Magnetic properties of self-assembled iron nanoparticle arrays

Nanoparticles of Fe were synthesized via thermal decomposition of iron pentacarbonyl, Fe(CO)₅, in the presence of surfactants. Heterogeneously nucleating particles from Pt seeds led to high moment, minimally oxidized Fe particles 4.5–9 nm in diameter. Homogeneous nucleation of particles in the presence of an excess of oleic acid led to formation of partially oxidized particles, consisting of an Fe core and an oxide shell, 9–19 nm in diameter. Once synthesized, the particles were dispersed in hexane, and the hexane evaporated from the dispersion. During the evaporation, the particles self-assembled to form particle superlattices. The size and quality of the particle arrays depended on particle and surfactant concentration and drying conditions. Transmission electron microscopy (TEM) was used to characterize the size and structure of both particles and particle superlattices. Structural evidence for magnetic interactions between particles in the arrays was observed. Samples of hcp superlattices of 6.6 nm, high moment Fe particles displayed a preference for odd numbers of layers. This was not observed in arrays of low moment particles, and has not been reported for non-magnetic particles. The magnetic properties of dilute particle suspensions and dried particle arrays were measured using a Quantum Design MPMS magnetometer. The hysteretic and remanent behavior of both the dispersions and dried assemblies were indicative of the existence of dipole interactions between particles. Differences in the magnetic behavior of dispersions and arrays indicated that dipole interaction effects depend on the size and structure of particle assemblies. Magnetizing interactions play a larger role in the large, close-packed arrays than in the smaller, loosely-associated clusters contained in the dispersions. The magnetizing effects in the arrays can be enhanced by decreasing the interparticle spacing. The arrays were also magnetically anisotropic, with magnetic properties depending on the angle between the applied field direction and the array plane.