IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 3397 Synthesis and Property of Core-Shell Ag@Fe O Nanoparticles Chih-Huang Lai , Member, IEEE, Tsung-Feng Wu , and Ming-Der Lan Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, R.O.C. Department of Physics, National Chung Hsing University, 250, Kuo Kuang Road, Taichung, Taiwan 40227, R.O.C. Ag nanoparticles with Fe O shells have been synthesized by the polyol method. The Ag@Fe O core-shell nanoparticles exhibited a superparamagnetic behavior at the temperature higher than the blocking temperature (100 K) and showed shifted loops under a field cooling process. The enhanced magnetic anisotropy was observed and was ascribed to the surface effect. Index Terms—Core-shell structure, exchange bias, Fe O nanoparticles, superparamagnetic. I. INTRODUCTION N ANOPARTICLES (NPs) have been extensively studied on the optical, electrical, and magnetic properties due to their quantum size effects and large surface area. Although appli- cations of magnetic NPs have been proposed in the areas of magnetic recording [1], biological sensors, biomedical [2], na- noelectronics [3], and catalysts [4], researchers still attempt to combine magnetic materials with others to exhibit additional functions. A core/shell structure has been demonstrated to be a new route to fabricate NPs with new characteristics that can not be achieved by using a single component. The transition metal NPs have been used as catalysts in the miscellaneous chemical reactions. For example, Klabunde et al. [5] reported that Fe O @MgO and Fe O @CaO significantly improved ef- ficiencies of SO adsorption and H S removal, respectively, comparing to pure MgO and CaO. Haruta et al. [6] also re- ported gold particles covered on iron oxides were greatly re- active with CO oxidation, and the similar reaction was also ob- served on the composites of silver with iron oxides. In addition, NPs of a core-shell structure exhibited peculiar magnetic prop- erties. Sun and coworkers [7] synthesized FePt NPs coated with MFe O shells such as Fe O and CoFe O , and demonstrated that magnetic properties could be tailored by the relative dimen- sions of core/shell thickness and chemical composition. Further- more, the thermal stability of magnetic NPs was improved by antiferromagnetic shells [8]. Relatively few reports have been published on the magnetic properties of NPs with a structure of a nonmagnetic core and a magnetic shell. In this work, we fab- ricated core-shell Ag@Fe O NPs, and the magnetic properties of NPs were investigated. II. EXPERIMENT We synthesized the Ag@Fe O core-shell structure by the two-step polyol method. First, the Ag NPs were made by reduc- tion of Ag(acac) (0.5 mmol) in the presence of 1,2-hexadecane- diol (0.4 g), oleic acid (160 L) and oleyl amine (224 L) in 10 mL of octyl ether. The solution was heated to 200 C and stirred magnetically for one hour. After cooling the solution, Digital Object Identifier 10.1109/TMAG.2005.855212 ethanol was added into the solution to separate products by cen- trifugation (6000 rpm, 10 min). The Ag nanoparticles could be re-dispersed in the hexane, an orange dispersion. 60 mg of Ag NPs dispersed in the hexane were used as the seeds and mixed with Fe(acac)3 (1 mmol), 1,2-hexadecanediol (1.3 g), oleic acid (80 nL), and oleylamine (80 L) in the phenyl ether solvent (20 mL). The mixture was heated at 100 C for 40 min to re- move hexane. Then the solution was heated at 200 C for 1 h, and continuously heated at 260 C for another 30 min. The black product was precipitated by injecting ethanol and centrifuging. Final product could be redispersed into hexane again. All syn- thesis was carried out under argon atmosphere. The phases of NPs were identified by using a powder X-ray diffractometer (XRD) at Cu K radiation. The assembly for X-ray diffraction was prepared by dropping the hexane disper- sion of NPs on Si substrates. The microstructure of NPs was observed with a transmission electron microscope (TEM) at an accelerating voltage of 120 kV. X-ray photoelectron spec- troscopy (XPS) was used to verify the valence state of Fe O . The UV-visible spectra were taken with calibration of hexane background absorption to measure surface plasmon absorption of NPs. M-H hysterises loops and M-T curves were obtained by using a superconducting quantum interference device (SQUID) magnetometer. All samples for magnetic measurements were prepared by drying NPs on Si substrates. The zero-field-cooled (ZFC) process was carried out in the absence of an external field when the sample was cooled from room temperature to 5 K, and the magnetization was subsequently recorded during a heating process. The field-cooled (FC) process was measured in the sim- ilar way except the existence of the external field of 100 Oe during the cooling of samples. III. RESULTS AND DISCUSSION Fig. 1 shows the X-ray diffraction patterns of the assemblies of Ag NPs Fig. 1(a), Fe O NPs [Fig. 1(b)], and core-shell Ag@Fe O NPs [Fig. 1(c)], respectively. Compared to Fig. 1(a) and (b), Fig. 1(c) is clearly composed of two sets of peaks: Ag and Fe O . Because it is not easy to differentiate the magnetite phase (Fe O ) from the maghemite phase ( -Fe O ) by only using XRD spectra, XPS was performed to identify the valance state of Fe in NPs. Two characteristic peaks were observed of Fe 2p3/2 at 710.9 eV and 2p1/2 at 723.8 eV, shown in Fig. 2. 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