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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5488-5490 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The static critical properties of classical, isotropic Heisenberg ferromagnets have been studied numerically. Using an improved cluster algorithm we performed extensive Monte Carlo simulations on L×L×L simple cubic and body-centered cubic systems with L≤40 over a temperature range covering the critical region. Temperature dependences of thermodynamic quantities were determined with an optimized multiple-histogram method, and both critical temperatures and critical exponents were extracted from finite-size scaling of several different thermodynamic quantities. It was found that within their respective error bars the critical exponents for both ferromagnets agree with each other and with field theoretical predictions.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 6215-6217 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have used Monte Carlo computer simulations to study the behavior of an Ising model consisting of two ferromagnetic layers with different interaction constants coupled weakly together. For the range of lattice sizes studied it appears as though the system undergoes a single transition at the transition temperature of an isolated layer with the stronger coupling, but substantial changes in the thermodynamic properties also occur near the transition temperature of an isolated layer with the weaker of the two couplings.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6091-6095 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Although classical spin systems in two dimensions cannot show usual long range order, they may exhibit unusual phase transitions of several different types. We present an overview of the work which has been done to understand both the statics and dynamics of these systems and offer some challenges for the future.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 6153-6157 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent developments in the use of histograms (measured discrete probability distributions) now make it possible to determine critical properties with high accuracy using Monte Carlo (MC) simulations combined with finite-size scaling analyses. The advantages and limitations of the use of histograms for extracting the maximum information from MC simulation data are described. Results from extensive MC studies of the critical properties of two important magnetic systems, the d=3 ferromagnetic Ising and Heisenberg models, are presented. It is shown that the careful use of histogram techniques can provide results comparable or superior to these obtained with other numerical methods while maintaining all the advantages of standard MC techniques.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 6158-6160 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An importance sampling Monte Carlo method has been used to simulate L×L×nD Ising superlattices consisting of n alternating layers of ferromagnetic and antiferromagnetic material each of which is D atomic planes thick. Spins within each plane are coupled ferromagnetically and the interface interations are assumed to be ferromagnetic. From data on layer properties, the phase diagram is determined as a function of temperature and magnetic field. A series of phase transitions and tricritical points are found.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5645-5647 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using spin dynamics simulations we have studied magnetic excitations in the randomly site-diluted, body-centered cubic, classical Heisenberg antiferromagnet at a low temperature. With site configurations and initial spin configurations generated by Monte Carlo simulations, equations of motion for spins in 40×40×40 systems with periodic boundary conditions and only nearest-neighbor interactions were integrated using a fourth-order predictor-corrector method. Neutron-scattering functions were then determined from the space-time Fourier transform of averaged spin-spin correlation functions convoluted with a resolution function. We found that at occupation fraction of magnetic ion p=1 only spin-wave excitations exist, while at p=0.245 (percolation threshold) only fracton excitations are present. In the intermediate region spin-wave excitations at long length scales cross over to fracton excitations at short length scales.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 5427-5429 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Employing Monte Carlo simulations, the equilibrium dynamic critical behavior or L×L×L simple cubic classical Heisenberg models with periodic boundary conditions are determined. From the long-time exponential decay of the time-displaced correlation function of the magnetization the linear (equilibrium) relaxation time τ is also determined. It is found that at the critical point Tc, τ grows like τ(approximately-equal-to)Lz with z=1.94±0.06. In the paramagnetic region a power-law divergence τ(approximately-equal-to)(T/Tc−1)−Δ was found with Δ=zν=1.38±0.04, which implies a dynamical critical exponent z=1.96±0.07. Both values are consistent with theoretical predictions and results of experimental studies concerning the dynamical critical behavior of Heisenberg magnets perturbed by order-parameter nonconserving forces.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 1444-1454 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A three-dimensional lattice gas model with nearest-neighbor attractive interaction confined to a slitlike adsorbing capillary of thickness D is studied by computer simulation, varying the chemical potential μ, temperature, T, as well as the strength of the short-range interaction between the walls of the capillary and the gas. We show that the chemical potential μc(D) at the condensation transition in the capillary is shifted relative to its bulk value μc(∞) according to the Kelvin equation, μc(D)−μc(∞)∝D−1, for large enough D. Deviations are found for small D, however (of the order of 10 lattice spacings), particularly under conditions where for μ=μc(∞) the surfaces of the capillary for D→∞ are wet. For D=16 lattice spacings we also locate a capillary condensation critical point, and study the distortion of the gas–fluid coexistence curve due to the attractive walls. Profiles of density and local energy across the capillary are studied, and data for the temperature dependence of the local density at the wall both in the gas and in the liquid phase are presented. It is shown that these quantities vary very smoothly near the wetting temperature TW of the bulk (D→∞) system.
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 5807-5809 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Monte Carlo simulations and ground-state calculations have been used to map out the phase diagram of a system of classical spins, on a simple cubic lattice, where nearest-neighbor pairs of spins are coupled via competing antiferromagnetic superexchange and ferromagnetic double-exchange interactions. For a certain range of parameters, this model is relevant for some magnetic materials, such as doped manganites, which exhibit the remarkable colossal magnetoresistance effect. The phase diagram includes two regions in which the two sublattice magnetizations differ in magnitude. Spin-dynamics simulations have been used to compute the time- and space-displaced spin–spin correlation functions, and their Fourier transforms, which yield the dynamic structure factor S(q,ω) for this system. Effects of the double-exchange interaction on the dispersion curves are shown. © 2000 American Institute of Physics.
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  • 10
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using Monte Carlo and spin-dynamics techniques,1 we studied the dynamic behavior of the body-centered-cubic classical Heisenberg antiferromagnet with single site anisotropy at the critical temperature. In order that we may directly compare our results to experiment, we performed the simulation with the anisotropy term set to match the degree of anisotropy found23 in both FeF2 (strong anisotropy) and MnF2 (weak anisotropy). Through the determination and analysis of the form of the neutron scattering function S(q,ω) at Tc we have studied the critical dynamics. We have compared our results to simulations1 for the isotropic case, and to experimental results23 and theory45 for the anisotropic case. We have found, in agreement with theory, an intense purely diffusive longitudinal component and a suppressed propogative transverse component. We were able to determine the transverse relaxation rate, Γ⊥, at the limit q→0 since at this limit Γ⊥→ωm, the characteristic frequency, which can be measured by the simulation.1 Γ⊥ was found to be noncritical in agreement with theory and experiment. We were able to make a quantitative comparison to experimental results of Γ⊥(0) for both FeF2 and MnF2. © 1997 American Institute of Physics.
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