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  • 2000-2004  (4)
  • 2001  (4)
  • PACS. 61.12.-q Neutron diffraction and scattering – 61.66.Fn Inorganic compounds – 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.) – 75.30.Et Exchange and superexchange interactions  (2)
  • PACS. 64.60.-i General studies of phase transitions – 64.60.Fr Equilibrium properties near critical points, critical exponents – 75.40.-s Critical-point effects, specific heats, short-range order  (2)
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  • 2000-2004  (4)
Year
Keywords
  • 1
    Electronic Resource
    Electronic Resource
    Subleading long-range interactions and violations of finite size scaling (2001)
    Springer
    The European physical journal 21 (2001), S. 251-268 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 64.60.-i General studies of phase transitions – 64.60.Fr Equilibrium properties near critical points, critical exponents – 75.40.-s Critical-point effects, specific heats, short-range order
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We study the behavior of systems in which the interaction contains a long-range component that does not dominate the critical behavior. Such a component is exemplified by the van der Waals force between molecules in a simple liquid-vapor system. In the context of the mean spherical model with periodic boundary conditions we are able to identify, for temperatures close above T c, finite-size contributions due to the subleading term in the interaction that are dominant in this region decaying algebraically as a function of L. This mechanism goes beyond the standard formulation of the finite-size scaling but is to be expected in real physical systems. We also discuss other ways in which critical point behavior is modified that are of relevance for analysis of Monte Carlo simulations of such systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510170201
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Two-point correlation function in systems with van der Waals type interaction (2001)
    Springer
    The European physical journal 23 (2001), S. 211-219 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 64.60.-i General studies of phase transitions – 64.60.Fr Equilibrium properties near critical points, critical exponents – 75.40.-s Critical-point effects, specific heats, short-range order
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: The behavior of the bulk two-point correlation function G( ;T| d ) in d-dimensional system with van der Waals type interactions is investigated and its consequences on the finite-size scaling properties of the susceptibility in such finite systems with periodic boundary conditions is discussed within mean-spherical model which is an example of Ornstein and Zernike type theory. The interaction is supposed to decay at large distances r as r - (d + σ), with 2 〈 d 〈 4, 2 〈 σ 〈 4 and d + σ≤6. It is shown that G( ;T| d ) decays as r - (d - 2) for 1 ≪r≪ξ, exponentially for ξ≪r≪r *, where r * = (σ - 2)ξlnξ, and again in a power law as r - (d + σ) for r≫r *. The analytical form of the leading-order scaling function of G( ;T| d ) in any of these regimes is derived.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510170070
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  • 3
    Electronic Resource
    Electronic Resource
    Crystal and magnetic structures of the oxyphosphates MFePO 5 (M = Fe, Co, Ni, Cu). Analysis of the magnetic ground state in terms of superexchange interactions (2001)
    Springer
    The European physical journal 22 (2001), S. 429-442 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 61.12.-q Neutron diffraction and scattering – 61.66.Fn Inorganic compounds – 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.) – 75.30.Et Exchange and superexchange interactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We have studied in detail the crystal and magnetic structures of the oxyphosphates MFePO5 (M: divalent transition metal) using neutron powder diffraction as a function of temperature. All of them are isomorphic to the mixed valence compound α-Fe2PO5 with space-group Pnma. No disorder exists between the two metallic sites. The M2+O6 octahedra share edges between them and faces with Fe3+O6 octahedra building zigzag chains running parallel to the b-axis that are connected by PO4 tetrahedra. The topology of this structure gives rise to a complex pattern of super-exchange interactions responsible of the observed antiferromagnetic order. The magnetic structures are all collinear with the spin directed along the b-axis except for M = Co. The experimental magnetic moments of Cu+2 and Ni2+ correspond to the expected ionic value, on the contrary the magnetic moment of Fe3+ is reduced, probably due to covalence effects, and that of Co2+ is greater than the spin-only value indicating a non negligible orbital contribution. Using numerical calculations we have established a magnetic phase diagram adapted for this type of crystal structure and determined the constraints to be satisfied by the values of the exchange interactions in order to obtain the observed magnetic structure as the ground state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510170093
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Neutron diffraction study of the magnetic ordering in the series R 2 BaNiO 5 (R = Rare Earth) (2001)
    Springer
    The European physical journal 24 (2001), S. 59-70 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 61.12.-q Neutron diffraction and scattering – 61.66.Fn Inorganic compounds – 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.) – 75.30.Et Exchange and superexchange interactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group Immm, a ≈ 3.8Å, b ≈ 5.8Å, c ≈ 11.3Å) is structurally characterised by the presence of flattened NiO6 octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures of R2BaNiO5 ( R = Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic structure characterised by the temperature-independent propagation vector = (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R3+ cations are close to the free-ion value. The magnetic moment of Ni2+ is around 1.4 , the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects and covalency. The thermal evolution of the magnetic structures from T N down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R3+, seems to occur below and very close to T N in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot be neglected.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510170022
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    Paper (German National Licenses)
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