Reductionn Of The Fcc To L10 Ordering Temperature For Self Assembled Fept Nano Particles Containing Ag

VOLUME 2, NUMBER 10, OCTOBER 2002 © Copyright 2002 by the American Chemical Society

Reduction of the fcc to L10 Ordering Temperature for Self-Assembled FePt Nanoparticles Containing Ag Shishou Kang,† J. W. Harrell,†,‡ and David E. Nikles*,†,§ Department of Physics and Astronomy, Department of Chemistry and Center for Materials for Information Technology, The UniVersity of Alabama, Tuscaloosa, Alabama 35487-0209 Received May 15, 2002; Revised Manuscript Received August 9, 2002

ABSTRACT [Fe49Pt51]88Ag12 nanoparticles were prepared by the simultaneous polyol reduction of platiunum acetylacetonate and silver acetate and the thermal decomposition of iron pentacarbonyl, giving 3.5 nm diameter FePt particles with Ag atoms substituted in the lattice. The addition of Ag promoted the fcc to tetragonal phase transition, thereby reducing the temperature required for this transition by some 100 to 150 °C compared with pure FePt nanoparticles. After heat treatment at 400 °C for 30 min, the coercivity of the films containing [Fe49Pt51]88Ag12 nanoparticles was more than 3400 Oe, while the films containing FePt nanoparticles were superparamagnetic. This decrease in phase transformation temperature allowed us to decrease the particle coalescence and loss in particle positional order seen when FePt nanoparticles were transformed at temperatures above 550 °C.

One of the Chemistry Highlights of 20001 was the report by S. Sun and workers at IBM of the synthesis, self-assembly, and magnetic recording performance of FePt nanoparticles.2 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. Before this application can be realized some basic materials science problems must be solved. One of the most challenging problems is the phase transformation from the fcc phase to the tetragonal (L10) phase. As prepared, the particles have an fcc structure and are superparamagnetic. The particles are * Corresponding author. E-mail: [email protected] † Center for Materials for Information Technology. ‡ Department of Physics and Astronomy. § Department of Chemistry. 10.1021/nl025614b CCC: $22.00 Published on Web 09/06/2002

© 2002 American Chemical Society

coated with organic surfactants (oleic acid and oleylamine) and can be dispersed in hydrocarbon solvents. When the dispersions are cast onto solid substrates and the solvent evaporates, the particles self-assemble into close-packed arrays. After heat treatment at temperatures above 500 °C, the particles transform to the tetragonal phase, having high magnetocrystalline anisotropy3 and giving films with high coercivity. The coercivity depends on the heating temperature, and for complete phase transformation, the particles must be heated to nearly 580 °C.4 However, when heated to temperatures above 550 °C, there is considerable particle coalescence and loss of particle positional order.5 Particle coalescence leads to increased switching volumes, which defeats the point of making small particles. The loss of positional order comes from the decomposition of the

Figure 1. TEM images of self-assembled [Fe49Pt51]88Ag12 nanoparticles: as prepared, 1a, and after heat treatment at 400 °C for 30 min, 1b. These are negative images with the particles appearing as white spots.

surfactant layers allowing particle motion. It would be highly desirable if the temperature required for the fcc to tetragonal phase transformation were lower, at least below temperatures where the particles coalesce, preferably below the temperature where the organic surfactants decompose. There have been many recent reports of the effect of additives on the thermal ordering of sputtered films of either CoPt or FePt alloys. Kitakami et al. have found that the addition of Sn, Pb, Sb, Bi, or Ag into sputtered CoPt films promotes the disordered fcc to ordered tetragonal transformation, resulting in significant reductions in the temperature required for ordering.6,7 Quite recently, Maeda et al., from Toshiba, observed a reduction of the ordering temperature for FePt sputtered films by adding Cu.8 For films containing [FePt]85Cu15, the coercivity was 5000 Oe after annealing at 300 °C, while the Hc for films containing FePt was only a few hundred Oe after annealing at 300 °C. These reports suggest a means of lowering the fcc to tetragonal phase transformation for FePt nanoparticles by adding either copper or silver. We set about preparing FePt nanoparticles containing either Ag or Cu. It is not obvious that the effect of Ag or Cu will be the same in chemically synthesized nanoparticles as in sputtered films. In addition, unlike sputtered thin films using a Ag or Cu target, a suitable chemical procedure had to be found for introducing Ag or Cu into FePt. Here we report the synthesis of [Fe49Pt51]88Ag12 nanoparticles and the beneficial effect of the added silver on the phase transformation temperature. Our initial attempts to add Ag to FePt used a modification of the procedure we used to prepare FexCoyPt100-x-y nanoparticles.9 This involved the simultaneous polyol reduction of platinum acetylacetonate and silver acetylacetonate and the thermal decomposition of iron carbonyl in octyl ether. Unfortunately, silver acetylacetonate was not adequately soluble in octyl ether. Using silver acetate as the silver source and phenyl ether as the reaction solvent, we were able to prepare [Fe49Pt51]88Ag12 nanoparticles as follows. A 50 mL three-necked round-bottom flask was equipped with magnetic stirring, a reflux condenser, a thermometer, a rubber septum, and an argon atmosphere. Teflon sleeves were used for all ground glass joints. To the flask was added 0.50 mmol platinum acetylacetonate, 0.25 mmol silver acetate, and 20 mL phenyl ether. The mixture was heated to 80 °C, 1034

