General properties of polymer systems

We prove the existence of the thermodynamic limit for the pressure and show that the limit is a convex, continuous function of the chemical potential.The existence and analyticity properties of the thermodynamic limit for the correlation functions is then derived; we discuss in particular the Mayer Series and the virial expansion.In the special case of Monomer-Dimer systems it is established that no phase transition is possible; moreover it is shown that the Mayer Series for the density is a series of Stieltjes, which yields upper and lower bounds in terms of Padé approximants.Finally it is shown that the results obtained for polymer systems can be used to study classical lattice systems.Work presented in partial fullfilment of the Ph. D. Thesis.     

Influence of ferroelectric domain walls on the Raman scattering process in lithium tantalate and niobate

We report changes in the Raman spectra at ferroelectric domain walls in near-stoichiometric LiNbO3 and stoichiometric LiTaO3. We find a decrease of intensity for the regular bulk Raman peaks along with increases of intensity in spectral regions that correspond to phonons, which propagate at an angle with respect to the incident light. In the backscattering geometry, such modes are not supported in the bulk crystal due to momentum conservation. We confirm that these changes are due to the domain wall itself and are independent of intrinsic defects or charging effects.     

Effect of fluctuations on the forced motion and interactions of domain walls in one-dimensional magnetic systems

Forced motion of a domain wall in the presence of fluctuations of external magnetic field and those of the parameters of the magnetic medium is studied. Calculations for the models of magnetic systems described by the sine-Gordon and Landau-Lifshitz equations are presented. It is shown that the driven motion of domain walls is characterized by the time-independent velocity distribution function which is used to calculate various statistical characteristics of the domain wall. Analysis of the mean velocity of the steady motion of the domain wall leads to the conclusion that the presence of a fluctuating magnetic field results in an increase of the effective relaxation constant of the magnetic system. In case of the sine-Gordon model the mean radiation power accompanying the forced motion of the domain wall is calculated. Inelastic interactions of two domain walls of opposite polarities are described.     

Influence of three-dimensional inhomogeneities of the magnetic parameters on the dynamics of vortex-like domain walls

The interaction of a vortex-like domain wall moving in an external magnetic field with a three-dimensional periodic chain of cubic volumes with high values of the saturation magnetization and magnetic anisotropy constant has been investigated theoretically. It has been found that the result of the interaction depends on the initial distance between the wall and the region of inhomogeneity of magnetic parameters at the moment of turning on the external magnetic field. The pinning of domain walls near the regions with high values of the saturation magnetization and magnetic anisotropy constant has been investigated, and the anisotropy of the corresponding depinning fields has been revealed. The method of investigation is the numerical micromagnetic simulation.     

The properties of spin echo signal in ferromagnetic domain walls

The domain-wall enhanced spin echo amplitude was calculated for the case of unequal lengths of the exciting pulses. The values thus obtained were compared with results of experiments performed on pure iron powder. Good agreement was obtained at exact resonance, but discrepancies were observed when frequency of exciting pulses differed from the NMR frequency. Special measurements were carried out in order to obtain the value of rf. field intensity inside the powder particles and the maximum enhancement factor related to this value was calculated.     

Effect of the micromagnetic structure of domain walls on the properties of an isolated domain in a thin magnetic film

The effect of the inner structure of domain walls on the time-independent parameters of an isolated stripe domain in a thin ferromagnetic film is studied. The adopted variant of the perturbation theory makes it possible to account, within a unified approach, for the contributions of the magnetostatic and exchange interactions. Zh. éksp. Teor. Fiz. 115, 1315–1325 (April 1999)     

Influence of long-range effects on the critical behavior of three-dimensional systems

The Padé-Borel resummation technique is used to describe field-theoretically, in the two-loop approximation, the behavior of Ising systems with long-range effects directly in a three-dimensional space. The renormalization-group equations are analyzed and the fixed points governing the critical behavior of the system are determined. It is shown that the long-range effects can bring about a change in both the regime of critical behavior and the kind of phase transition.     

Influence of inner and outer walls electromagnetic properties on the onset of a stationary dynamo

To study the onset of a stationary dynamo in the presence of inner or outer walls of various electromagnetic properties, we propose a simple 1D-model in which the flow is replaced by an alpha effect. The equation of dispersion of the problem is derived analytically. It is solved numerically for walls of different thicknesses and of electric conductivity and magnetic permeability different from those of the fluid in motion. We also consider walls in the limit of infinite conductivity or permeability.     

