Synthesis and magnetic properties of gold coated iron oxide nanoparticles

We report on synthesis, structural, and magnetic properties of chemically synthesized iron oxide (Fe3O4) and Fe3O4@Au core-shell nanoparticles. Structural characterization was done using x-ray diffraction and transmission electron microscopy, and the magnetite phase of the core (similar to 6 nm) and fcc Au shell (thickness of similar to 1 nm) were confirmed. Magnetization (M) versus temperature (T) data at H=200 Oe for zero-field-cooled and field-cooled modes exhibited a superparamagnetic blocking temperature T-B similar to 35 K (40 K) for parent (core-shell) system. Enhanced coercivity (H-c similar to 200 Oe) at 5 K along with nonsaturating M-H loops observed for Fe3O4@Au nanoparticles indicate the possible role of spin disorder at the Au-Fe3O4 interface and weak exchange coupling between surface and core spins. Analysis of ac susceptibility (chi' and chi '') data shows that the interparticle interaction is reduced upon Au coating and the relaxation mechanism follows the Vogel-Fulcher law. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3059607]