NANO LETTERS
High Coercivity of Ordered Macroporous FePt Films Synthesized via Colloidal Templates
2005 Vol. 5, No. 7 1525-1528
Ferry Iskandar,† Toru Iwaki,‡ Toshiyuki Toda,§ and Kikuo Okuyama*,† Department of Chemical Engineering, Graduate School of Engineering, Hiroshima UniVersity, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan, Hiroshima Joint Research Center for Nanotechnology Particle Project, Japan Chemical InnoVation Institute, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530 Japan, and Toda Kogyo Corp., 4-1-2 Funairiminami, Naka-ku, Hiroshima 730-0847, Japan Received May 9, 2005; Revised Manuscript Received May 27, 2005
ABSTRACT The preparation of a three-dimensionally (3D) ordered macroporous iron−platinum (FePt) film derived from monodisperse FePt nanoparticles (approximately 3 nm in diameter) and polystyrene latex particles (254 nm in diameter) is described. The prepared film has a hexagonally ordered porous structure and coercivity up to 10 kOe after annealing at a temperature of 600 °C. We also found that size of FePt particles was maintained at around 3 nm, even after annealing at a temperature of 600 °C.
Materials with three-dimensional (3D) ordered porous structures have attracted considerable attention from the researcher over the past decade.1 These materials hold promise for use as photonic crystals,2 advanced coatings, advanced catalysts,3 and in a variety of other current and future applications. We previously reported on the preparation of silica films and silica spherical particles containing ordered macropores using a dip-coating method and a spray drying method, respectively.4-6 Polystyrene (PS) particles and silica nanoparticles colloids were used as precursors. An approach for creating materials by these methods is to replicate the PS colloidal crystal that serves as template. The voids between PS particles are infiltrated by silica nanoparticles that solidify, and the templates can then be removed. Materials containing highly ordered pores prepared by those methods exhibit a low effective refraction index as well as a low dielectric constant. FePt nanoparticles have recently attracted the interest of researchers for possible applications in ultrahigh-density magnetic recording media, due to the high magnetocrystalline anisotropy (108 erg/cm3) of the face-centered tetragonal (fct) FePt phase.7-9 A coercivity of up to 9 kOe at room temperature has been reported for fct phased FePt nanoparticles, and this value became almost double at very low temperature. The coercivity can be easily controlled by * Corresponding author. E-mail:
[email protected]. † Hiroshima University. ‡ Japan Chemical Innovation Institute. § Toda Kogyo Corp. 10.1021/nl050863w CCC: $30.25 Published on Web 06/07/2005
© 2005 American Chemical Society
changing the fraction of Fe and Pt atoms by appropriately changing the molar fraction of the precursors. For FePt particles produced from a solution phase, the coercivity, as well as the crystallinity, can be tuned by postannealing the as-grown nanoparticles at different temperatures. In addition to magnetic recording media, FePt particles also have been usually used as a catalyst.10 In this letter, we report on the preparation of a highly ordered porous FePt film using monodisperse FePt nanoparticles and polystyrene latex particles as colloidal templates. The process involves the preparation of 3D ordered PS particles on a wafer substrate and the infiltration of FePt nanoparticles into the voids between the PS particles. After they solidify, the templates are then removed by a heat
Figure 1. Schematic diagram of the preparation of an FePt porous film derived from FePt nanoparticles and PS particles.
Figure 2. TEM images of as-synthesized FePt nanoparticles.
