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IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
Magnetic Properties of Cobalt and Cobalt–Platinum Alloy Nanoparticles Synthesized Via Microemulsion Technique Amar Kumbhar, Leonard Spinu, Fabrice Agnoli, Kai-Ying Wang, Weilie Zhou, and Charles J. O’Connor
Abstract—Metallic cobalt and cobalt–platinum alloys of various nanometer sizes have been synthesized via the microemulsion technique and their magnetic properties have been characterized. Preparation of cobalt and cobalt–platinum alloy nanoparticles was achieved by reducing aqueous metallic salts confined in the polar regions of the reverse micelle of cetyltrimethyl bromide (CTAB) with sodium borohydride. These particles are further coated with gold by reducing aqueous gold salts with borohydride. The dc susceptibility data of 15 nm gold coated Co, CoPt and CoPt3 particles exhibit a blocking temperature of 4 K, 80 K and 106 K and coercivity of 20 Oe, 200 Oe, and 415 Oe at 10 K, respectively. Annealing these samples at 400 C further enhanced their magnetic properties. The two cobalt–platinum alloys have been synthesized and characterized by x-ray powder diffraction and transmission electron microscopy. Index Terms—Core–shell, nanoparticles, reverse micelle, superparamagnetism.
I. INTRODUCTION
T
HE DEMAND for smaller materials for high density storage media is the fundamental motivation for fabrication of nanoscale magnetic materials. Since the crystallite size distribution and interparticle spacing have great impact on magnetism, the ideal synthesis must provide control over all these parameters. Water-in-oil microemulsion, an increasingly popular method is used to yield nanoparticles with highly uniform morphologies [1], [2]. Depending on the size of micelle, particles range from 2–30 nm and range from superparamagnetic to ferromagnetic at larger sizes [3]. In this paper, we will discuss the magnetic properties of goldcoated cobalt and cobalt–platinum alloy core–shell nanoparticles, grown in reverse micelle solution. Long-range ferromagnetic exchange enhancement is achieved by the addition of iron and cobalt to platinum in low concentrations to form alloys. This enhancement is due to positive spin polarization of the 4d-electrons around the localized magnetic impurities [4]. Ferromagnetic materials like Fe Pt, FePt [5] and CoPt are of current interest in magnetic storage devices due to their high coercivities. Exchange enhancement as well as high magnetocrystalline anisotropy is responsible for the increase in coercivity [6].
II. EXPERIMENTAL A. Materials All chemicals were purchased from Aldrich Chemicals (Milwaukee, WI) and used without further purification. Distilled and deionized water was used throughout the synthesis. B. Preparation of Nanoscale Metallic and Alloy Particles All syntheses were performed in argon using a Schlenk technique. Nanometer size particles were synthesized using reverse micelles of cetyltrimethylammonium bromide (CTAB), using 1-butanol as the co-surfactant and octane as the oil phase [7]–[9]. To this solution, aqueous solution containing the metal ions was added. The molar ratio of water to surfactant governs the size of the reverse micelle. For example, to form a reverse micelle of size 15 nm, the molar ratio of water to [H2O]/[CTAB]) must be adjusted to surfactant (CTAB) ( 15 nm. The metal and alloy nanoparticles are formed within the reverse micelle by the reduction of metallic salts using sodium borohydride as a reducing agent. Sequential synthesis afforded by reverse micelles has been exploited for the synthesis of core–shell nanoparticles. Metal/alloy as the core and gold as the shell. Initially, two such solutions were prepared: one with metal salts (0.425 mmols per 2.5 g of aqueous solution) and another with reducing agent NaBH (2.9 mmols per 2.5 g of aqueous solution). Then both solutions were mixed and stirred for two hours. Excess of borohydride was used to suppress oxidation of cobalt by water. Also, shining UV light throughout the reaction prevents oxidation of the metal and alloy nanoparticle. For the alloyed particles, the molar ratios were prepared in the aqueous phase in the proportions as the desired molar ratio, e.g., 50 : 50 ratio for CoPt, 3 : 1 for CoPt . Further, the micelles were expanded to accommodate a passivating gold shell by reducing 0.06 M aqueous gold salt. The magnetic particles were isolated using magnetic field, and the surfactant was removed by successive washing with chloroform/methanol (1 : 1) and drying in vacuum. The resulting powder was black in color. All solids were annealed at 400 C for 4 hours under argon. The synthesis scheme is presented in Fig. 1. C. Instrumentation
Manuscript received October 13, 2000. This work was supported by DARPA through Grant MDA972-97-1-0003. The authors are with the AMRI, UNO, New Orleans, LA 70148 USA (e-mail:
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A Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer was used for the magnetization measurements. Discussion of calibration techniques and general magnetic susceptibility measurements are reported elsewhere [10]. A Phillips–Norelco x-ray
0018–9464/01$10.00 © 2001 IEEE
KUMBHAR et al.: MAGNETIC PROPERTIES OF COBALT AND COBALT–PLATINUM ALLOY NANOPARTICLES SYNTHESIZED
Fig. 1. Synthesis scheme for preparation of gold-coated metal and alloy core-shell nanoparticles.
