Synthesis And Phase Transition Of Self Assembled Fepd And Fepdpt Nano Particles

JOURNAL OF APPLIED PHYSICS

VOLUME 95, NUMBER 11

1 JUNE 2004

Synthesis and phase transition of self-assembled FePd and FePdPt nanoparticles Shishou Kang, Zhiyong Jia, David E. Nikles, and J. W. Harrella) Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama 35487-0209

共Presented on 6 January 2004兲 Fe54Pd46 nanoparticles were prepared by the simultaneous chemical reduction of palladium acetylacetonate and iron chloride. The particle size can be tunable from 2 to 10 nm by controlling the amount of surfactants. Similarly, Fe50Pdx Pt50⫺x (x⫽8, 15, 25兲 nanoparticles were prepared by the simultaneous reduction of palladium acetylacetonate, platinum acetylacetonate, and iron chloride. The average diameter for the Fe50Pdx Pt50⫺x particles was always 3.5 nm and independent of the amount of surfactants. Films of the particles were cast onto silicon wafers from hydrocarbon dispersion. The coercivity of Fe54Pd46 nanoparticles increases with annealing temperature up to 550 °C, indicating fcc to fct phase transition. After further increasing the annealing temperature, the coercivity of the Fe54Pd46 nanoparticles decreased, suggesting the formation of a soft magnetic phase. This new phase (Fe3 Pd) was confirmed from x-ray diffraction measurements. For Fe50Pdx Pt50⫺x nanoparticles, the coercivity increases to more than 10 kOe with annealing temperature up to 650 °C (x⫽8). With increasing Pd content, the coercivity of the Fe50Pdx Pt50⫺x nanoparticles decreased, as expected since the anisotropy energy of bulk FePd material is only one third of that of bulk FePt. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1669306兴

INTRODUCTION

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. The nanoparticles were prepared using a modification of the procedure reported by Sun et al.8 The synthetic experiments were carried out using standard airless procedures. A solution of platinum acetylacetonate 共0–0.45 mmol兲, palladium acetylacetonate 共0.05–0.5 mmol兲, iron chloride 共0.5 mmol兲, and 1,2-hexadecanediol 共1.5 mmol兲 in 20 ml phenyl ether was heated to 80 °C in a three-necked round-bottom flask under a nitrogen atmosphere with magnetic stirring. After these chemicals were completely dissolved in phenyl ether, the mixture was heated up to 150 °C for 20 min. To this solution was added via syringe oleic acid, oleylamine, TBP, and TBA. Then LiBEt3 H 共1.0 M THF solution, 2 ml兲 was slowly dropped into the mixture. The black dispersion was stirred at 200 °C for 5 min under N2 to remove low boiling solvent and then was heated to reflux at 260 °C for 20 min. The heat source was then removed and the dispersion was allowed to cool to room temperature. The inert gas protected system could then be opened to ambient environment. The black product was precipitated by adding 40 ml ethanol. The mixture was centrifuged to isolate the particles from the brown supernatant. The particles were redispersed in hexane, precipitated with ethanol, and isolated by centrifuging. The particles were dispersed in a 50/50 mixture of hexane and octane 共10 mg particles in 1 ml solvent兲, containing 0.1 ml of a 50/50 mixture of oleylamine and oleic acid. The dispersion was dropped onto a silicon wafer, and the solvent was allowed to evaporate to give films for x-ray diffraction 共XRD兲 and magnetic characterization. The dispersion was then further diluted and dropped onto a carbon-coated copper

The requirements for ultrahigh density magnetic recording have driven the development of magnetic thin film media with smaller grains, higher coercivity, and minimal exchange coupling between neighboring grains.1 FePt and CoPt based hard magnetic nanoparticle arrays have been viewed as a promising candidate for future recording media applications.2 Recently, Sun and workers at IBM reported the synthesis, self-assembly, and magnetic recording performance of FePt nanoparticles.3 This report has generated considerable interest in the use of self-assembled FePt nanoparticles for recording media in future high-density data storage magnetic hard disk drives. This solution phase-based synthesis offers a convenient approach to monodisperse FePt nanoparticles with the size being controlled down to only a few nanometers. We have previously reported modifications of the synthetic procedure reported by Sun et al. to prepare FePtM (M⫽Co, Cu, Ag, and Au兲 nanoparticles.4 –7 Here we describe the synthesis of FePd and FePdPt nanoparticles. The L1 0 phases of these alloys also have a high magnetocrystalline anisotropy.2 EXPERIMENT

