1,405 research outputs found
Monte Carlo Study of the Finite Size Effects on the Magnetization of Maghemite Small Particles
Monte Carlo simulations of a model for -FeO (maghemite)
single particle of spherical shape are presented aiming at the elucidation the
specific role played by the finite size and the surface on the anomalous
magnetic behaviour observed in small particle systems at low temperature. The
influence of the finite-size effects on the equilibrium properties of extensive
magnitudes, field coolings and hysteresis loops is studied an compared to the
results for periodic boundaries. It is shown that for the smallest sizes the
thermal demagnetization of the surface completely dominates the magnetization
while the behaviour of the core is similar to that of the periodic boundary
case, independently of . The change in shape of the hysteresis loops with
demonstrates that the reversal mode is strongly influenced by the presence
of broken links and disorder at the surface.Comment: Presented at the 8th Joint MMM-Intermag Conference, San Antonio,
Texas, 7-11 January 2001. Session HF-09. To be published in J. Appl. Phys. 3
pages, 3 figure
Magnetic Domains and Surface Effects in Hollow Maghemite Nanoparticles
In the present work, we investigate the magnetic properties of ferrimagnetic
and noninteracting maghemite (g-Fe2O3) hollow nanoparticles obtained by the
Kirkendall effect. From the experimental characterization of their magnetic
behavior, we find that polycrystalline hollow maghemite nanoparticles are
characterized by low superparamagnetic-to-ferromagnetic transition
temperatures, small magnetic moments, significant coercivities and
irreversibility fields, and no magnetic saturation on external magnetic fields
up to 5 T. These results are interpreted in terms of the microstructural
parameters characterizing the maghemite shells by means of an atomistic Monte
Carlo simulation of an individual spherical shell model. The model comprises
strongly interacting crystallographic domains arranged in a spherical shell
with random orientations and anisotropy axis. The Monte Carlo simulation allows
discernment between the influence of the structure polycrystalline and its
hollow geometry, while revealing the magnetic domain arrangement in the
different temperature regimes.Comment: 26 pages, 8 figures. In press in Phys. Rev.
Population dynamics and reproductive phenology of a harvestman in a tidal freshwater wetland
There is a great amount of ecological information for terrestrial arthropods in several types of habitats, but few studies have focused on populations living in tidal freshwater wetlands. During a two-year field survey, we studied the temporal dynamics of the harvestman Discocyrtus prospicuus inhabiting a freshwater wetland exposed to predictable tides and unpredictable floods. We also explored the effects of temperature, precipitation, and tide level on the population dynamics and reproductive phenology. Our findings are markedly different from those reported in other harvestman species living in different habitats and also from conspecific populations living in the mainland. Adults, subadults, and juveniles remain active throughout the year, and a long breeding season was observed. However, the number of adults, subadults, juveniles, and egg clutches showed clear variations over the year without a consistent seasonal pattern. Contrary to the general pattern in harvestmen, no climatic variables were significant predictors of abundance fluctuations. We discuss the potential masking effect of unpredictable flood events, obscuring the relationship between abundance and abiotic factors. In addition, given that climatic conditions seem to favor harvestmen activity throughout the year, we also hypothesized that abundance variations could be driven mainly by biotic rather than by abiotic interactions.Fil: Iglesias, Patricia Paola. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Pereyra, MartÃn Oscar. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Nordeste. Instituto de BiologÃa Subtropical. Instituto de BiologÃa Subtropical - Nodo Posadas | Universidad Nacional de Misiones. Instituto de BiologÃa Subtropical. Instituto de BiologÃa Subtropical - Nodo Posadas; Argentin
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