Chen Copt And Fepd

JOURNAL OF APPLIED PHYSICS

VOLUME 91, NUMBER 10

15 MAY 2002

Synthesis of spherical FePd and CoPt nanoparticles Min Chena) and David E. Nikles Department of Chemistry and Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama 35487-0209

Co48Pt52 nanoparticles were prepared by the simultaneous chemical reduction of platinum acetylacetonate and the thermal decomposition of cobalt tricarbonyl nitrosyl. Similarly, Fe50Pd50 nanoparticles were prepared by the simultaneous reduction of palladium acetylacetonate and the thermal decomposition of iron pentacarbonyl. The average diameter for the Co48Pt52 particles was 7 nm, while the average diameter for the Fe50Pd50 particles was 11 nm. Films of the particles were cast onto silicon wafers from hydrocarbon solution. Only after annealing at 700 °C for three hours did the Co48Pt52 films transform to the L10 phase. However there was no evidence for the tetragonal phase for the films containing Fe50Pd50 particles after annealing at 700 °C for three hours. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1456406兴

关 Co共CO兲3 NO兴 was purchased from Strem Chemicals. Ethanol, hexane and octane were purchased from Fisher Scientific. The carbon-coated copper grids 共400-count model No. 2450兲 were purchased from Structure Probe Inc. CoPt nanoparticles: Platinum acetylacetonate 共0.50 mmol兲 and 1,2-hexadecanediol 共1.5 mmol兲 were dissolved in 20 mL dioctyl ether in a three-necked round bottom flask with magnetic stirring under nitrogen at 100 °C. After the Pt共acac兲2 completely dissolved, Co共CO兲3 NO 共1.0 mmol兲, oleic acid 共0.50 mmol兲, and oleylamine 共0.50 mmol兲 were added to the reaction mixture. The mixture was heated to 286 °C 共the boiling point of dioctyl ether兲 and the mixture allowed to reflux for 30 minutes. The reaction mixture was allowed to cool to room temperature, giving a black dispersion. Ethanol 共20 mL兲 was added to precipitate the particles. The particles were isolated by centrifuging, then washed with ethanol. FePd nanoparticles: Palladium acetylacetonate 共0.50 mmol兲 and 1,2-hexadecanediol 共1.5 mmol兲 was dissolved in 20 mL of dioctyl ether in a three-neck flask with magnetic stirring at 100 °C under N2 protection. After the Pd共acac兲2 , completely dissolved, iron pentacarbonyl 共0.75 mmol兲, oleic acid 共0.50 mmol兲, and oleylamine 共0.50 mmol兲 were added to the reaction mixture. The mixture was heated to reflux and allowed to reflux for 30 minutes. After refluxing for 30 minutes, the reaction mixture was allowed to cool to room temperature and 20 mL ethanol was added to precipitate the particles. The particles were isolated by centrifuging and washed twice with ethanol to remove the byproducts. Film deposition: A mother dispersion was prepared by dispersing 200 mg particles in 20 mL of a 1:1 mixture of hexane and octane, containing 0.05 mL of oleic acid and 0.05 mL of oleylamine. The mother dispersion was dropped onto a Si 共100兲 wafer having its native oxide. The solvent was allowed to evaporate to give films. The mother dispersion was diluted four times with the mixture of 共1:1兲 hexane and octane. The diluted dispersion was then dropped onto a carbon-coated copper TEM grid for electron microscopy. Films were annealed under vacuum (⬍1⫻10⫺7 Torr) for 30 minutes. Alternatively the films were annealed under flowing

