Light reflection at the interface between a transparent insulator and magnet with an arbitrary orientation of magnetization

The boundary problem for a semi-infinite magnetic medium with an arbitrary orientation of the magnetization vector has been solved. The polarization and intensity characteristics of the reflected light wave are analyzed. It is shown that the variations of the incidence angle, magnetization orientation, and polarization plane substantially affect the polarization characteristics and give rise to typical features in the dependences studied.     

Thermal expansion of Sm-Tb orthoferrites in the region of the spin reorientation transition

A new model of spin reorientation is discussed on the basis of the analysis of experimental data on thermal expansion of samarium-terbium orthoferrite and the earlier studies of its magnetic and crystallooptical properties. During the orientational phase transitions, the zero values of the thermal and optical parameters of rare earth orthoferrites correspond to the alignment of the magnetization vector along the [110] crystallographic directions. For orthorhombic Sm-Tb orthoferrite crystals, these directions coincide with the twofold axes in the plane.     

Number of optical axes in absorbing triclinic crystals

The possible numbers of optical axes in absorbing triclinic crystals have been considered. Most of these crystals possess four circular optical axes; however, there may be crystals having one isotropic optical axis, two or three circular optical axes, or simultaneously one isotropic and one or two circular optical axes. It is shown that particular cases significantly differ from the general case of crystal with four circular axes. The dependence of the form of the complex permittivity tensor on the number of optical axes is presented. The ellipticities of eigenwaves and the transmitted-light ellipticity in the case of normal incidence of a wave with a right- or left-hand circular polarization are calculated for these crystals.     

Anisotropic Optical and Transport Properties of Trans-(Ch)X

We have produced fully oriented highly crystal 1ine trans-(CH)_X films, which exhibit dramatic polarization effects for light polarized parallel or perpendicular to the oriented (CH)_X-chains. The polarization behavior in the vicinity of the bandgap and of the characteristic IR-modes, including those which are introduced as a result of doping with ASF5 is presented. DC-conductivity measurements allow to deduce the conductivity anisotropy of δ_‖/δ_⊥ ～ 100 for undoped and I₂-doped trans-(CH)_x at 300 K.     

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.     

Spin glass behavior in complexes of iron doped coordinated polymers

Magnetization measurements were performed on metal polycarbosilazane complexes polymers doped with Fe(II) and Fe(III)-ions. Zero field cooled (ZFC) and field cooled (FC) magnetization for Fe(II) doped polymer were found to bifurcates at a freezing temperature T_g, showing typical spin glass behavior but at much higher fields (～kOe) than what is commonly observed in metallic spin glass(～Oe). However, the ZFC magnetization of Fe(III) doped polymer did not reveal any spin-glass behavior. The magnetization curves showed very little Hysteresis effects and saturate at about 40 kOe at 2 K then drops sharply at 20 K. The saturation magnetization continues to drop gradually up to room temperature ～300 K.     

Investigation for ferroelectric domains by scanning electron microscopy

Examples for the direct observation of domain structures in ferroelectric crystals by means of scanning electron microscopy are given. The results for TGS and AFB crystals with direction of the polarization vector perpendicular to the observed surface are presented with respect to temperature changes, to domain enlargement under the influence of an external electric field and to the influence of X-ray- and γ-radiation on the domain formation.     

Properties of nonlinear elastic waves propagating along acoustic axes

Amplitudes and polarization of nonlinear elastic harmonics exited along an arbitrary acoustic axis of the conic type have been studied as functions of the rotation angle of the polarization vector of the degenerate reference wave. When the reference polarization is rotated by an angle of π, the resulting rotation angle for the polarization vector of the second harmonic turns out to be equal to either zero or 2π or ?2π. The actual type of the behavior of the second harmonic propagating along the given acoustic axis is determined by an algebraical criterion for elastic moduli of the second and the third order for the medium. For polarization of the reference wave, we determined the orientations corresponding to the maximum and minimum for the amplitude of the second harmonic. Similar results were obtained for different cases of collinear interactions between degenerate waves, in particular, for the excitation of longitudinal harmonics. The possible control of nonlinear interactions with the use of degenerate-wave polarization in optics and acoustooptics is also discussed.     

