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2007, 111, 7231-7234 Published on Web 05/03/2007
Hydrogen-Induced Crystal Structural Transformation of FePt Nanoparticles at Low Temperature Masafumi Nakaya,† Masayuki Kanehara,† Miho Yamauchi,‡ Hiroshi Kitagawa,‡ and Toshiharu Teranishi*,† Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan, and Department of Chemistry, Faculty of Science, Kyushu UniVersity, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan ReceiVed: March 20, 2007; In Final Form: April 17, 2007
The hydrogen-induced crystal structural transformation of FePt nanoparticles from a chemically disordered face-centered-cubic (fcc) structure to a chemically ordered L10 structure was first demonstrated at around 300 °C. The hydrogen treatment at low temperature led to not complete but considerable suppression of the nanoparticle coalescence. The obtained L10-FePt nanoparticles exhibited ferromagnetic property with large coercivity at room temperature. The ordering degree of FePt nanoparticles was dependent on both the hydrogen pressure and treatment time.
In the phase diagram of bimetal materials, the stable phase is determined by the composition and temperature. The bulk FePt alloy with an Fe content of ∼50 atom % has a facecentered-cubic (fcc) structure at temperatures lower than 500 °C, while annealing above 500 °C transforms the crystal structure of the FePt alloy from fcc to the chemically ordered L10 structure, which is known to cause the ferromagnetic property.1 Similarly, in the case of chemically synthesized FePt nanoparticles, the as-synthesized particles generally have a chemically disordered fcc structure, while annealing above 600 °C under either a vacuum, an inert gas, or a reducing atmosphere transforms the crystal structure from fcc to the chemically ordered L10 phase.2-12 The obtained L10-FePt nanoparticles are expected to be applied to permanent magnetic devices, such as ultrahigh-density recording media, and the hard magnetic phase of nanocomposite magnets, owing to high uniaxial magnetocrystalline anisotropy and high coercivity. However, such a high ordering temperature always leads to a coalescence of nanoparticles, which changes both the particle size and the positional order of the FePt nanoparticle superlattice. Many efforts have been dedicated to prevent coalescence of the nanoparticles, for example, the reduction of ordering temperature by adding a third element to the FePt nanoparticles13,14 or the confinement of FePt nanoparticles into a SiO2 shell15 or NaCl fine powder.16 Unfortunately, these techniques have drawbacks, such as the contamination coming from a third element or the time-consuming treatment including the confinement and extraction of the FePt nanoparticles. Therefore, a novel facile technique to efficiently transform the crystal structure of FePt nanoparticles without any impurities at low temperature is required. Recently, we have found that hydrogen gas effectively works as a phase-transformation agent for FePt nanoparticles * To whom correspondence should be addressed. E-mail: teranisi@chem. tsukuba.ac.jp. Phone: +81-29-853-4011. Fax: +81-29-853-6503. † University of Tsukuba. ‡ Kyushu University.
10.1021/jp072201w CCC: $37.00
at temperatures below 500 °C. This hydrogen effect was observed for the bulk FePt films17 but not for chemically prepared FePt nanoparticles. Here, we report a facile technique to transform the crystal structure of FePt nanoparticles from fcc to L10 by hydrogen treatment (storage and release of hydrogen) at temperatures below 500 °C, where the hydrogen pressure, treatment temperature, and treatment time were systematically altered. We demonstrated that the storage of hydrogen atoms in FePt nanoparticles promotes the diffusion of Fe and Pt atoms. The FePt nanoparticles were synthesized by the polyol reduction of Pt(acac)2 and Fe(acac)3 in a mixture of oleic acid and oleylamine under a nitrogen atmosphere. The metal precursors, oleic acid, and distilled oleylamine were placed together in a flask, and the mixture was subjected to a degassing process including evacuation at room temperature. The flask was flushed with nitrogen three times and then quickly heated to 140160 °C to completely dissolve the precursors with vigorous stirring under a nitrogen flow. When the solution temperature reached 140-160 °C, 1,2-hexadecanediol was added to the solution, and the mixture heated to 240-250 °C within ca. 2 min. After stirring the solution for 30 min at the same temperature, the heat source was removed to allow the black solution to cool down to room temperature. The obtained black precipitate was dissolved in hexane and reprecipitated with ethanol for purification. The resulting black precipitate was redispersed in hexane containing oleic acid and oleylamine as stabilizers, and nitrogen gas was bubbled through the solution to remove oxygen. The hydrogen treatment was performed in the following way. The FePt nanoparticles dispersed in hexane were transferred to a glass capillary. After evaporating the solvent, the capillary was inserted into a sample holder. Under a vacuum, the sample was heated to treatment temperature (280-400 °C) and kept at the same temperature for 30 or 60 min under vacuum. The FePt © 2007 American Chemical Society
7232 J. Phys. Chem. C, Vol. 111, No. 20, 2007
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Figure 1. XRD patterns of FePt nanoparticles before and after hydrogen treatment at 10 MPa at 400 °C for 3 h.
