Yu. N. Skryabin

Statistical Mechanics of Magnetically Ordered Systems

Handsomely produced monograph provides graduate students and researchers with elegantly lucid accounts of some modern aspects of the topic to which the title refers. The five chapters bear these titles: Statistical mechanics of the Heisenberg ferromagnet; Statistical mechanics of electronic models o

Structural state of expanded graphite prepared from intercalation compounds

The structural state of nanocrystalline samples of expanded graphite is investigated using X-ray diffraction and neutron diffraction analyses. The expanded graphite samples are prepared by a rapid thermal decomposition of intercalation compounds of oxidized graphite based on fluorinated graphite, graphite oxide, and graphite aminofluoride. It is established that the main phase of expanded graphite belongs to the hexagonal crystal system (space group P63/mmc) and that carbon atoms in the structure occupy the 2b and 2c positions. The unit cell parameters and the unit cell volume in the structure of expanded graphite samples are larger than those in the structure of massive graphite.

Small-angle neutron scattering from samples of expanded carbon

The subatomic structure of expanded graphite has been investigated by small-angle neutron scattering. Samples were synthesized during quick thermal decomposition of intercalated compounds based on oxidized graphite. They had a low bulk density (up to 0.1 g/cm3) and were characterized by considerable small-angle scattering. It has been established that majority of the volume of expanded graphite samples is occupied by participles with characteristic sizes in two ranges: from 6 to 8 nm and from 20 to 30 nm. Small particles have properties of a surface fractal with the dimension Ds = 2.4–2.6, whereas the larger particles are mainly smooth and have the dimension Ds = 2.0–2.1. The specific surface of the samples studied was determined from the small-angle scattering data.

Dirac Points, Spinons, and Spin Liquid in Twisted Bilayer Graphene

Twisted bilayer graphene is an excellent example of highly correlated system demonstrating a nearly flat electron band, the Mott transition and probably a spin liquid state. Besides the one-electron picture, analysis of Dirac points is performed in terms of spinon Fermi surface in the limit of strong correlations. Application of gauge field theory to describe deconfined spin liquid phase is treated. Topological quantum transitions, including those from small to large Fermi surface in the presence of van Hove singularities, are discussed.

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 Rm 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.

Multiple small-angle neutron scattering in ferromagnets

Abstract The conclusions of the multiple small-angle neutron scattering theory, developed recently by Maleyev, Pomortsev, and Skryabin (MPS) for the Born parameter α⪢1 are experimentally confirmed by magnetic scattering from domain structure of pure Fe and Ni, as well as Fe65Ni35 alloy. A critical thickness L0 is found at which the multiple refraction transforms to multiple Fraunhofer diffraction. This crossover is accompanied by variation of the neutron beam broadening w versus sample thickness L, from w ∼ √L to w ∼ L.

Atomic and Superatomic Structures of Solid Solutions of Zirconium and Calcium Oxides

Solid solutions of the ZrO2-CaO system with 7, 9.6, and 20 mol % of calcium oxide are analyzed by X-ray and neutron diffraction and small-angle neutron scattering. Samples are synthesized by the sol-gel method and annealed at the temperature T = 600°C for 4 h. All samples have a tetragonal structure (P42/nmc space group). The size of crystallites is 10–20 nm. Small-angle neutron scattering curves are characterized by a peak near the momentum transfer q = 0.5 nm−1. The existence of the peak is caused by correlation between solid nanoparticles in the samples. Density correlation functions are numerically computed from the experimental small-angle scattering cross-sections by the inverse Fourier transform. Correlation lengths, concentrations of scattering nanoparticles, and specific surface areas of the samples are determined.

Formation of exotic states in the s–d exchange and t–J models

Different scenarios of the implementation of the two-band model in systems of strongly correlated electrons, including frustrated magnetic systems, high-temperature superconductors, and Kondo lattices, are considered. The interaction of current carriers with magnetic moments in the representations of pseudofermions or Schwinger bosons describing the spinon excitations is studied on the basis of the derived Hamiltonians of the s–d exchange and t–J models within the formalism of many-electron Hubbard X operators.

Effects of magnetic anisotropy and exchange in Tm 2 Fe 17

Neutron diffraction experiments have been carried out to study the magnetocrystalline anisotropy of two (2b and 2d) Tm sublattices and four (4f, 6g, 12j, and 12k) Fe sublattices in ferrimagnetic compound Tm2Fe17 (space group P63/mmc). We have determined the temperature dependence of the magnitude and orientation of magnetization for each of the thulium and iron sublattices in the range (10–300) K. A spontaneous rotation (at about 90 K) of the Tm and Fe sublattice magnetizations from the c-axis to the basal plane is accompanied by a drastic change in the magnetization magnitude, signifying a large magnetization anisotropy. Both Tm sublattices exhibit an easy-axis type of the magnetocrystalline anisotropy. The Fe sublattices manifest both the uniaxial and planar anisotropy types. The sublattice formed by Fe atoms at the 4f position reveals the largest planar anisotropy constant. The Fe atoms at the 12j position show a uniaxial anisotropy. We find that the inelastic neutron scattering spectra measured below and above the spin-reorientation transition are remarkably different.

Determination of Texture Degree of NdFeB-Magnets by Means of Neutron Diffraction

Six samples of Nd-Fe-B-permanent magnets, distinguished in the texture degree, have been measured by neutron diffraction in the reflection and transmission regimes. The texture in the samples was formed by application of impulses of external magnetic field 0H = 4 T. Analysis of the obtained neutron diagrams by means of the Fullprof program allowed quantitative determination of the texture degree and conclusion to be made that the texture degree grows with an increase in numbers of magnetic field impulses.