Effect of domain walls on the propagation of microwaves

The propagation of microwaves in ferrimagnets is affected mainly by the gyrotropic terms of the magnetic permeability tensor, due in this frequency range to ferromagnetic and exchange resonances. It is shown that the change in the gyrotropic terms in the domain wall region leads to the existence of a localized mode propagating close to the domain wall. The domain wall constitutes an optical inhomogeneity for the microwave. The reflectivity and transmittivity of a domain wall are calculated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 22–26, June, 1985.The authors are grateful to G. S. Krinchik for discussion of the paper.     

Influence of domain structure on the nonlinear polarization properties of A 2 BX 4-group crystals

Harmonic analysis of repolarization in A ₂ BX ₄-group crystals in the ferroelectric phase near phase-transition temperature T _c is used to separate the contributions to the harmonic-component amplitudes in the output signal of a Sawyer-Tower circuit from charging the reference capacitor with capacitive and conductive current through such a crystal. Harmonic analysis of dielectric hysteresis loops obtained by placing a specimen in an electric field with harmonically changing strength reveals general regularities in the evolution of the domain walls of A ₂ BX ₄-group crystals. It is established that the domain structure affects the nonlinear polarization properties of our specimens. The dependence of the potential relief of ferroelectric ion motion in a harmonically changing electric field is plotted on the basis of the experimental results.     

Effect of magnetoelectrical interactions on the multiferroic domain walls

The antiferromagnetic domain structure of a multiferroic has been investigated in the presence of a ferroelectric domain structure. It has been demonstrated that an inhomogeneous magnetoelectric (flexomagnetoelectric) interaction leads to pinning of antiferromagnetic domain walls at the walls of the ferroelectric domains and to a change in the structure of antiferromagnetic domain walls.     

Phase transition and critical properties of spin-orbital interacting systems

Phase transition and critical properties of Ising-like spin-orbital interacting systems in 2-dimensional triangular lattice are investigated. We first show that the ground state of the system is a composite spin-orbital ferro-ordered phase. Though Landau effective field theory predicts the second-order phase transition of the composite spin-orbital order, however, the critical exponents obtained by the renormalization group approach demonstrate that the spin-orbital order-disorder transition is far from the second-order, rather, it is more close to the first-order. The unusual critical behavior near the transition point is attributed to the fractionalization of the composite order parameter.     

Thermal motion of one-dimensional domain walls in monolayers of a polar polymer observed by Video-STM

Scanning tunneling microscopy (STM) has been used to investigate monolayers of the ferroelectric copolymer polyvinylidenefluoride/trifluoroethylene P(VDF/TrFE) showing images of ordered polymer monolayers. By scanning with video frame rate, direct observation of the motion of onedimensional domain walls was also possible for the first time. The images clearly show domain walls normal to the polymer chains. From measurements of the temperature dependence of the domain wall velocities the activation energy for the thermally generated kink motion was estimated. These results are compared with theoretical models describing domain wall motion in ferroelectric PVDF.     

Static properties of asymmetric vortex-like domain walls in magnetically uniaxial thick films

The structure and energy of asymmetric vortex-like Bloch and Néel walls in a magnetically uniaxial film with an easy magnetization axis lying in the film plane are investigated by numerically minimizing the total energy within the rigorous micromagnetic approach and the two-dimensional model of the magnetization distribution. The calculations are performed over wide ranges of film thicknesses b (up to b = 1 μm) and magnetic parameters of the films. It is established that the asymmetric vortex-like domain walls are the most universal wall structures in the films under consideration. In magnetically uniaxial films, unlike in magnetically multiaxial films, the asymmetric Bloch walls are always stable.     

Magnetooptic setup for investigating the dynamic properties of domain walls in thin ferromagnetic films

A magnetooptic Kerr setup is designed for investigating the dynamic properties of domain walls in thin ferromagnetic films subjected to an external magnetic field in the temperature range 20–150°C. With this setup, the method of interrupted magnetization is implemented and the magnetic (domain) structure is visualized based on the meridional Kerr effect. The domain structure is displayed on a PC monitor using a Nikon DXM 1200 high-resolution digital video camera. A dedicated software makes it possible to automatize measurements and data processing.