Study of structural order in porphyrin-fullerene dyad ZnDHD6ee monolayers by electron diffraction and atomic force microscopy

The structure of porphyrin-fullerene dyad ZnDHD6ee monolayers formed on the surface of aqueous subphase in a Langmuir trough and transferred onto solid substrates has been studied. The data obtained are interpreted using simulation of the structure of isolated molecules and their packing in monolayer and modeling of diffraction patterns from molecular aggregates having different sizes and degrees of order. Experiments on the formation of condensed ZnDHD6ee monolayers are described. The structure of these monolayers on a water surface is analyzed using π-A isotherms. The structure of the monolayers transferred onto solid substrates is investigated by electron diffraction and atomic force microscopy. The unit-cell parameters of two-dimensional domains, which are characteristic of molecular packing in monolayers and deposited films, are determined. Domains are found to be organized into a texture (the molecular axes are oriented by the [001] direction perpendicular to the substrate). The monolayers contain a limited number of small 3D domains.     

Ferroelectricity and antiferroelectricity in CuCrO2-type multiferroic semiconductors

Multiferroic semiconductors of the CuCrO₂ type are considered. These materials, in the presence of free charge carriers, allow for the existence of opposite domains (in the ferroelectric FE _d phase), along with ordinary 180° domains (in the FE phase). The magnetization phase transition in a chiral multiferroic, allowing for piezoelectric effects in an antiferromagnet with a layered triangular structure which result in an incommensurate (helicoidal) spin structure, is phenomenologically described. The behavior of the ferroelectric polarization in the considered phases is characterized. The antiferroelectric is considered a system of parallel layers alternating in chirality sign with oppositely directed polarization vectors. The possibility of antiferroelectric phase transition due to the specific features of the dipole-dipole interaction is discussed.     

Neutron nano-spin-echo spectrometer based on magnetic nanostructures

A neutron spin-echo spectrometer based on spin precessors in the form of magnetic layered nanostructures is described. A model of a spin-echo spectrometer is developed on beam no. 9 in the IBR-2 reactor. In this model, spin precession occurs during motion of neutrons in a magnetic field and their double reflection from Al(30 nm)/Fe(15 nm)/Al(120 nm)/Cu(150 nm) magnetic layered structures. The obtained spectrometer parameters make it possible to investigate excitations in films with a wave vector oriented along the neutron beam direction in the range from 10^?3 to 10^?1 Å^?1 and perpendicularly to the beam in the range from 10^?4 to 10^?5 Å^?1.     

A Polymorph Crystal Structure of Hexaphenyl Observed in Thin Films

Highly oriented thin films of hexaphenyl — which are used in organic opto‐electronic applications — are characterised in terms of their crystal structures. Two different crystal structures of hexaphenyl (C₃₆H₂₆) are observed when the films are prepared by physical vapour deposition at various substrate temperatures. If the substrate is kept at room temperature, hexaphenyl crystallises within a structure which is already known from single crystal investigations. However, when the thin films are grown at a substrate temperature of 160°C a new crystalline phase appears. This structure was characterised by X‐ray and transmission electron diffraction. Due to the strong preferred orientation of the crystallites within the thin films, the lattice constants as well as main features of the new crystal structure could be determined. The lattice is indexed as monoclinic with: a = 7.98Å, b = 5.54Å, c = 27.64Å and β = 99.8°. The new crystal structure has high similarity to the already known crystal structure: Both structures are built by layers of hexaphenyl molecules, within one layer the aromatic planes of the hexaphenyl molecules are packed in a herringbone pattern. The characteristic feature of the new structure is that the long axes of the hexaphenyl molecules are arranged absolutely perpendicular to the layers, whereas, within the already known structure the long axes show an tilt angle of 17° to the layer normal direction.     

Epitaxial Mn-doped ZnO diluted magnetic semiconductor thin films grown by plasma-assisted molecular-beam epitaxy

A growth window for the Mn effusion cell temperature (T_Mn) is demonstrated for epitaxial Mn-doped ZnO (MnZnO) thin films grown on sapphire substrates using molecular-beam epitaxy. Within the growth window, the films are ferromagnetic with the largest saturated magnetization occurring at T_Mn=700 °C. The Curie temperature of these MnZnO diluted magnetic semiconductor thin films is above room-temperature. The ferromagnetism is weakly anisotropic. Well-resolved near-band-edge photoluminescence emissions dominate the spectra at both low- and room-temperatures. No evident spin polarization on the carriers was detected with the magneto-photoluminescence studies. Magnetoresistance and anomalous Hall effects of the MnZnO thin films were studied. The anomalous Hall coefficient shows a quadratic dependence on the resistivity.     

