756 research outputs found
Nature of phase transition in magnetic thin films
We study the critical behavior of magnetic thin films as a function of the
film thickness. We use the ferromagnetic Ising model with the high-resolution
multiple histogram Monte Carlo (MC) simulation. We show that though the 2D
behavior remains dominant at small thicknesses, there is a systematic
continuous deviation of the critical exponents from their 2D values. We observe
that in the same range of varying thickness the deviation of the exponent
is very small from its 2D value, while exponent suffers a larger
deviation. Moreover, as long as the film thickness is fixed, i. e. no finite
size scaling is done in the direction perpendicular to the film, the 3D
values of the critical exponents cannot be attained even with very large (but
fixed) thickness. The crossover to 3D universality class cannot therefore take
place without finite size scaling applied in the direction, in the limit of
numerically accessible thicknesses. From values of exponent obtained
by MC, we estimate the effective dimension of the system. We conclude that with
regard to the critical behavior, thin films behave as systems with effective
dimension between 2 and 3.Comment: 8 pages, 17 figures, submitted to Phys. Rev.
Frustration Effects in Antiferromagnetic FCC Heisenberg Films
We study the effects of frustration in an antiferromagnetic film of FCC
lattice with Heisenberg spin model including an Ising-like anisotropy. Monte
Carlo (MC) simulations have been used to study thermodynamic properties of the
film. We show that the presence of the surface reduces the ground state (GS)
degeneracy found in the bulk. The GS is shown to depend on the surface in-plane
interaction with a critical value at which ordering of type I coexists
with ordering of type II. Near this value a reentrant phase is found. Various
physical quantities such as layer magnetizations and layer susceptibilities are
shown and discussed. The nature of the phase transition is also studied by
histogram technique. We have also used the Green's function (GF) method for the
quantum counterpart model. The results at low- show interesting effects of
quantum fluctuations. Results obtained by the GF method at high are
compared to those of MC simulations. A good agreement is observed.Comment: 11 pages, 19 figures, submitted to J. Phys.: Condensed Matte
A profiling analysis of contributions of cigarette smoking, dietary calcium intakes, and physical activity to fragility fracture in the elderly
Fragility fracture and bone mineral density (BMD) are influenced by common and modifiable lifestyle factors. In this study, we sought to define the contribution of lifestyle factors to fracture risk by using a profiling approach. The study involved 1683 women and 1010 men (50+ years old, followed up for up to 20 years). The incidence of new fractures was ascertained by X-ray reports. A “lifestyle risk score” (LRS) was derived as the weighted sum of effects of dietary calcium intake, physical activity index, and cigarette smoking. Each individual had a unique LRS, with higher scores being associated with a healthier lifestyle. Baseline values of lifestyle factors were assessed. In either men or women, individuals with a fracture had a significantly lower age-adjusted LRS than those without a fracture. In men, each unit lower in LRS was associated with a 66% increase in the risk of total fracture (non-adjusted hazard ratio [HR] 1.66; 95% CI, 1.26 to 2.20) and still significant after adjusting for age, weight or BMD. However, in women, the association was uncertain (HR 1.30; 95% CI, 1.11 to 1.53). These data suggest that unhealthy lifestyle habits are associated with an increased risk of fracture in men, but not in women, and that the association is mediated by BMD
Re-orientation Transition in Molecular Thin Films: Potts Model with Dipolar Interaction
We study the low-temperature behavior and the phase transition of a thin film
by Monte Carlo simulation. The thin film has a simple cubic lattice structure
where each site is occupied by a Potts parameter which indicates the molecular
orientation of the site. We take only three molecular orientations in this
paper which correspond to the 3-state Potts model. The Hamiltonian of the
system includes: (i) the exchange interaction between nearest-neighbor
sites and (ii) the long-range dipolar interaction of amplitude
truncated at a cutoff distance (iii) a single-ion perpendicular
anisotropy of amplitude . We allow between surface spins, and
otherwise. We show that the ground state depends on the the ratio
and . For a single layer, for a given , there is a critical value
below (above) which the ground-state (GS) configuration of molecular axes
is perpendicular (parallel) to the film surface. When the temperature is
increased, a re-orientation transition occurs near : the low- in-plane
ordering undergoes a transition to the perpendicular ordering at a finite ,
below the transition to the paramagnetic phase. The same phenomenon is observed
in the case of a film with a thickness. We show that the surface phase
transition can occur below or above the bulk transition depending on the ratio
. Surface and bulk order parameters as well as other physical quantities
are shown and discussed.Comment: 7 pages, 11 figures, submitted for publicatio
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