whereupon it turned brown. Then 0.50 mmol oleic acid, 0.50 mmol oleylamine, and 1.4 mL (1.1 mmol) iron pentacarbonyl were added by syringe. The septum was replaced with a glass stopper and the reaction mixture heated to the reflux temperature of phenyl ether (∼260 °C). After refluxing for 30 min, the reaction mixture was allowed to cool to room temperature, giving a dark dispersion. Ethanol (25 mL) was added to precipitate the particles, and the particles were isolated by centrifuging. The particles were redispersed in hexane, precipitated with ethanol, and isolated by centrifuging. The particles were dried at room temperature under vacuum to give 100 to 200 mg. The dispersion and precipitation purified the particles; however, we did not characterize the content of the washings. For comparison, we also prepared Fe53Pt47 nanoparticles using the procedure of S. Sun and co-workers.2 The particles were dispersed in a 50/50 mixture of hexane and octane that included small amounts of oleic acid and oleylamine. The dispersion was dropped onto a carbon-coated copper TEM grid (200 mesh from SPI) and the solvent evaporated to give a self-assembled film. The particle composition, [Fe49Pt51]88Ag12, was determined by energydispersive X-ray analysis on a Philips model XL 30 scanning electron microscope. The particles assembled into a hexagonal closed-packed array as shown in Figure 1a, similar to that seen for FePt and FexCoyPt100-x-y nanoparticles.9,10 They had an average diameter of 3.5 nm and a chemically disordered fcc structure (Figure 2, curve a). The (111) lattice spacing was expanded to 229 pm for the [Fe49Pt51]88Ag12 nanoparticle relative to 225 pm for Fe53Pt47, indicating that silver was substituting into the FePt lattice. Heat treatments were done in a Lindberg tube furnace with flowing argon (containing 2% hydrogen) for 30 min at temperatures ranging from 300 to 500 °C. The sequence of X-ray diffraction curves in Figure 2 shows the progress of the fcc to L10 phase transformation as a function of temperature. Careful examination of curve c in Figure 2 shows that very weak (001) and (011) peaks for the tetragonal FePt phase appeared after heat treatment at 350 °C. A shoulder appeared at the low 2θ side of the FePt (111) peak indicating that Ag was phase separating from the particles. After heat treatment at 400 °C, curve d in Figure 2, the (001), (011), and (200) diffraction peaks for the FePt L10 phase had clearly emerged, and the Nano Lett., Vol. 2, No. 10, 2002

Figure 2. X-ray diffraction data for the films containing selfassembled [Fe49Pt51]88Ag12 nanoparticles: as-prepared (curve a) and after heat treatment at 300 °C (curve b), 350 °C (curve c), 400 °C (curve d), 450 °C (curve e), or 500 °C (curve f).

Figure 3. Plot of d-spacing for the (111) diffraction peak as a function of annealing temperature for films containing selfassembled [Fe49Pt51]88Ag12 (closed circles) or Fe53Pt47 (open circles) nanoparticles.

Ag (111) diffraction near 2θ ) 38° was more apparent. The fcc to tetragonal phase transformation was beginning around 350 °C. The plots in Figure 3 compare values of the (111) lattice spacing for [Fe49Pt51]88Ag12 and Fe53Pt47 as a function of heat treatment temperature. For [Fe49Pt51]88Ag12 the (111) lattice spacing approaches the bulk value for tetragonal FePt (d111 ) 219.7 pm) at lower heat treatment temperatures than for Fe53Pt47. It appears that the Ag atoms leave the FePt lattice at temperatures less than 400 °C, leaving lattice vacancies. These vacancies increase the mobility of the Fe and Pt atoms, thus enhancing the kinetics for the phase transformation. Although the X-ray diffraction data clearly show that the Ag left the FePt particles, there was no indication in the TEM for the formation of separate Ag particles. Indeed, we do not know where the silver went, perhaps it remained on the surface of the particles. We plan to answer this question in future work. In Figure 4 is a comparison of the in-plane magnetic hysteresis curves (measured on a Princeton Measurements Nano Lett., Vol. 2, No. 10, 2002

Figure 4. Magnetic hysteresis curves for films containing selfassembled [Fe49Pt51]88Ag12 (solid curve) or Fe53Pt47 (dashed curve) nanoparticles after heat treatment at 400 °C for 30 min.