Iron chloride 关 FeCl2 •4H2 O兴 , platinum acetylacetonate 关 Pt(acac) 2 兴 , palladium acetylacetonate 关 Pd(acac) 2 兴 , lithium triethylborohydride 共superhydride, LiBEt3 H) in tetrahydrofuran 共THF兲 solution 共1.0 M兲, 1,2-hexadecanediol, phenyl ether, tributylphosphine 共TBP兲, tributylamine 共TBA兲, oleic acid, and oleylamine were purchased from Aldrich. Ethanol, a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

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© 2004 American Institute of Physics

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J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004

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FIG. 1. TEM images of as-made: 共a兲 3 nm FePd and 共b兲 9 nm FePd nanoparticles.

transmission electron microscope 共TEM兲 grid for electron microscopy. The films were annealed under flowing argon containing 5% 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 XRD 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 19 kOe saturating field. RESULTS AND DISCUSSION

As-prepared, the FePd particles were superparamagnetic and could be dispersed in hydrocarbon solvents. The particle size can be tuned from 2 to 10 nm by adjusting the amount of surfactants. For example, 0.5 mmol oleylamine/oleic acid gives an average particle size of 9 nm, while 0.5 mmol TBA/ TBP with 0.5 mmol oleylamine/oleic acid gives an average particle size of 3 nm 共see Fig. 1兲. It is clear that the distribution of particle sizes in Fig. 1共b兲 was greater than that of in Fig. 1共a兲, indicating the need for further work to narrow the particle size distribution for large particles and achieve a highly ordered particle assembly. The films on silicon wafers were annealed to transform the particles to the tetragonal (L1 0 ) phase. Since the minimal magnetically stable particle size for FePd is about 5 nm at room temperature,2 we focus the phase transition on the large FePd particles. After heat treatment, the FePd particles partially transform to the fct structure with sufficient anisotropy to be ferromagnetic at room temperature. Shown in Fig. 2 are the room temperature in-plane hysteresis loops for 9

FIG. 2. Room temperature hysteresis loops for 9 nm FePd nanoparticles annealed at: 共a兲 400 °C, 共b兲 450 °C, 共c兲 500 °C, 共d兲 550 °C, 共e兲 600 °C, and 共f兲 650 °C for 60 min.

nm FePd nanoparticles annealed at 400, 450, 500, 550, 600, and 650 °C for 60 min. Here we were not able to determine values of M s , because of the unknown amount of surfactants in the films. From Fig. 2, it is obvious that coercivity of FePd nanoparticles was strongly dependent on the annealing temperature. The coercivity increased with the annealing temperature up to 550 °C. Upon further increasing the annealing temperature, the coercivity decreased, suggesting a new soft phase Fe3 Pd formed at high temperature. This was confirmed from XRD measurements as described below. The sequence of x-ray diffraction curves in Fig. 3 illustrates the development of the chemically ordered L1 0 phase of 9 nm FePd nanoparticles as a function of annealing temperature. The phase transformation can be seen by the evolution of the 共001兲 and 共110兲 peaks with increasing annealing temperature. Moreover, we can clearly see the splitting of the 共200兲 and 共002兲 peaks due to the fct structure of the samples after annealing at 550 °C as indicated in Fig. 3. With further increase in the annealing temperature, the particles were partially transformed to the L1 2 Fe3 Pd phase, while the coercivity decreased dramatically. Since the average composition of the particles is Fe54Pd46 , the formation of the L1 2 – Fe3 Pd phase would suggest that the excess Pd might be seen in the XRD spectra. However, the amount of excess Pd would be small since the L1 2 phase can form with Fe content as low as 65 at. %. Nevertheless, a careful examination of the spectrum in Fig. 3 共inset兲 for the sample annealed at 650 °C shows a

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FIG. 3. XRD patterns for 9 nm FePd nanoparticles annealed at 450, 550, and 650 °C for 60 min. Inset is an enlarged view of the XRD pattern for the sample annealed at 650 °C.