Structural,ferroelectric and magnetic properties of Bi0.85Sm0.15FeO3 perovskite

Room temperature crystal structure, ferroelectric and magnetic properties of polycrystalline Bi_0.85Sm_0.15FeO₃ samples were investigated. X‐ray diffraction study shows that the compound possesses a dominant PbZrO₃‐like orthorhombic structure with √2a ×2√2a ×2a superlattice (a is the parameter of the cubic perovskite subcell). In contrast to piezoresponse force microscopy data demonstrating some features characteristic of ferroelectrics, polarization vs. electric field measurements reveal the behavior expected for nonpolar materials. Investigation of magnetic properties confirms that 15% samarium substitution suppresses the spin modulation typical of BiFeO₃ and induces the appearance of spontaneous magnetization. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three Dimensionally Oriented Polyacetylene

Changes in the dimensions of oriented polyacetylene under isomerisation and iodine doping are presented. We show that the polymer chains are parallel to the fiber axis. In roll-oriented samples the relative change in dimensions in anisotropic in all three directions and this is atributed to an oredeing of the fibers with respect to the crystalline axes of the polymer. The volume depends libearly on iodine concentration in agreement with a model of inh omogeneous doping where it is assumed that for intermediate iodine concentrations the fibers consist of a fully doped surface layer surrounding the undoped interiour.     

Morphology of Pristine Poly (Acetylene) Obtained by Luttinger's Catalyst

Poly (acetylene) was jprepared as freestanding mechanically stable film using Luttinger's catalyst. The cis isomer formed initially is stable during storage at room temperature over many days. Electron micrographs show a loose conglomerate of irregularly shaped platelets with a diamjeter of several ten nanometers and a thickness of less than 10 nm. Electron diffraction on the cis form shows the chain axes perpendicular to the lamella surface. Annealing conver ts the polymer into the trans isomer. Morphological changes are not observed. The chanins however tilt in the course of the phase transition associated with the isomerization. Films of the material obtained by Luttinger's cataltyst show essentially the same electrical and mechanical behavior as the ones prepared by a Shirakwa-technique.     

Reducing Inversion Twinning in Single Crystal Growth of GaPO4

Gallium orthophosphate (GaPO₄) single crystals were grown by the reverse temperature gradient method from phosphoric acid solutions under hydrothermal conditions. Twins after (110) were studied by etching faces having been cut perpendicular to one of the twofold axes. Based on the determination of the twin boundary position as well as on the knowledge of the growth rates of different crystallographic forms, a few faces have been chosen to be quite promising for growing high‐quality GaPO₄ single crystals if they are offered at the referring seed crystal. From the characterization of the grown crystals conditions have been found, which may lead to the reduction of the inversion twin number during the growth process.     

Gyrotropic absorbing crystals of lower symmetries

The specific features of polarization of natural waves propagating in crystals near their optical axes have been studied successively in transparent gyrotropic, absorbing, and gyrotropic absorbing crystals of classes 222, 2, 1, m, and mm2. The dependences of the ellipticities of natural waves and the azimuth of polarization of light transmitted through a crystalline plate (the case of normal incidence) on the orientation of the optical-axis plane of the plate axes with respect to the normal to surface are calculated for these crystals. These characteristics are compared for crystals of axial and planal classes with two different orientations of the optical-axis plane with respect to the symmetry elements of crystal.     

Magnetic structure of Er5Si3 at T ≥ 20 K

The symmetry analysis of magnetic structures of the Er₅Si₃ compound, possible at temperatures of 20 and 25 K, has been performed to interpret the experimental elastic neutron scattering data obtained at these temperatures. It was shown that the smallest dispersion factor R _m at these temperatures corresponds to the modulated magnetic structure in which the magnetic moments of Er atoms are directed along the a₃ axis of the unit cell and form an antiferromagnetic longitudinal spin wave characterized by the wave vector k = μb₃, where μ is approximately 0.264 and 0.274 at 20 and 25 K, respectively. The analysis of the temperature behavior of the magnetic reflection intensities demonstrated that the Er₅Si₃ compound is paramagnetic at 30 K.     

Polarization characteristics of polydimethylsiloxane-based composites during first-order phase transitions

Crystallization of polydimethylsiloxane (PDMS) filled with insulating carbon has been studied in an inhomogeneous temperature field. It is shown that the material polarization during the melt-crystal transitions depends on the concentration of the filler. This results in a decrease of the temperature coefficient of polarization with the concentration of dielectric carbon. The temperature dependences of thermodynamic functions are calculated for the crystallization range of the crystals under study.