INTRODUCTION

Magnetic recording technology has made tremendous gains in data storage density, partially by scaling down the grain size of thin film media. However, further scaling to support future increases will bring the grain sizes near the superparamagnetic limit. This had led to a search for new materials and new microstructures for longitudinal recording. The L1 0 phase of FePt has a very high magnetocrystalline anisotropy (K u ⫽6.6– 10⫻107 erg/cm2 ). 1 Sun et al. have prepared spherical FePt nanoparticles by the simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl.2 As prepared the particles were spherical, superparamagnetic and had a distorted facecentered cubic structure. The particles had a very narrow size distribution. They were dispersed in hexane, the dispersion cast onto a silicon wafer to dry to highly ordered, selfassembled films consisting of close-packed particles. Upon annealing at temperatures above 550 °C, the particles transformed to the L1 0 共tetragonal兲 phase. The annealed films were ferromagnetic with coercivities that depended on the annealing conditions. They were able to record on the films and demonstrated that the thermal stability factor (KV/kT ⬎48) was suitable for a thin film medium. This demonstration has aroused our interest in the synthesis and selfassembly of FePt nanoparticles. Earlier we modified the synthetic procedure reported by Sun et al. to prepare Fex Coy Pt100⫺x⫺y nanoparticles.3,4 Here we describe the synthesis of CoPt and FePd nanoparticles. The L1 0 phases of these alloys also have a high magnetocrystalline anisotropy and FePd K u ⫽1.8 共CoPt K u ⫽4.9⫻107 erg/cm3 7 ⫻10 erg/cm3 兲.1 EXPERIMENT

Iron pentacarbonyl 关 Fe共CO兲5 兴 , platinum acetylacetonate 关 Pt共acac兲2 兴 , palladium acetylacetonate 关 Pd共acac兲2 兴 , 1,2hexadecanediol, octyl ether, oleic acid, and oleyl amine were purchased from Aldrich. Cobalt tricarbonyl nitrosyl, a兲

Electronic mail: [email protected]

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© 2002 American Institute of Physics

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J. Appl. Phys., Vol. 91, No. 10, 15 May 2002

M. Chen and D. E. Nikles

TABLE I. Effect of the amount of Co(CO) 3 NO and Pt(acac) 2 charged to the reaction on the composition of the as-prepared particles. Co(CO) 3 NO 共mmol兲

Pt(acac) 2 共mmol兲

Composition

0.75 1.0

0.50 0.50

Co25Pt75 Co48Pt52

argon containing 2% hydrogen in a tube furnace. The particle composition was determined by energy dispersive x-ray analysis on a Philips model XL 30 scanning electron microscope. Images of the magnetic particles were obtained on a Hitachi model H-8000 transmission electron microscope. Thin film x-ray diffraction measurements 共␪-2␪ scans兲 were made on a Rigaku model D/MAX-2BX thin film diffractometer. Magnetic hysteresis curves were measured on a Princeton Micromag 2900 alternating gradient magnetometer using a 18 kOe saturating field. RESULTS AND DISCUSSION

Our initial attempts to prepare CoPt nanoparticles used two approaches, 共1兲 simultaneous reduction of cobalt acetylacetonate and platinum acetylacetonate or 共2兲 reduction of platinum acetylacetonate and thermal decomposition of dicobalt octacarbonyl, Co2 (CO) 8 . The first approach gave a mixture of ferromagnetic cobalt-rich, CoPt particles and superparamagnetic Pt-rich CoPt particles. The cobalt-rich particles precipitated from the reaction mixture, while the Pt-rich particles remained in dispersion. The second approach gave large cobalt particles that precipitated from the reaction mixture. Co2 (CO) 8 was not very soluble in dioctyl ether and it decomposed heterogeneously to give Co particles. We found success in preparing CoPt nanoparticles by the simultaneous chemical reduction of platinum acetylacetonate and thermal decomposition of cobalt tricarbonyl nitrosyl. This procedure was analogous to that used to prepare FePt nanoparticles by Sun et al., except Co共CO兲3 NO was substituted for Fe共CO兲5 .

FIG. 1. TEM micrograph of Co48Pt52 nanoparticles.

TABLE II. Effect of annealing conditions on the order parameter and coercivity of films containing Co48Pt52 nanoparticles. Temperature 共°C兲 Time 共min兲 Condition a 共pm兲 c 共pm兲 S H c 共Oe兲

