Study of anomalous behavior in solution synthesized iron nanoparticles
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The magnetic and physical properties of oxide-free, ligand passivated, iron nanoparticles were studied using superconducting quantum interference device (SQUID) magnetometry and synchrotron based X-ray radiation. Particles used for this study ranged in diameter between 2 and 10 nm, which made it possible to distinguish between bulk and surface effects in the nanoparticles’ properties. Additionally, the effects of two different weakly interacting ligands (2,4-pentanedione and hexaethylene glycol monododecylether) on the nanoparticles’ behavior were studied. The results of this study were compared to theoretical predictions of magnetic transition metal behavior in both thin films and nanoparticles, as well as experimental results from measurements of transition metal clusters formed in an inert carrier gas and measured with a Stern-Gerlach magnet. Magnetometry revealed that the iron nanoparticles have a magnetocrystalline anisotropy an order of magnitude greater than bulk iron. At the same time, these particles exhibit a saturation mass magnetization up to 209 Am2/kg, which is only slightly lower than bulk iron. The structural properties of these particles were characterized using high energy X-ray diffraction analyzed using the atomic pair distribution function method (PDF). The PDF analysis indicates that the Fe particles have a distorted and expanded form of the bcc lattice, which could, at least in part, explain the magnetocrystalline anisotropy of these nanoparticles. X-ray absorption fine structure (XAFS) was used to study the surface properties of the iron nanoparticles and further characterize their structural properties. XAFS showed that oxidized species of iron exist at the nanoparticles’ surface and can be attributed to iron/ligand interactions. The percentage of oxidized species scales with the surface to volume ratio of the nanoparticles, and therefore appears limited to the nanoparticle surface. The layer of Fe(II) species present at the nanoparticles’ surface accounts for the reduction in saturation mass magnetization values (when compared to bulk iron) observed in these particles. XAFS analysis also provided further confirmation of the nanoparticles’ expanded crystalline lattice.