3094 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007 Static and Dynamic Magnetic Properties of Composite Au-Fe O Nanoparticles Natalie A. Frey , Sanyadanam Srinath , Hariharan Srikanth , Chao Wang , and Shouheng Sun Department of Physics, University of South Florida, Tampa, FL 33647 USA School of Physics, University of Hyderabad, Hyderabad, Andhra Pradesh 500046, India Department of Chemistry and Division of Engineering, Brown University, Providence, RI 02912 USA Fe O (9 nm) grown epitaxially on one or several facets of a seed Au particle (8 nm) leads to two distinct composite nanoparticle structures dubbed “dumbbell-” and “flower-like.” These particles exhibit novel magnetic behavior such as large magnetic anisotropy compared to the single component Fe O . We have studied the static and dynamic magnetic properties using temperature and field- dependent dc magnetization and RF transverse susceptibility. While the magnetic properties of dumbbell-like particles are relatively conventional, field-cooled M-H loops of flower-shaped particles as well as the transverse susceptibility show large shifts along the field axis indicative of exchange bias. Index Terms—Exchange bias, magnetic anisotropy, magnetic nanoparticles, transverse susceptibility. I. INTRODUCTION M AGNETIC nanoparticles have been the focus of much interest as they show properties that are dramatically dif- ferent than that of bulk magnetic systems. In such small parti- cles, surface effects and particle shapes play a large role in the observed magnetic behavior. Recently, the study of magnetic nanoparticles has focused on more complicated structures such as core-shell particles [1] and nanocomposites [2]. Nanoparticles of Fe O and Au have been extensively studied as individual systems: Fe O for its magnetic properties and Au for its optical and electronic properties as well as its desirable surface chemistry. The joining of these two materials in a unique nanostructure would be interesting from applications perspec- tive as well as for the opportunity to learn more about the fun- damental physics of composite nanostructures. Due to the lat- tice constant of Fe O [ 8.35 A) being very nearly double that of Au ( 4.08 A)], it turns out that Au and Fe O can be grown as epitaxial composite nanoparticles. Based on the chem- istry of the reaction, Fe O can grow on one facet or multiple facets of an Au seed particle. The former results in a so-called “dumbbell-like” nanoparticle and the latter in a “flower-like” nanoparticle. (Henceforth, we will refer to them as just dumb- bells and flowers). Besides being composed of two technolog- ically important materials thus rendering them “bifunctional,” such structures represent new surface spin configurations for the much studied Fe O nanoparticles. Both the dumbbell and flower Au-Fe O structures have been synthesized and details have been reported in an earlier work [2]. In this paper, we specifically focus on the static and dynamic magnetic charac- terization and the influence of interfacial interactions on the col- lective magnetization of these particles. II. NANOPARTICLE PREPARATION AND CHARACTERIZATION The dumbbell Au-Fe O nanoparticles were prepared by de- composing iron pentacarbonyl Fe(CO) , over the surface of Au Digital Object Identifier 10.1109/TMAG.2007.893846 Fig. 1. (a) ZFC-FC curves for Au-Fe O dumbbells. Inset shows the TEM image of Au (8 nm)-Fe O (9 nm) dumbbells. (b) ZFC-FC curves for flowers with the TEM image shown in the inset. nanoparticles, followed by oxidation in air. By changing the sol- vent from a nonpolar hydrocarbon to a slightly polarized solvent (e.g., diphenyl ether), the geometrical configuration of the com- posite particles evolves from dumbbell to flower [2]. The insets of Fig. 1 show transmission electron microscopy (TEM) images of the nanoparticles with the Au particles ap- pearing darker due to the high electron density of Au. In the inset of Fig. 1(a), the TEM image of the dumbbell particles with 8-nm Au and 9-nm Fe O particles is shown. The inset of Fig. 1(b) shows the TEM of flower nanoparticles with individual Au and 0018-9464/$25.00 © 2007 IEEE