Giant domain walls in a ferromagnet

Giant domain walls with a width of ∼7 μm are observed on the surface of a ferromagnet — an amorphous magnetically soft alloy. A magnetooptic investigation shows that the walls have a Néel structure in the subsurface region. The subsurface structure of these walls differs substantially from that of the narrower walls previously observed in iron, Permalloy, and amorphous materials. According to the theoretical model of Scheinfein and co-workers, which relates the width of an asymmetric Bloch wall in the bulk with the width at the surface, the width of the wall in the bulk is estimated to be 3–4 μm. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 528–530 (25 October 1999)     

Critical fields and currents in a weakly nonlinear medium near the percolation threshold

For fields above a critical value the expansion of the conductivity in powers of the field ceases to be valid and the weak-nonlinearity approximation no longer works. The density behavior of the critical fields in strongly inhomogeneous media near the percolation threshold is found on the basis of two criteria—an average criterion and a local criterion. The parameter values of the medium for which crossover—a change of the critical behavior—occurs are determined. Similar calculations are performed for the critical currents. Zh. Tekh. Fiz. 68, 5–8 (June 1998)     

Polar magneto-optic Kerr effect in the near-light field of a small nonmagnetic particle

A theory of elastic light scattering by a small resonantly polarizing particle located near a flat surface of a magnetic medium has been developed. The effective polarizability of the particle was calculated self-consistently taking account of the dynamic “image forces” in all orders of perturbation theory in the interaction of the particle with the demagnetized ferromagnet, and the magneto-optic perturbation was determined to first order in the magnetization. In the case of a ferromagnet magnetized perpendicular to the surface, the light conversion factors and the magneto-optic corrections to the transverse cross sections of all processes in which the scattering of light by a particle and the polar magneto-optic Kerr effect are elementary events, have been calculated. The results, including an analysis of the near-field contribution to light scattering, comprise the physical foundation for constructing a theory of near-field magneto-optic spectroscopy of ferromagnets and magnetic structures. Fiz. Tverd. Tela (St. Petersburg) 39, 560–567 (March 1997)     

Effect of conduction electrons on the law of approach to saturation of a metallic ferromagnet with surface pinning of the magnetic moment

The effect of conduction electrons on the magnetization curve of a metallic ferromagnet with surface pinning of the magnetic moment is investigated theoretically. The electronic contribution is due to the rearrangement of the discrete spectrum of charge carriers trapped by the nonuniform magnetic induction of such a ferromagnet, and it is a kind of diamagnetic effect that appreciably decreases the volume-averaged magnetization of the ferromagnet. A powerlaw dependence H ^−3/4 on the external magnetic field H is obtained according to the law of magnetization approach to saturation. This dependence is due to the contribution of the conduction electrons. Fiz. Tverd. Tela (St. Petersburg) 41, 647–653 (April 1999)     

Adhesion of metals and semiconductors analyzed by a dielectric formalism

This paper discusses a model of the adhesive interaction of metals and semiconductors, based on a dielectric formalism and using the concepts of collective excitations — plasmons of the electron-ion system. Expressions are obtained in terms of the jellium model in the longwavelength approximation for the adhesion energy and the adhesive interaction force and are determined via the dispersion dependences of the energies of surface plasma oscillations for various materials whose surfaces are separated by a gap of arbitrary magnitude. The adhesion energies and the adhesive interaction forces are calculated for a number of simple and transition metals and semiconductors, and the adhesion characteristics are also obtained for the contact of the given materials with an insulating medium. Fiz. Tverd. Tela (St. Petersburg) 39, 964–967 (June 1997)     

Magnetostatic surface waves produced by an inhomogeneity of the anisotropy with a turning point of the spectral function on a ferromagnet surface

