stress, Δτ_sn, at superconducting transition on ageing time and antimony concentration was obtained.     

DISTORTIONS INDUCED BY A MAGNETIC FIELD IN PLANAR ALIGNED SAMPLES OF SMECTIC-C LIQUID CRYSTALS

Abstract

This article presents some preliminary theoretical results for the onset of the Helfrich-Hurault transition in smectic-C liquid crystals induced by a magnetic field applied parallel to the smectic layers. An energy in terms of the smectic layer displacement u is minimized via averaging to enable the calculation of a critical field strength H _c for the onset of layer distortions.     

Elliptic NdCrO3 microplate by a simple one‐step hydrothermal synthesis route and its characterization and magnetic properties

Elliptic NdCrO₃ microplates were synthesized by a simple and facile one‐step hydrothermal method of processing temperature 280 °C for 3 days. The products prepared in this paper have been characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), X‐ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR) and field‐emission scanning electron microscopy (FESEM). The magnetic properties of the final sample are also studied. The XRD pattern indicates the pure orthorhombic phase for NdCrO₃ particles, the XPS, XRF and FTIR results further demonstrate the composition and purity of the final product. A possible growth mechanism for elliptic NdCrO₃ microplates is proposed. Through the investigation of magnetic properties, it can be generally concluded that the orthorhombic elliptic NdCrO₃ microplates exhibit typical behaviors of magnetic transition, spin reorientation transition and magnetic exchange bias. The Néel temperature is 218 K and the spin reorientation transition temperature is 46 K. The hysteresis loop under 5 K shows that the value of exchange bias field (H_ex) is 12 Oe and the shift of remanent magnetization (ΔM) is 0.008 emu/g, respectively.     

Magnetic Order in Organic Superconductors

We have undertaken an electron spin resonance study at both low and high magnetic field on the superconducting phase of the ambient pressure superconductor (BEDT-TTF>₂I₃, (ET)₂I₃, over the temperature range 1-300 K and hydrostatic pressure range from 0 to 2 kbar. At ambient pressure the ESR results are consistent with the picture of (ET)₂I₃ as a metal. Superconductivity is observed at 1.6 K via low field ESR. Application of modest pressures strongly suppresses the superconducting transition temperature. At pressures above about 0.3 kbar the superconductivity is suppressed in favor of an as-yet-unidentified magnetic state whose onset is at 7K. Through an analysis of the microwave ESR lineshape we find that the microwave conductivity over the temperature range 5-50 K is in agreement with dc measurements.     

Numerical investigation of magnetic field effect on pressure in cylindrical and hemispherical silicon CZ crystal growth

The effect of axial magnetic field of different intensities on pressure in silicon Czochralski crystal growth is investigated in cylindrical and hemispherical geometries with rotating crystal and crucible and thermocapillary convection. As one important thermodynamic variable, the pressure is found to be more sensitive than temperature to magnetic field with strong dependence upon the vorticity field. The pressure at the triple point is proposed as a convenient parameter to control the homogeneity of the grown crystal. With a gradual increase of the magnetic field intensity the convection effect can be reduced without thermal fluctuations in the silicon melt. An evaluation of the magnetic interaction parameter critical value corresponding to flow, pressure and temperature homogenization leads to the important result that a relatively low axial magnetic field is required for the spherical system comparatively to the cylindrical one.     

Magnetic properties of disordered oxides with iron and manganese ions

Magnetic properties of disordered oxides involving oxide glasses have been investigated. Spin glass-like transition is observed for Fe₂O₃–TeO₂ and MnO–TeO₂ glasses in temperature dependence of dc susceptibility, although magnetic transition does not take place for MnO–TeO₂ glasses with low concentration of manganese ion above 2 K at least. The mechanism of magnetic transition observed for Fe₂O₃–TeO₂ glasses is discussed on the basis of dc susceptibility obtained under conditions of field cooling and zero field cooling, magnetic field dependence of magnetization at low temperatures, and frequency dependence of spin-freezing temperature derived from temperature dependence of ac susceptibility. Magnetic properties of disordered ZnFe₂O₄ thin film prepared by a radio frequency sputtering method have been also studied. The disordered ZnFe₂O₄ thin film exhibits ferrimagnetic behavior with high magnetization even at room temperature. At the same time, the thin film shows spin-freezing at around 320 K. Temperature dependence of nonlinear susceptibility leads to a conclusion that this transition is explainable in terms of superparamagnetism with magnetic interaction among clusters.     

Ball solitons in the kinetics of magnetic first-order phase transitions: A new mechanism of phase reconstruction

Magnetic ball solitons can arise at a spin-flop transition induced by a magnetic field in antiferromagnets with uniaxial anisotropy (as a result of energy fluctuations). Soliton states are possible in a wide range of amplitudes and energies, including negative energies with respect to the initial phase state. When an antiferromagnet is in the metastable state, the probability of formation of ball solitons becomes the highest if the energy of such states is close to zero. Ball solitons continuously evolve, transforming into macroscopic domains of a new magnetic phase.     

Low-temperature Magnetic Properties of Terbium Orthoferrite Single Crystals

Spontaneous and by an external magnetic field induced magnetic phase transitions in TbFeO₃ have been experimentally studied. Precision investigations of velocities of transmission and attenuation of sound wave were made, the processes of formation and rearrangement of magnetic structure examined and its change under the influence of external magnetic field explored. This has been made possible by the development of methods of controlled growth of rare-earth orthoferrites resulting in the formation of high-quality single crystals of required size with reproducible physical properties. The measurements were taken in the temperature range from 0.45 to 8.0 K. From the threshold Geld values a H_a  T phase diagram for TbFeO₃ has been plotted.     

Neutron diffraction investigation of the atomic and magnetic structures of MnAs at high pressures

The atomic and magnetic structures of the MnAs compound are investigated using neutron diffraction at high pressures up to 38 kbar in the temperature range 15–300 K. It is found that, as the temperature decreases under high pressure, the MnAs compound undergoes a spin-reorientation transition from the ferromagnetic orthorhombic phase to a new phase whose magnetic moment has both the ferromagnetic and antiferromagnetic components. The structural and magnetic parameters of the high-pressure phases of the MnAs compound are determined.     

Kinetic investigations within the transition region from polyhedral to skeletal growth mode (I). Growth kinetics of the basal face of zinc single crystals under diffusion conditions

The effect of diffusion in the vapour phase on the kinetics of growth of zinc single crystals in the presence of argon has been studied. The shift of the basal face in normal direction as a function of the time has been measured at constant temperature and supersaturation and argon pressures varying within the range 5–250 Torr as well as in vacuum 1 × 10⁻⁶ Torr. It has been established that the crystal size R changes with the time t following a linear and a parabolic law in the kinetic and the diffusion regimes, respectively. The kinetic critical size R_k for the transition from a kinetic to a diffusion regime has been experimentally obtained. A relatively good agreement is found between the experimentally established and theoretically calculated values of the critical size. The problems related to the transition of the basal faces of zinc single crystals from a kinetic to a diffusion growth mode is discussed.     

Evidence for Superconductivity in Nb2SC0.90 Carbosulfide

Abstract

Evidence for bulk superconductivity in Nb₂SC_0.90 carbosulfide is presented. Diamagnetic volume fraction of Nb₂SC_0.90 was precisely determined with low applied magnetic fields which were corrected by 6N lead rod as an internal standard. Based on magnetic field dependence of magnetic moment at several constant temperature's below T_C = 5 K, Nb₂SC_0.90 is found to be a type II superconductor with relatively high H _C2 and low H_C1.