Chizhikov V.A.,RAS Shubnikov Institute of Crystallography |
Dmitrienko V.E.,RAS Shubnikov Institute of Crystallography
Physical Review B - Condensed Matter and Materials Physics | Year: 2012
The spiral magnetic order in cubic MnSi-type crystals is described analytically using the model of classical Heisenberg ferromagnetics with an extra interaction of the Dzyaloshinskii-Moriya (DM) type between neighboring atoms. For all shortest Mn-Mn bonds the DM vectors D are expressed via the D vector of an arbitrary chosen bond, in accordance with the P2 13 space group of these crystals and bond directions. After minimization of the magnetic energy it is found how the propagation vector k of magnetic helices depends on D: for any helix direction k=2(D x-2D y-D z)/(3J) where J is the exchange interaction constant and k vector is in the units of the inverse lattice constant. The propagation number k determines both the sign and strength of global spiraling, whereas locally, within a unit cell, the helical order is shown to be frustrated so that the twist angles between neighboring ferromagnetic layers may be even of different signs. Conical deformations of helices caused by an arbitrary directed external magnetic field are also considered within the same model. The critical field of helix unwinding is found and it is shown that even in the unwound state there remains a residual periodic canting of magnetic moments with the canting angle proportional to (D x+D z)/J which can be measured by diffraction methods. It is also demonstrated how the usually used continuous picture of moment distribution can be obtained from the discrete one in a coarse grain approximation. © 2012 American Physical Society.
Pikin S.A.,RAS Shubnikov Institute of Crystallography |
Lyubutin I.S.,RAS Shubnikov Institute of Crystallography
Physical Review B - Condensed Matter and Materials Physics | Year: 2012
The conditions for occurrence of the magnetoelectric, magnetoelastic, and piezomagnetic effects in crystals of the langasite family with a triangular magnetic lattice are considered on the basis of the phenomenological model. It is shown that the iron-containing langasites with a triangular magnetic lattice may have piezoelectric properties above the Nèel temperature T N, but they are not ferroelectrics. Occurrence of either ferroelectric or both ferroelectric and piezoelectric states is possible below T N, and these crystals can be considered as multiferroics with a helicoid of magnetic moments oriented along the c axis. The observation of electric polarization p z in these crystals at T
Lider V.V.,RAS Shubnikov Institute of Crystallography
Physics-Uspekhi | Year: 2014
Various configurations of the X-ray crystal interferometer are reviewed. The interferometer applications considered include metrology, the measurement of fundamental physical constants, the study of weakly absorbing phase objects, time-resolved diagnostics, the determination of hard X-ray beam parameters, and the characterization of structural defects in the context of developing an X-ray Michelson interferometer. The three-crystal Laue interferometer (LLL-interferometer), its design, and the experimental opportunities it offers are given particular attention. ©2014 Uspekhi Fizicheskikh Nauk Russian Academy of Sciences.
Petukhov B.V.,RAS Shubnikov Institute of Crystallography
Journal of Experimental and Theoretical Physics | Year: 2012
The statistical Kolmogorov–Mehl–Johnson theory of solidification is generalized with allowance for the effect of obstacles creating delays for the propagation of new-phase boundaries, as applied to one-dimensional systems. An equation is derived to describe the process kinetics and is used to calculate the time dependence of the fraction of a transformed substance. The modification of the kinetics caused by changes in the obstacle density and the obstacle-induced delay time is studied. The theory can be applied to the extended contacts in large-scale integration circuits, biological macromolecules, and many other systems. © 2012 Pleiades Publishing, Inc.
Sobolev B.P.,RAS Shubnikov Institute of Crystallography
Crystallography Reports | Year: 2012
The manifestation of gross nonstoichiometry in MF m-RF n systems (m < n ≤ 4) has been studied. Fluorides of 34 elements, in the systems of which phases of practical interest are formed, are chosen. To search for new phases of complex composition, a program for studying the phase diagrams of the condensed state (~200 systems) has been carried out at the Institute of Crystallography, Russian Academy of Sciences. The main products of high-temperature interactions of the fluorides of elements with different valences (m n) are grossly nonstoichiometric phases of two structural types: fluorite (CaF 2) and tysonite (LaF 3). Systems of fluorides of 27 elements (M 1+ = Na, K; M 2+ = Ca, Sr, Ba, Cd, Pb; R 3+ = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; R 4+ = Zr, Hf, Th, U) are selected; nonstoichiometric M 1 -xR xF m(1 -x) + nx phases, which are of greatest practical interest, are formed in these systems. The gross nonstoichiometry in inorganic fluorides is most pronounced in 80 MF 2 -RF 3 systems (M = Ca, Sr, Ba, Cd, Pb; R are rare earth elements). The problems related to the growth of single crystals of nonstoichiometric phases and basic fields of their application as new fluoride multicomponent materials, the properties of which are controlled by the defect structure, are considered. © 2012 Pleiades Publishing, Inc.
