The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigated by means of spin dynamics simulations coupled to molecular dynamics, using a distance-dependent exchange interaction. Finite-size effects in the total magnetization as well as the influence of the free surface and the surface/core proportion of the nanoclusters are analyzed in detail for a wide temperature range, going beyond the cluster and bulk Curie temperatures. Comparison is made with experimental data and with theoretical models based on the mean-field Ising model adapted to small clusters, and taking into account the influence of low coordinated spins at free surfaces. Our results for the temperature dependence of the average magnetization per atom M(T ), including the thermalization of the transnational lattice degrees of freedom, are in very good agreement with available experimental measurements on small Fe nanoclusters. In contrast, significant discrepancies with experiment are observed if the translational degrees of freedom are artificially frozen. The finite-size effects on M(T ) are found to be particularly important near the cluster Curie temperature. Simulated magnetization above the Curie temperature scales with cluster size as predicted by models assuming short-range magnetic ordering. Analytical approximations to the magnetization as a function of temperature and size are proposed.

Natural gas (NG) is an interesting primary fuel; its larger-scale use is hindered by the difficulties of storing it under high pressures or low temperatures; a viable alternative is its storage via physisorption in porous materials. Most NG adsorption studies have focused on adsorption of pure methane, its primary component. Here we investigate the influence of heavier alkanes commonly found in NG (propane, ethane) on the adsorption process. We present the results of extensive molecular dynamics simulations of mixtures of methane–propane and methane–ethane at T = 300 and 400 K and P = 0–1500 bar in slit-shaped pores with interlayer spacings H = 8–20 Å. We observed that heavier hydrocarbons adsorb preferentially but remain mobile, which is promising for the intended application. We also solved a common problem with simulations of molecules with high adsorption affinity: the difficulty to determine their partial pressure. We developed an Arrhenius-type relationship allowing the calculation of these partial pressures from relationships between energy distributions of the different molecules in the simulations in conditions where a direct determination of these is impractical or impossible.