I. V. Pleshakov

Magneto-optic effects in silica core microstructured fibers with a ferrofluidic cladding

Studies of magneto-optic effects in ferrofluid-filled silica core microstructured optical fibers are reported. Amplitude, frequency, and polarization dependencies of magneto-optic responses were measured for both the transverse and longitudinal geometries of the applied AC magnetic field. Magneto-optic responses of two types, i.e., the fast and slow one, were observed. A qualitative model based on the Neel rotation for the fast response and on particle motion and agglomeration for the slow response is proposed.

The effect of a pulsed magnetic field on the nuclear spin echo signal in ferrite

A pulsed magnetic field produces shifts of domain walls in a magnetically ordered material (ferrite) and thus affects the nuclear spin echo signal in this material. The suppression and restoration of a response signal have been observed using exciting sequences with pulses of different polarities. The established dependence of the spin echo suppression coefficient on the external constant magnetic field confirms the hypothesis that the observed phenomena are related to the domain walls.

The statistical distribution of magnetic critical currents determined by HTSC film morphology

An analysis is made of the geometric and morphological properties of YBCO superconducting films and of their effect on the magnetic and transport phenomena. A study is presented of the statistical characteristics of critical currents derived from the variation of trapped magnetic flux induced by passing a pulsed transport current. It is shown that the critical currents in the materials under study have a specific statistical distribution, whose main properties are due to the morphology of their structure and can be determined by geometric-probability analysis. The superconducting film is considered as a percolation system. An empirical normal-phase cluster-area distribution function was used to derive the distribution function of magnetic critical currents, which describes adequately the experimental data on how transport current affects trapped magnetic flux. The critical current for transition of a film to the resistive state has been calculated.

Laser polarization-optical detection of the magnetization process of a magnetically ordered crystal

This paper demonstrates the use of laser radiation with deep polarization modulation as a method of investigating a magnetically ordered substance. Using a model sample—single-crystal iron borate—as an example, it is shown that this technique can be used to study magnetization processes. A qualitative picture is given of the formation of the polarization-optical response in the sample being magnetized, and the corresponding experimental characteristics are obtained. Sudden changes of the response are recorded that are associated with abrupt reconstruction of the domain structure. The approach thus developed can be useful in studies of the domain structure and of other features of the structure of materials for optoelectronics, nonlinear and magnetooptics, laser engineering, etc.

Transport current-activated flux creep in high-Tc superconductor thin films

This paper discusses the relaxation of a magnetic flux trapped in a superconducting film under the action of a transport current. The evolution of the magnetization after the first jump, at the moment the Lorentz force appears, has been found to obey the creep laws. The effective pinning potential has been obtained as a function of transport current. The distribution of the pinning forces is presented and its statistical properties considered. The standard deviation of the pinning forces is the same for films prepared using the same technique, while the expectation values characterize the attainable critical current density. A criterion for estimating the quality of thin films from the distributions of the pinning forces is formulated.

Influence of a pulsed field on the excitation of nuclear spin system in a domain wall of a magnetically ordered material

The formation of a spin echo signal from nuclei located in domain walls of a ferrite sample exposed to magnetic field pulses at the instants corresponding to the spin-system excitation has been considered. The effects related to the influence of magnetic field pulses on the domain structure are found. It is shown that they can be observed by scanning the sample in the domain-wall vicinity.

Investigation of composite structure of magnetically ordered material-semiconductor composite based on cobalt and porous silicon

Preliminary data are reported concerning the character of magnetically ordered material particles formed in porous silicon matrix using a special chemical incorporation technology. According to this, the internal volume of a porous silicon matrix is filled with cobalt chloride, which is subsequently reduced to a magnetic phase by means of a chemical reaction with sodium tetraborohydride. The obtained composite samples have been studied by the methods of electron microscopy and magnetometry.

Electric field induced by collective vortex creep in superconductors with fractal clusters of normal phase

The influence of the collective creep of magnetic flux on the electric field induced in a superconducting composite with fractal cluster structure is considered. Current–voltage (I–V) characteristics of these superconductors are determined with allowance for the influence of the fractal dimensionality of boundaries of the normal phase clusters and the height of pinning barriers on the nonlinearity of I–V characteristics at small transport currents. A relationship is established between the collective pinning and vortex glass state that is formed in superconductors with a fractal cluster structure. It is shown that the intensity of an electric field induced in the case of collective creep is smaller than that for Anderson–Kim creep.

Investigation of polarization magnetooptic responses of a low-concentration ferrofluid

A highly sensitive laser polarization-optical technique was developed and applied to investigations of a ferrofluid, which was placed in a magnetic field. A colloid solution of magnetite in kerosene was used as a sample. It was shown that the magnetooptic response of this substance is reliably registered at a volume solid-phase content of down to ~10–5. It was revealed that within a very wide range of changes in the solution concentration, a quadratic dependence of the observed birefringence value on the low-intensity field (up to several tens of oersted) is retained.

Phonon echo in high-temperature superconductors as a nonlinear magnetoacoustic phenomenon

The nonlinear behavior of a phonon echo signal in high-temperature superconductors is studied experimentally. It is demonstrated that conventional consideration of anharmonic effects is insufficient for comprehensive explanation of the echo signal behavior. It is shown that the nonlinear properties of the phonon echo should be taken into account when the phonon echo is used as a means for studying superconductor parameters. An increase in the elastic nonlinearity in iron-doped yttrium ceramics is found.