Kaminskii A.A.,RAS Shubnikov Institute of Crystallography
Doklady Physics | Year: 2013
The basic χ(3) nonlinear laser properties of tetragonal vanadates (YVO4, GdVO4, ErVO4, YbVO 4, LuVO4, Y0.5Gd0.5VO4, and Gd0.5Lu0.5VO4), which represent a novel class of SRS-active crystals, are presented. The presence of combined χ(3) active phonon modes in the GdVO4 crystal at a cryogenic temperature (∼9 K) is reported. © 2013 Pleiades Publishing, Ltd.
Dmitrienko V.E.,RAS Shubnikov Institute of Crystallography |
Chizhikov V.A.,RAS Shubnikov Institute of Crystallography
Physical Review Letters | Year: 2012
A symmetry analysis of the Dzyaloshinskii-Moriya (DM) interaction in MnSi-type crystals reveals a nontrivial antiferromagnetic pattern of tilted Mn moments remaining even after an unwinding of the ground-state helix by a strong magnetic field. The remaining tilts are caused by that component of the DM vector which is perpendicular to the component responsible for helical spiraling; both components are evaluated and related to the atomic structure using a simple model. It is shown that the tilting should induce pure magnetic reflections 00(=2n+1) in neutron or x-ray magnetic scattering. In addition, the DM-induced antiferromagnetic ordering is important for the core structure of intrinsic defects, for the spectra of magnetic resonances, and generally for a better understanding of spin-orbit interaction in this type of magnetics. © 2012 American Physical Society.
Yakushkin E.D.,RAS Shubnikov Institute of Crystallography
JETP Letters | Year: 2014
Change in the dielectric response of a uniaxial ferroelectric in a static magnetic field has been detected. The effect has been observed in a polydomain ferroelectric crystal and is likely attributed to the action of the magnetic field on the domain wall pinning centers. © 2014 Pleiades Publishing, Inc.
Gorkunov M.V.,RAS Shubnikov Institute of Crystallography |
Osipov M.A.,University of Strathclyde
Soft Matter | Year: 2011
In the framework of molecular mean-field theory we study the effect of nanoparticles embedded in nematic liquid crystals on the orientational ordering and nematic-isotropic phase transition. We show that spherically isotropic nanoparticles effectively dilute the liquid crystal medium and decrease the nematic-isotropic transition temperature. At the same time, anisotropic nanoparticles become aligned by the nematic host and, reciprocally, improve the liquid crystal alignment. The theory clarifies the microscopic origin of the experimentally observed shift of the isotropic-nematic phase transition and an improvement of the nematic order in composite materials. A considerable softening of the first order nematic-isotropic transition caused by strongly anisotropic nanoparticles is also predicted. © 2011 The Royal Society of Chemistry.
Ilyushin G.D.,RAS Shubnikov Institute of Crystallography
Structural Chemistry | Year: 2012
The current state of the art related with the selforganization of crystal-forming systems, where long-range order spontaneously appears in the arrangement of structural units of any nature (micro- and macromolecules or atomic clusters), is considered. Three partially overlapping stages of self-organization of a system accepted in physical models of "order-disorder" kinetic transitions are matched to those used in supramolecular chemistry. An algorithmically constructed model of transition from disordered to hierarchically ordered systems is considered. The geometrical and topological modeling of density fluctuations of n-atomic species (clusters) A n in a crystal-forming medium is carried out. Clusters A n of a higher level of the system self-organization (the time moment t i) were determined as assemblies of specially selected clusters at the lower level (the time moment t (i-1)). Such clusters are built of equivalent clusters and, therefore, have a hierarchical structure; i.e., the simplest clusters are integrated into the clusters of the next higher level. Also, the mechanism of self-assembly of symmetrically and topologically different chains and microlayers (in the form of planar nets) from cyclic clusters An is considered in the model system. The 45 obtained nets correspond to 11 uninodal Shubnikov nets and new binodal nets. Algorithms are presented for combinatorial and topological analysis to search for precursor clusters and restore a three-dimensional net of covalent and non-covalent bonds in a crystal structure by the matrix (cluster) self-assembly mechanism. The developed model is universal. The mechanism of self-assembly has been modeled for O 3 (ozone), C 6H 6 (benzene), C 2H 2 (acetylene), NaAlSi 3O 8 (albite), K 2.25Na 0.31Ca 2.25Ba 1.44 (Al 11.5Si 30.5O 84H 1.4) (H 2O) 25 (paulingite, PAU), B(OH) 3, H 3B 3O 6, H 2SeO 3, the Friauf-Laves structure family(which counts in 1,400 of binary and ternary compounds): MgCu 2 (cF24), MgZn 2 (hP12), and MgNi 2 (hP24), the icosahedral structures: B 12, C 20H 20, C 60, ZrZn 22 (cF184), and NaCd 2 (cF1192) Samson Phase. © Springer Science+Business Media, LLC 2012.