Magnetostatic surface waves with fixed frequency and wave vector are predicted to exist in a ferromagnet with an inhomogeneity of the magnetic anisotropy such that the spectral function has a turning point on the surface. This result is most important for the case when an external magnetic field magnetizes the ferromagnet perpendicular to its surface. The frequency of the surface wave is determined by the frequency of the magnetostatic volume wave at the surface of the ferromagnet, and the wave vector is determined by the surface values of the local magnetic anisotropy field and its derivative. Zh. Tekh. Fiz. 68, 118–123 (June 1998)     

Formation of a bistructure of a solid in a computer experiment

Molecular-dynamics was used to investigate the structural changes occurring in a three-dimensional solid when the solid is transferred from an amorphous into a crystalline state. Crystal cells of a new type — pentadecahedrons with five square lateral faces and ten regular triangular faces at the vertices of a cell — were found for the first time in a computer experiment. It is shown that a bistructure consisting of crystal cells of different types, including cells with five-fold symmetry axes, are stable in the solid. Fiz. Tverd. Tela (St. Petersburg) 40, 1919–1924 (October 1998)     

Formation of a chaotic layer of nonlinear resonance by a two-frequency perturbation

A new effect [V. V. Vecheslavov, Zh. éksp. Teor. Fiz. 109, 2208 (1996) (JETP 82, 1190 (1996)]—the appearance of low-frequency secondary harmonics in the separatrix mapping of a system—is discussed in detail for the example of a pendulum with a two-frequency perturbation. It is shown that there exist regions of values of the perturbation parameters where these harmonics make the main contribution to the formation of the chaotic layer of the fundamental resonance. The results of analytical and numerical determinations of the amplitudes of the secondary harmonics are compared. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 989–994 (25 June 1996)     

Spin excitons — a new mechanism of superconducting pairing in copper oxides

The Fermi and Bose quasiparticle spectrum in copper oxides is studied in a many-band p-d model taking account of the strong electronic correlations. It is shown that hole-doped systems possess a Bose mode — a spin exciton — which is associated with the singlet-triplet excitation of the two-hole ground-state term of CuO₄ clusters. Intercluster hopping leads to fermion-boson interaction with a spin exciton as the intermediate boson. Such a mechanism does not exist for n-type systems. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 23–28 (10 July 1996)     

Stochastic resonance between limit cycles. Spring pendulum in a thermostat

The effect of white noise on phase synchronization is studied numerically for a classical model of a spring pendulum with a multiple ratio of the frequencies of small oscillations (Vitt-Gorelik model). It is shown that in the model investigated a Fermi resonance regime occurs for a system in a thermostat. A new type of nonlinear dynamics is found — stochastic resonance between limit cycles. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 585–589 (25 April 1999)     

Spectrum of flexural oscillations of a 180-degree domain wall in yttrium iron garnet

The long-wave asymptotic behavior of the spectrum of flexural oscillations of a monopolar domain wall in a cubic ferromagnet is found. The experimental results, numerical calculations, and theoretical results obtained for uniaxial ferromagnets are compared. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 10, 797–802 (25 May 1996)     

Effect of a photonic band gap in the optical range on solid-state SiO2 cluster lattices — opals

It is shown that synthetic opals — cubic face-centered lattices of SiO₂ clusters — are systems which exhibit a number of properties of photonic crystals in the visible-light range. By filling the voids (pores) in such lattices with different materials it is possible to vary the optical contrast of the medium and to obtain crystals of both the lattice of spheres type and its three-dimensional replica. It is shown that under conditions of identical optical contrast and in the presence of an additional optical inhomogeneity of the spheres, the transparency of the lattice of spheres is lower than that of its replica based on homogeneous media. A refractive index modulation of 1.266 was achieved in the lattice of spheres. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 496–501 (10 April 1996)     

Spectroscopy of GaAs/AlGaAs microstructures with submicron spatial resolution using a near-field scanning optical microscope

