The formation of magnetic nanoclusters is simulated using molecular dynamics simulations. In general, metals can not be described by a simple pair potential approach. Therefore, the embedded atom method is used to account for the specific properties of the investigated materials. In combination with an alloy model proposed by Johnson, the embedded atom method allows for investigations with multiple substituents making it a feasible tool to explore crystalline formations in magnetic materials. The temperature is adjusted and controlled by a Nosé-Hoover thermostat. To test the developed simulation tool, bulk material properties such as the equilibrium lattice constant or the thermal expansion coefficient are determined for different published potentials. Furthermore, the experimentally approved core-shell formation of CoAg is simulated. The calculation of the annealing process yields a clear segregation of the Ag atoms towards the surface. The crystal planes of the core do not show a unique stacking order. To investigate the layering of the core, an automated analysis tool is employed. CoAg core-shell formations exhibit an intermixture between hexagonal closed packed fractions and a face centered cubic structure in the considered parameter range.
Magnetically, nanoparticles behave differently as compared to bulk materials. The high surface-to-volume ratio of nanoclusters influences the magnetic properties drastically. Additionally, the morphology of a nanocluster influences the effective magnetocrystalline anisotropy. The facets of aggregates consisting of a few thousand atoms depend on the systems size and is a direct consequence of the surface energy minimization. A magnetic model is introduced to account for the peculiarities of nanoclusters. A long-range formulation of the exchange energy accounts for deviations in the exchange energy appearing for atoms close to the surface. Surface atoms exhibit less neighbors than non-surface atoms, leading to lower exchange energies. Therefore, the magnetocrystalline anisotropy is modeled differently for surface atoms and non-surface atoms. The magnetocrystalline aniso-tropy of non-surface atoms is described by using local anisotropy axes depending on the fluctuations of the neighboring atoms. Subsequently, thermal fluctuations arising from the motion of the atoms in the magnetic system are explored. The specific formulation of the phonon dependent magnetic model leads to an implicitly coupled phonon-magnon system. The additional fluctuations arising from the varying positions of the atoms do not significantly change the magnetic behavior. In contrast, using a long-range exchange formulation yields a coercive field 20% smaller as compared to a standard next neighbor exchange model.