treatment. The resulting ordered porous FePt film exhibits magnetic properties with coercivity values as high as 10 kOe, after reannealing at 600 °C in Ar/H2 atmosphere. The FePt nanoparticles used here were synthesized by Elkins’s method using a combination of reduction and the thermal decomposition of ferric acetylacetonate, Fe(acac)3, and platinum acetylacetonate, Pt(acac)2, by 1,2 hexadecanediol in dioctyl ether as the solvent.8 A slightly modified synthetic procedure is as follows: Using standard airless techniques in an argon atmosphere, a mixture of 0.3 mL of oleic acid, 0.3 mL of oleylamine and 1.8 g of 1,2 hexadecanediol, and 200 mL of dioctyl ether were added to a 300 mL, four-necked reaction flask containing a PTFE coated
magnetic stirring bar. 0.47 g of Fe(acac)3 and 0.4 g of Pt(acac)2 were added into the flask and the contents stirred while purging with Ar gas, and the solution was then heated until all the contents had dissolved. The flask was then heated to 290 °C at a rate of approximately 10 °C per minute. The color changed from reddish-brown to black as the temperature approached 180 °C. The flask was maintained at a refluxing temperature of 290 °C for 30 min before naturally cooling to room temperature under a flow of argon gas. The nanoparticles were first precipitated by adding ethanol and followed by centrifugation at 18 000 rpm in 1 h. The supernatant was discarded and the precipitate redispersed in hexane. A small amount of oleylamine and oleic acid can be added at this point as an aid in redispersing the nanoparticles. After a second centrifugation, the supernatant was redispersed in hexane and stored under refrigeration. Figure 1 shows a schematic diagram of the preparation of an ordered porous FePt film by the present method. Monodispersed polystyrene latex particles, dispersed in aqueous medium (254 nm in diameter, Japan Synthetic Rubber, JSR Co. Ltd), were used. In the preparation, PS particles were first assembled to be ordered on the wafer substrate using a dip coating method. After drying the aqueous suspension of PS particles, the FePt nanoparticles were infiltrated into the voids of the PS ordered particles. After drying at ambient conditions, the composite film was heated to 350 °C for 20 min to remove the PS particles, resulting in the formation of an ordered porous FePt film. The porous FePt film was reheated to a temperature of 600 °C under a flow of H2 and Ar to form a fct phased structured of FePt. An induced couple plasma (ICP) Seiko SPS-4000 was used to characterize the composition of the Fe and Pt atoms in the samples. Magnetization was measured using a Quantum Design PMS superconducting quantum interference device
Figure 3. SEM images and XRD patterns of hexagonally ordered porous FePt film prepared. (a) FePt porous film and (b) FePt nanoparticles before annealing process; (c) FePt film, (d) FePt nanoparticles, and (e) XRD pattern of FePt nanoparticles in powder form after annealed at temperature of 600 °C for 30 min. 1526
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(SQUID) magnetometer and a high magnetic field Kerr effect measurement apparatus (model BH-620-LP-RP, Neoark Corp.) with a maximum magnetic field of 25 and 3 T, respectively. Scanning electron microscope (SEM) images were recorded using an Hitachi S5000, and the transmission electron microscope (TEM) pictures were recorded using JEOL JEM-2010. For TEM measurements, the samples were dispersed in hexane before placing them on the TEM grids. Figure 2 shows a typical TEM image of as-synthesized FePt nanoparticles prepared by the present method. The nanoparticles had a quite uniform shape with the size distribution as shown in inset of Figure 2. The mean particle size was 2.6 nm with a standard deviation 1.1, indicating that the particles show a good monodispersity. The ICP analysis of this sample indicated a final particle composition of Fe58Pt42. The fraction of Fe and Pt was similar to the molar fraction of Fe precursor relative to that of the Pt precursors. From the XRD patterns the fct phase was produced by annealing the as-prepared sample at 600 °C under an Ar/H2 atmosphere. The annealed samples generated sharp peaks, confirming the presence of (face-centered cubic) fcc to facecentered tetragonal (fct; L10 phase) conversion. Figure 3a and 3b show the ordered macroporous of fcc FePt film at different magnifications before the annealing process to form fct FePt. From the figures, the macropores appear to be ordered hexagonally; indicating that self-organized of PS was not disrupted during the infiltration of FePt nanoparticles into the voids of the PS particles. The SEM image also showed that the near pores were connected by smaller holes around 70 nm in diameter size. A similar morphology has been observed in silica films prepared using similar methods, as reported by us in a previous study. The pore size could be easily controlled by changing the size of the starting PS particles. From the high magnification image (Figure 3b), FePt nanoparticles with diameter size around 3 nm were observed. This indicates that after the PS particles were removed at a temperature of 350 °C, the size of FePt nanoparticles was not changed. Figure 3c and 3d show the ordered macroporous of fct FePt film after annealing at a temperature of 600 °C. The hexagonally ordered porous structure was not changed. From the high magnification image (Figure 3d), the sizes of FePt nanoparticles were also found to be similar to the initial size as shown in Figure 2. We conclude that the sustained diameter size might be caused by the oxidation process during the evaporation of PS particles at a temperature of 350 °C, that oxide-coated FePt particles were formed to a considerable extent, and this oxide material was resistant to the sintering process between FePt nanoparticles during the annealing process. The measurement of crystal structure of the FePt nanoparticles in macroporous film has been tried, but the XRD pattern could not be obtained due to the low sensitivity of the XRD equipment. Therefore, we tried to evaluate the XRD pattern of FePt nanoparticles in powder form after annealed at 600 °C. As shown in Figure 3e, the measured XRD pattern indicates the L10 phase. We considered that the phase of FePt nanoparticles in the macroporous film was also converted to the L10 ferromagnetic phase after annealed at temperature of 600 °C. Nano Lett., Vol. 5, No. 7, 2005
Figure 4. Hysteresis loops of the (a) FePt nanoparticles and (b) ordered macroporous FePt film after annealing in Ar/H2 at 600 °C for 30 min.
