Large enhancements of magnetic anisotropy in oxide-free iron nanoparticles

Magnetic characterization of spherical, oxide-free, bcc iron nanoparticles synthesized with β-diketone surfactants has been performed. The results of this characterization, which included particles with diameters ranging between 2 and 5nm show that the nanoparticles have an average anisotropy of 1.9×106±0.3×106J/m3, which is more than an order of magnitude greater than the magnetocrystalline anisotropy of bulk iron. Despite their unusually large anisotropy, these particles can have saturation magnetizations of up to 210Am2/kg (slightly lower than bulk iron). High-energy X-ray diffraction data indicates that the Fe particles have a distorted bcc lattice, which could, at least in part, explain the magnetic behavior of these nanoparticles. Dipolar coupling between particles, while present, is weak and cannot account for the high anisotropy of these nanoparticles.