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Fig. 3. A typical XRD powder pattern of gold-coated CoPt alloy. TABLE I SUMMARY OF MAGNETIC DATA OBTAINED ON THE AS PREPARED AND ANNEALED GOLD COATED COBALT AND COBALT–PLATINUM ALLOYS
Fig. 2. The transmission electron micrograph of as prepared gold-coated CoPt . The bar represents 25 nm.
attachment on JEOL 2010 TEM. Characteristic peaks of gold, cobalt and platinum were present. Powder X-ray diffraction patterns of the reverse micelle synthesized materials revealed average crystallite size of 20 nm calculated using the Sherrer formula and comparing the full width at half maximum (FWHM) of the most intense peak of the sample to an internal reference standard. A typical plot of XRD powder pattern is given in Fig. 3. In this figure the characteristic peaks of gold and CoPt alloys appear. This is characteristic in both the as prepared and annealed samples. In agreement with the XRD data, TEM images reveal uniformly sized particles with little aggregation. B. Magnetic Properties
diffractometer with a graphite monochromator and the photomultiplier tube (PMT) detector was used to obtain x-ray diffraction (XRD) plots. Transmission electron microscopy (TEM) imaging was performed on a JEOL 2010 transmission electron microscope. III. RESULTS AND DISCUSSION A. Structural Properties Morphology of the as prepared and the annealed samples was determined using TEM. A representative TEM image of as prepared gold-coated CoPt nanoparticles is presented in Fig. 2. The average size was determined to be 20 nm, TEM image shows that the particles are uniform in size and are spherical in shape. Images of other samples were obtained, but are not illustrated due to space constraints. Nominal elemental analysis was performed using a energy dispersive analysis by X-ray (EDAX)
The dc-susceptibility and hysteresis measurements were performed on powdered samples of the nanoparticles. Magnetic measurements performed on powdered samples reveal that the particles have a single domain structure and are superparamagnetic at room temperature. Magnetic properties of the goldcoated cobalt and cobalt–platinum alloy nanoparticles are summarized in Table I. A representative plot of the temperature dependent susceptibility of a sample of 15 nm CoPt coated with gold particles measured at 100 Oe is shown in Fig. 4. In all as prepared samples, the magnetization plots exhibit a cusp in the zero field cooled (ZFC) susceptibility at the blocking tempera. Above , in the superparamagnetic regime, the partiture cles align freely with the field during the measuring time and depart from the ZFC susceptibility at a temperature near the ZFC maximum and increases below this temperature. The values of both blocking temperature and coercivity increase as the Pt content increases.
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
Fig. 4. A dc-susceptibility plot of gold-coated CoPt . The measurements were performed using a 100 Oe measuring field.
temperature: 500 Oe (Fig. 5) versus 25 Oe at 300 K. We suppose that the observed effect of annealing on the magnetic properties for CoPt/Au, is caused by ordering in their cores. Annealing the as prepared disordered face-centered cubic (fcc) structure into ordered face-centered tetragonal (fct) structure, which has a high uniaxial magnetocrystalline anisotropy ( ) is responsible for the enhanced coercivity at 300 K for the goldcoated CoPt nanoparticles. Thus, water-in-oil microemulsion technique provides a useful method of synthesizing monodisperse core–shell nanoparticles. Magnetic data on the samples reveal that the particles are superparamagnetic, single domain, and have a narrow size distribution. Annealing the as prepared samples increases the intrinsic coercivity for the gold-coated CoPt nanoparticles. Studies on size dependent magnetic properties and effect of different annealing temperatures on the magnetic properties are in progress. Also its utility as a giant magneto-resistive material is being explored. ACKNOWLEDGMENT The authors would like to thank Dr. John B. Wiley from Department of Chemistry, UNO, for providing the furnace to anneal these samples. REFERENCES
Fig. 5. A magnetization vs. field plot of the annealed sample of gold-coated CoPt nanoparticles at 300 K. The magnetization was measured as the field oscillates between 50 to 50 kOe. The inset shows hysteresis at lower fields, showing the coercivity of 500 Oe.
0
For the as prepared samples, above the blocking temperature, in the superparamagnetic regime, no coercivity or remanence is observed. Annealing of the Co/Au and CoPt /Au composites at 400 C for 4 hours did not cause any significant change in the magnetic behavior. In contrast, the CoPt/Au samples showed an increase in coercivity, which is especially noticeable at higher
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