Direct imaging of self-assembled magnetic nanoparticle arrays: Phase stability and magnetic effects on morphology

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Abstract
Chemically stabilized 7.0 nm Fe and 3.9 nm FePt nanoparticles have been self-assembled into thin arrays, and imaged by transmission electron microscopy. The local packing structures and stacking sequences of the arrays were uniquely identified by a combination of bright-field imaging and projected potential image simulations. Close-packed structures, hexagonal close-packed (hcp) and face-centered cubic (fcc), were observed for 7.0 nm Fe nanoparticles, while a loosely packed body-centered cubic (bcc) structure was detected for 3.9 nm FePt nanoparticles as well as fcc and hcp. The bcc structure formation is driven by the large vibrational entropy of particles and the soft interparticle repulsions originating from competing enthalpic and entropic effects on surfactant molecule conformation. The sole anomaly noted is a preference for forming hcp arrays with odd numbers of layers observed for Fe nanoparticles. This is attributed to the thickness- and layer-diameter-dependent magnetic coupling between superferromagnetically ordered array layers.