Field-induced layering of confined ferromagnetic nanoparticles: the role of attractive interactions

Abstract

We present Molecular Dynamics simulation results for confined ferromagnetic nanoparticles (film geometry) under the influence of strong, homogeneous magnetic fields. We focus on the role of short-ranged attractive interactions between the particles for the structure formation in thin films consisting of four to five monolayers. To this end we compare simulation results for Stockmayer particles (Lennard-Jones plus dipolar interactions) with corresponding results for dipolar soft spheres, where the short-ranged potential is purely repulsive. At moderate and large average densities, both systems exhibit pronounced layering. Based on the corresponding normal pressure oscillations we show that the field-induced effects on the layer formation in this density range are essentially independent of the short-range interactions. The dipolar soft sphere system merely has a stronger tendency to form a crystal-like lateral structure upon application of a parallel field. More pronounced differences occur at lower densities and not too high dipolar coupling strengths within the vapour–liquid region of the confined, zero-field Stockmayer system. Applying a perpendicular field, the Stockmayer system develops a blob structure absent in the corresponding repulsive system. Finally, for very large values of both, dipolar coupling parameter and field strength of a perpendicular field, we find evidence for a field-induced in-plane crystallisation of the layered systems.