Size- and temperature-dependent magnetization of iron nanoclusters

Abstract

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.

Spin-lattice dynamics model with angular momentum transfer for canonical and microcanonical ensembles

“…By removing the periodicity of the system, different Curie temperatures can be observed between the static and dynamic lattice case, due to the surface effects that induce a variation of the effective exchange constant in the order of 10%. Santos et al 59 have performed a systematic comparison of the finite size effects of the magnetisation of static and dynamic lattice calculation and their results showed differences between the two even in the case of periodic systems. This suggests that the individual parameterisation of the exchange and inter-atomic potential can largely influence the behaviour of the equilibrium magnetisation.…”

Spin-lattice model for cubic crystals

“…Previous studies have been investigating the relevance of the Néel model to simulate surface effects in 3-d magnetic metals 52,53 . Future work could leverage our framework to develop surface interaction models for spinlattice simulations of magnetic nanoparticles 54 .…”

Temperature-dependent magnetism in Fe foams via spin-lattice dynamics

“…Here SLD simulations may prove useful. This technique has been used to study magnetism in Fe nanoparticles, the effect of vacancies on spin waves, the influence of magnetism on phase transitions, and other phenomena [12][13][14][15][16][17][18][19].…”