600 30 vacuum 384 384 0 145

700 60 tube furnace 380 375 0.487 466

700 180 tube furnace 381 371 0.972 630

The reaction gave a dispersion of CoPt particles that could be isolated by adding ethanol and centrifuging. The composition of the particles depended on the relative amount of Co共CO兲3 NO and Pt共acac兲2 added to the reaction mixture, Table I. For the first case, when the amount was 0.75 mmol Co共CO兲3 NO to 0.5 mmol Pt共acac兲2 , the composition of the particles was Co25Pt75 . For the second case, where he molar ratio was 1.0–0.5, the composition was Co48Pt52 . We chose to focus attention on the particles with the composition Co48Pt52 . As-prepared the Co48Pt52 particles were superparamagnetic and could be dispersed in hydrocarbon solvents. TEM images of films cast on carbon-coated copper TEM grids, Fig. 1, showed an average particle size of 7 nm. This was twice as large as the FePt particles we prepared using the procedure of Sun et al.5,6 The distribution of particle sizes was greater for the Co48Pt52 particles, indicating the need for further work to narrow the particle size distribution and achieve a highly ordered particle assembly. The films on silicon wafers were annealed to transform the particles to the tetragonal (L1 0 ) phase. Calculating the ordering parameter, S, using Eq. 共1兲, where c and a are the unit cell parameters obtained by x-ray diffraction quantified the degree of tetragonal ordering. S⫽

共 1⫺c/a 兲 . 1⫺0.973

共1兲

FIG. 2. TEM micrograph of Fe50Pt50 nanoparticles.

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J. Appl. Phys., Vol. 91, No. 10, 15 May 2002

M. Chen and D. E. Nikles

TABLE III. Effect of annealing conditions on the order parameter and coercivity of films containing Fe50Pd50 nanoparticles. Temperature 共°C兲 Time 共min兲 Condition a 共ppm兲 c 共pm兲 S H c 共Oe兲

As-prepared 0 vacuum 386 386 0 12

550 30 vacuum 383 383 0 685

600 30 vacuum 383 383 0 548

700 60 Ar– H2 381 381 0 421

700 180 Ar– H2 379 379 0 297

Only after annealing for 3 h at 700 °C, Table II, did the particles transform to the tetragonal phase with a high degree of tetragonal ordering. The annealed films had a relatively low coercivity. Iron palladium alloy nanoparticles were prepared by the simultaneous reduction of palladium acetylacetonate and the thermal decomposition of iron pentacarbonyl. This procedure was entirely analogous to that used to prepare FePt nanoparticles by Sun et al. In this case Pd共acac兲2 substituted for Pt共acac兲2 . The reaction gave a dispersion of FePd particles that could be isolated by adding ethanol and centrifuging. The composition was Fe50Pd50 . As-prepared the Fe50Pd50 particles were ferromagnetic but still could be dispersed in hydrocarbon solvents. TEM images of films cast on carboncoated copper TEM grids, Fig. 2, showed an average particle size of 11 nm with a relatively broad distribution of particle sizes. The films on silicon wafers were annealed in an at-

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tempt transform the particles to the tetragonal (L1 0 ) phase. Even after annealing for three hours at 700 °C, Table III, there was no evidence for the tetragonal phase. The coercivity increased to 685 Oe after annealing at 550 °C, but then decreased for film annealed at higher temperatures. The procedure described by Sun et al. for the synthesis of FePt nanoparticles can be modified to prepare FeCoPt,3,4 CoPt or FePd nanoparticles. This opens up a rich new area of magnetic particle chemistry by providing a general route to the synthesis of spherical particles in the size range 3–10 nm. The particle size distribution CoPt and FePd particle described here needs to be narrowed before we can realize the particle self-assembly into close-packed films seen with FePt or FeCoPt particles. ACKNOWLEDGMENT

This work was supported by the NSF Materials Research Science and Engineering Center Award No. DMR-9809423. 1

D. Weller, A. Moser, L. Folks, M. E. Best, W. Lee, M. F. Toney, M. Schwickert, J.-U. Thiele, and M. F. Doerner, IEEE Trans. Magn. 36, 10 共2000兲. 2 S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287, 1989 共2000兲. 3 M. Chen and D. E. Nikles, Mater. Res. Soc. Symp. Proc. 674, U4.7 共2001兲. 4 M. Chen and D. E. Nikles, NanoLetters 共to be published兲. 5 M. Chen and D. E. Nikles, Mater. Res. Soc. Symp. Proc. 674, U4.8 共2001兲. 6 J. W. Harrell, S. Wang, D. E. Nikles, and M. Chen, Appl. Phys. Lett. 79, 4393 共2001兲.

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