shoulder on the low angle side of the Fe3 Pt 共111兲 peak that may be attributed to the Pd 共111兲 peak. Iron platinum palladium ternary alloy nanoparticles were prepared by the simultaneous reduction of platinum acetylacetonate, palladium acetylacetonate, and iron chloride. The reaction gave a dispersion of FePdPt particles that could be isolated by adding ethanol and centrifuging. Here we fixed the composition of Fe at about 50% and changed the relative composition of Pd and Pt. The particle size of the FePdPt nanoparticles was independent of the amount of surfactants and was always 3.5 nm. The films on silicon wafers were annealed in an attempt to transform the particles to the tetragonal (L1 0 ) phase. As illustrated in Table I, the coercivity of the FePdPt nanoparticles depended on the annealing temperature and the content of Pd. For Fe50Pd8 Pt42 nanoparticles, the coercivity increased to more than 10 kOe with annealing temperature up to 650 °C. With increasing Pd con-

TABLE I. Effect of annealing conditions on the coercivity 共Oe兲 of the FePdPt nanoparticles. 180

60

Time 共min兲 Temperature (°C)

400

450

500

550

600

650

Fe50Pd8 Pt42 Fe50Pd15Pt35 Fe50Pd25Pt25

151 67 77

111 78 39

180 384 108

538 695 618

5621 3454 2039

⬎12400a ⬎10690a ⬎8412a

a

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J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004

Minor loop.

tent, the coercivity of the Fe50Pdx Pt50⫺x nanoparticles decreased, since the anisotropy energy of bulk FePd material is only one third of that of bulk FePt.2 The results presented here underscore the utility of the new procedure developed by Sun et al. to prepare FePt nanoparticles.8 The synthesis of FePd and FePdPt nanoparticles involved the simultaneous reduction of iron chloride, palladium acetylacetonate, and/or platinum acetylacetonate. The FePd particle size can be controlled from 2 to 10 nm, while the FePdPt particle size is always 3.5 nm. Detailed structural and magnetic analyses of these nanoparticles show that the L1 0 FePd phase was developed with annealing temperature up to 550 °C, with the coercivity reaching a maximum of ⬃3.5 kOe. After further increasing the annealing temperature, a soft L1 2 Fe3 Pd phase develops, which decreases the coercivity of the films. For FePdPt nanoparticles, the coercivity increases with annealing temperature, as well as with the content of Pt. Along with FePt, L1 0 FePd and FePdPt nanoparticles are promising materials for ultrahigh density magnetic recording media. Because of its high anisotropy, fully ordered L1 0 FePt cannot currently be used for magnetic media because of write field limitations. Although the anisotropy of FePt can reduced by partial chemical ordering or by changing the stoichiometry, this may lead to large anisotropy field distributions.9 The ability to tune the anisotropy of L1 0 FePtPd nanoparticles by changing the Pt/Pd ratio offers an alternative means of controlling the anisotropy. Further investigations of the magnetic properties of these nanoparticles are underway. ACKNOWLEDGMENT

This work was supported by the NSF Materials Research Science and Engineering Center Award No. DMR-0213985. D. Weller and A. Moser, IEEE Trans. Magn. 35, 4423 共1999兲. 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. 3 S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287, 1989 共2000兲. 4 M. Chen and D. E. Nikles, Nano Lett. 2, 211 共2002兲. 5 S. Kang, J. W. Harrell, and D. E. Nikles, Nano Lett. 2, 1033 共2002兲. 6 X. Sun, S. Kang, J. W. Harrell, and D. E. Nikles, J. Appl. Phys. 93, 7337 共2003兲. 7 S. Kang, Z. Jia, D. E. Nikles, and J. W. Harrell, IEEE Trans. Magn. 39, 2753 共2003兲. 8 S. Sun, S. Anders, T. Thomson, J. E. E. Baglin, M. F. Toney, H. F. Hamann, C. B. Murray, and B. D. Terris, J. Phys. Chem. B 107, 5419 共2003兲. 9 S. Wang, S. S. Kang, J. W. Harrell, X. W. Wu, and R. W. Chantrell, Phys. Rev. B 68, 104413 共2003兲. 1 2

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