Magnetostatic interactions in highly ordered self-assembled arrays of epsilon-cobalt nanoparticles

Interparticle magnetostatic interactions are studied in magnetic nanoparticle assemblies. &epsis;-Co nanoparticles obtained by solution chemistry techniques were used for these studies. These particles are highly monodisperse and self-assemble to form macroscopic ordered structures. 2D arrays (monolayers) were prepared via a modified Langmuir film technique and 3D arrays ( crystals) by colloidal crystallization. The interaction between the particles in the arrays is purely magnetostatic due to the surfactant layer on the particle surface that keeps particle cores well-separated for exchange interaction to become significant. Evidence of interparticle magnetic correlations is observed and quantitative estimates on correlation lengths are obtained in 2D and 3D arrays using the Small Angle Neutron Scattering (SANS) and Electron Holography, respectively. Dipolar coupled domains are found to be much larger (∼17 μm) in 2D monolayers compared to 3D crystals (∼70 nm). The differences in domain sizes are explained by shape anisotropy due to dimensionality and anisotropy averaging effects due to random crystallographic orientation of particles in the array. Macroscopic zero-field cooled (ZFC) measurements performed on MPMS-SQUID magnetometer were used to estimate the net anisotropy in the two structures. A change in curvature is observed in Arrott plots similar to exchange-based ferromagnets. The dipolar Curie temperature thus measured is consistent with theoretical estimates and is much higher than the blocking temperature of the particles. The macroscopic magnetic properties of the highly ordered nanoparticle assemblies with purely dipolar coupling are consistent with a very soft nanocrystalline ferromagnet.