Inert Gas Condensation of Iron and Iron-Oxide Nanoparticles
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Inert Gas Condensation of Iron and Iron-Oxide Nanoparticles C. Baker,1 S. Ismat Shah,1,3,4 S.K. Hasanain,2 B. Ali,2 L. Shah,2 G. Li,3 T. Ekiert,3 and K.M. Unruh3 (1) Department of Materials Science and Engineering, University of Delaware, Newark, DE (2) Department of Physics, Quaid-i-Azam University, Islamabad, Pakistan (3) Department of Physics and Astronomy, University of Delaware, Newark, DE (4) Fraunhofer Center for Manufacturing and Advanced Materials, Newark, DE
Abstract An inert gas condensation technique has been used to prepare nanometer-sized particles of metallic iron by evaporation and agglomeration in a flowing inert gas stream. The resulting Fe nanoparticles were protected from complete oxidation either by the formation of a thin Fe-oxide surface passivation layer or by immersion in an oil bath. X-ray diffraction and transmission electron microscopy measurements indicated that the nanoparticles were typically between 10 and 20 nm in size, that the thickness of the Fe-oxide surface passivation layer was between 3 and 4 nm, and that the oil immersed samples exhibited a significant smaller volume fraction of Fe-oxides than did the surface passivated samples. Room temperature magnetization measurements were also carried out and the coercivity and saturation magnetization of the surface passivated and oil immersed samples determined. Although the coercivities and saturation magnetization values of both samples were very similar, the Fe/Fe-oxide samples exhibited a single component hysteresis loop while the Fe/oil samples exhibited a two component loop.
Introduction Nanometer-sized Fe particles are pyrophoric and therefore must be protected from complete oxidation if they are to be exposed to atmospheric oxygen. The most common passivation procedure has been to form an Fe-oxide surface layer by slowly exposing the as-prepare nanoparticles to an oxygen containing environment.1 Because the Fe-oxides are either antiferromagnetic (FeO, α-Fe2O3) or ferrimagnetic (γ-Fe2O3, Fe3O4),2 however, the physical and chemical properties of these Fe/Fe-oxide nanoparticles reflect not only the properties of the Fe core but the properties of the passivation layer as well. For example, the hysteresis loops of these nanoparticles have often been observed to be shifted with respect to zero applied field when field cooled through the Néel temperature of the surface oxide due to an exchange coupling between the ferromagnetic core and the antiferromagnetic (or ferrimagnetic) surface oxide.3 Greater insight into the effects of finite size on the magnetic properties of small Fe particles, without the added complication of a core/shell exchange coupling, could be obtained by the study of particles passivated by a non-magnetic surface layer. Several schemes for achieving this have been reported in the literature including, for example, various chemical and physical methods for producing C, SiO2, and Au coatings.4-6 An alternative approach involves immersing the Fe nanoparticles into an oil prior to their exposure to air. This
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