Figure 5. Plots of coercivity as a function of heat treatment temperature for films containing self-assembled [Fe49Pt51]88Ag12 (solid circles) or Fe53Pt47 (open circles) nanoparticles. Substitution of Ag of in to FePt nanoparticles to give self-assembled [Fe49Pt51]88Ag12 nanoparticles (a) allowed transformation from the superparamagnetic fcc to the ferromagnetic L10 phase after heat treatment at 400 °C for 30 min (b).

model 2900 alternating gradient magnetometer) for [Fe49Pt51]88Ag12 and Fe53Pt47 films heat treated at 400 °C. The film containing the Fe53Pt47 was superparamagnetic, while the film containing the [Fe49Pt51]88Ag12 was ferromagnetic with an apparent coercivity of 3400 Oe. The actual value probably exceeds 3400 Oe because the maximum field on our AGM (19 000 Oe) was not sufficient to completely saturate the loop. The curves in Figure 5 compare the coecivity of the [Fe49Pt51]88Ag12 and Fe53Pt47 films as a function of heat treatment temperature. Consistent with the trends in Figure 3, Hc increases more rapidly with increasing heat treatment temperature for the [Fe49Pt51]88Ag12 films. Hc for the film heated to 350 °C was more than 1000 Oe, indicating a significant amount of phase transformation at this temperature. In fact, the film heat treated at 300 °C had a small coercivity (200 Oe), indicating a small degree of transformation. 1035

If self-assembled films of tetragonal FePt nanoparticles are to be used as granular thin film media, a means of transforming the particles to the tetragonal phase, while maintaining the small particle size and the high degree of particle positional order, must be found. The results reported here are a very important step toward that goal. Including Ag in the FePt particles has allowed transformation of the particles from the fcc to the high coercivity tetragonal phase at a temperature some 100 to 150 °C lower than that for FePt particles. An immediate benefit of the lowering transformation temperature was a significant improvement in the ability to maintain the particle positional order. In Figure 1b is a TEM image of the film containing [Fe49Pt51]88Ag12 nanoparticles after heat treatment at 400 °C. There was some degradation in particle positional ordering; however, this was not as great as the loss in positional ordering for FePt films heat treated at 600 °C.4,5 Furthermore, there was less particle coalescence for the heat treated film containing [Fe49Pt51]88Ag12 nanoparticles. δM curves obtained from isothermal and dc magnetic remanence measurements show negative values of δM for the films heat treated at or below 400 °C.11 Negative values of δM are consistent with magnetostatic interactions, while positive values of δM would indicate exchange interactions due to sintering.12

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Acknowledgment. This work was supported by the NSF Materials Research Science and Engineering Center award number DMR-9809423. References (1) Borman, S. Chem. Eng. News 2000, 78(51), 24-31. (2) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (3) Weller, D.; Moser, A.; Folks, L.; Best, M. E.; Le, W.; Toney, M. F.; Schweickert, M.; Thiele, J.-L.; Doerner, M. F. IEEE Trans. Magnetics 2000, 36(1), 10-15. (4) Harrell, J. W.; Wang, S.; Nikles, D. E.; Chen, M. Appl. Phys. Lett. 2001, 79(26), 4393-4395. (5) Dai, Z. R.; Sun, S.; Wang, Z. L. Nano Lett. 2001, 1(8), 443-447. (6) Chen, C.; Kitakami, O.; Okamoto, S.; Shimada, Y. Appl. Phys. Lett. 2000, 76, 3218. (7) Kitakami, O.; Shimada, Y.; Oikawa, K.; Daimon, H.; Fukamichi, F. Appl. Phys. Lett. 2001, 78, 1104. (8) Maeda, T.; Kai, T.; Kikitsu, A.; Nagase, T.; Akiyama, J. Appl. Phys. Lett. 2002, 80(12), 2147-2149. (9) Chen, M.; Nikles, D. E. Nano Lett. 2002, 2(3), 211-214. (10) Chen, M.; Nikles, D. E. Mater. Res. Soc. Symp. Proc. 2001, 674, U4.8. (11) Harrell, J. W.; Nikles, D. E.; Kang, S. S.; Wang, S.; Wu, X. W. Presented at the International Conference on Fine Particle Magnetism, Pittsburgh, PA, August 2002, manuscript submitted for publication in J. Magn. Magn. Mater. (12) Zeng, H.; Sun, S.; Vedantam, T. S.; Liu, J. P.; Dai Z.-L.; Wang, Z.-L. Appl. Phys. Lett. 2002, 80, 2583-2585.

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Nano Lett., Vol. 2, No. 10, 2002