A near-field scanning optical microscope (NSOM), which we built, is used to investigate 1–5-μm wide stripes with a 10-nm thick layer—a quantum well — on a GaAs surface. A map of the photoluminescence intensity is obtained synchronously with the topographic profile of the structures. The measured spatial distribution of the photoluminescence intensity is described satisfactorily in a model that takes into account carrier diffusion in the layer and the existence of a region with a short carrier lifetime near the side boundaries of the layer. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 523–527 (10 April 1996)     

Adiabatic Raman polariton in a Bose condensate

The Raman interaction of optical fields with a Bose condensate is studied in the adiabatic regime. A superposition of operators is found — one annihilating an atom in a metastable state and the other annihilating a photon in resonance with a transition from the ground state to an excited state — which is an adiabatic invariant of the problem (Raman polariton). Possible applications for Bose-condensate diagnostics and development of atomic lasers are proposed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 473–477 (10 October 1996)     

Observation of a single laser-excited center at the point of a crystalline needle

The points of lithium fluoride needles are investigated by laser photoelectronic projection microscopy. A situation in which a single atomic-size source of electrons — an F ₂ color center — is observed in the region near the point is realized. As a result of the good fluorescence properties of these centers, these needles can be used as the active element of a scanning fluorescence microscope employing resonance transfer of electronic excitation energy. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 441–444 (25 March 1997)     

High-pressure microwave discharge in an above-breakdown field. Streamer branching

A new effect — the splitting of the tips (branching) of a microwave streamer arising from a pre-existing dense plasma cloud in an above-threshold electric field — is obtained numerically on the basis of a planar two-dimensional model. The causes of this phenomenon and the factors suppressing it are found. An expression is obtained for the value of the cloud radius above which branching occurs. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 15–19 (10 January 1999)     

Theory of scanning near-field magnetooptical microscopy

An analytical theory of scanning near-field magnetooptical microscopy is developed. The theory is based on the elastic scattering of light by small, resonantly polarizable particles, which are used to scan the plane surface of a nonuniformly magnetized medium. The effective polarizability of the particles is calculated with the effect of dynamic “image forces” taken into account in all orders of perturbation theory with respect to the interaction of the particle with a demagnetized ferromagnet, and the magnetooptical perturbation is calculated to first order in the magnetization. The major contributions to the magnetooptical light scattering for a ferromagnetic structure magnetized perpendicular to the surface are found, including a quasistatic approximation for the near-field particle-magnet interaction. The optical size resolution of a magnetic (dielectric) inhomogeneity is estimated. Zh. Tekh. Fiz. 68, 86–91 (July 1998)     

Self-organization of the critical state in a two-dimensional multijunction SQUID under closed boundary conditions

A model of a closed system with self-organization is presented. This is a simplified model of a multijunction SQUID in an ac magnetic field. In our closed system, a self-organized critical state is realized on account of the fact that current dumping, which gives rise to self-organization in open systems, is replaced here by a fundamentally different mechanism — annihilation of the currents. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 2, 119–125 (25 January 1999)     

On the possibility of investigating the dispersion of the energy bands of a crystal using resonant x-ray emission spectra

It is shown that, when the dispersion of the energy bands of crystals is investigated by resonant x-ray emission (RXE), an ambiguity can arise in the determination of k _tr — the transition point in k space — as a result of the characteristic features of the band structure and the value of the matrix elements. It is proposed that calculations of the distribution of partial contributions to interband optical transitions in the Brillouin zone (BZ) be used as a means of obtaining a simplified estimate of k _tr. The basic properties of this distribution are examined for lithium hydride, which is a convenient model object having a simple band structure and interband transitions which have been studied in detail. It is shown that the k points of the general position, which form a complex figure in the BZ, make the main contribution to the interband transitions. The contribution of high-symmetry points is much smaller mainly because of their small representativeness. This make it easier to understand the reason for the possible nonmonotonic variation of k _tr in RXE measurements of the dispersion of the energy bands of crystals or the sharp changes in the resonance-fluorescence intensity. Fiz. Tverd. Tela (St. Petersburg) 41, 1597–1601 (September 1999)