Figure 4 shows the room-temperature hysteresis loop coercivity for FePt nanoparticles and ordered porous films produced using them. The coercivity of both samples reaches a large value of 10 kOe at room temperature. This indicates that both samples have the high magnetocrystalline anisotropy of the fct FePt phase. The coercivity of the FePt nanoparticles is also in agreement with findings reported in ref 8. As far as we know, this constitutes the first report of an ordered macroporous FePt film with high coercivity. In conclusion, we succeeded in preparing a highly ordered porous iron-platinum (FePt) film derived from FePt nanoparticles and polystyrene latex particles. The prepared porous FePt film showed magnetic properties with a coercivity as high as 10 kOe at room temperature, after annealing at 600 °C in an Ar/H2 atmosphere. We expect that these porous films can be used for various applications related to magnetic materials in which a high porosity, a low dielectric constant, and a low refractive index are needed. The relation between the photonic crystal effect, due to the periodicity of macroporous materials, and the magnetic effect is an interesting 1527
one. Further investigations in this area are expected. The other hand, this type of porous materials could also be useful for high efficiency catalysts due to its high porosity and high surface area. Acknowledgment. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO)’s Nanotechnology Materials ProgramsNanotechnology Particle Project based on a fund provided by the Ministry of Economy, Trade, and Industry (METI), Japan. A Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship for Ferry Iskandar is gratefully acknowledged. The authors thank to Professor Dr. Toshiro Takabatake from Department of Quantum Matter, Graduate School of Advanced Sciences of Matter, Hiroshima University, for SQUID measurements. The authors also thank Kanae Yuasa, from Toda Kogyo Corp., for assistantship.
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References (1) Velev, O. D.; Kaler, E. W. AdV. Mater. 2000, 12, 531. (2) Yablonovitch, E. Phys. ReV. Lett. 1987, 58, 2059. (3) Yan, H.; Blandford, C. F.; Holland, B. T.; Parent, M.; Smyrl, W. H.; Stein, A. AdV. Mater. 1999, 11, 1003. (4) Iskandar, F.; Mikrajuddin; Okuyama, K. Nano Lett. 2001, 1, 231. (5) Iskandar, F.; Abdullah, M.; Yoden, H.; Okuyama, K. J. Appl. Phys. 2003, 93, 9237. (6) Iskandar, F.; Abdullah, M.; Yoden, H.; Okuyama, K. J. Sol-Gel Sci. Technol. 2004, 29, 41. (7) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (8) Elkins, K. E.; Vedantam, T. S.; Liu, J. P.; Zeng, H.; Sun, S.; Ding, Y.; Wang, Z. L. Nano Lett. 2003, 3, 1647. (9) Iwaki, T.; Kakihara, Y.; Toda, T.; Abdullah, M.; Okuyama, K. J. Appl. Phys. 2003, 94, 6807. (10) Bertolini, J. C.; Massardier, J. Catal. Lett. 1991, 9, 183.
NL050863W
Nano Lett., Vol. 5, No. 7, 2005