Figure 2. XRD patterns of FePt nanoparticles (a) before and after hydrogen treatment at 10 MPa for 1 h at (b) 280 °C, (c) 290 °C, (d) 300 °C, and (e) 320 °C.
nanoparticles were then treated with hydrogen gas at an applied pressure of 0.1-10 MPa for 5-300 min and then evacuated for 30 min using a rotary pump. The FePt nanoparticles were finally cooled to room temperature under reduced pressure. The crystal structure was measured using an X-ray diffractometer (Cu) or synchrotron X-ray diffractometer. The magnetic property was measured with a superconducting quantum interference device (SQUID) at room temperature. The hydrogen storage ability of 5.1 nm Fe49Pt51 nanoparticles (abbreviated as FePt nanoparticles afterward) protected by oleic acid and oleylamine (Supporting Information Figure S1) was first examined at room temperature. The hydrogen pressure content of the storage curve (Supporting Information Figure S2) demonstrates that the ratio of stored hydrogen to metal (H/M) was slowly raised at pressures lower than 103 Pa. A further increase in hydrogen pressure led to a drastic increase in the H/M value, suggesting the hydrogen storage ability of the FePt nanoparticles, regardless of Fe and Pt having no hydrogen storage ability themselves. However, room-temperature hydrogen treatment at hydrogen pressures from 0.1 to 10 MPa did not induce the crystal structural transformation of the fcc FePt nanoparticles. In order to promote the diffusion of hydrogen atoms in FePt nanoparticles, the FePt nanoparticles were first treated with hydrogen at 10 MPa and 400 °C for 3 h. Figure 1 shows the X-ray diffraction (XRD) patterns of the FePt nanoparticles before and after hydrogen treatment. The L10 ordering peaks (indicated by arrows) were clearly observed for the hydrogentreated nanoparticles, which exhibit a (111) peak at 41.06° (d111 ) 0.2198 nm), corresponding to a completely ordered value, and a coercivity (Hc) of 8 kOe at room temperature. These results clearly reveal that the high-pressure hydrogen has a potential to transform the crystal structure of the FePt nanoparticles from fcc to L10 at around 400 °C, as observed in the case where a third element is added.12,13 The main advantage of hydrogen treatment is that no contamination was left behind after the crystal structural transformation. However, the above condition, i.e., high temperature and extremely high pressure, led to a coalescence of the FePt nanoparticles from 4.0 nm (assynthesized) to 18 nm (hydrogen-treated), as confirmed by the Scherrer formula using a full width at half-maximum (fwhm) width for the (111) peak. Then, we tried to lower the ordering temperature down from 400 °C to around 300 °C, keeping the hydrogen pressure at 10 MPa and treatment time at 1 h. Figure 2 presents the XRD patterns of the FePt nanoparticles before and after 10 MPa hydrogen treatment at 280-320 °C. When the nano-
TABLE 1: Room-Temperature Magnetic Properties of FePt Nanoparticles after the Continuous and Intermittent 0.1 MPa Hydrogen Treatments at 300 °C treatment type
time or cycle
continuous
1h 3h 5h once 3 times 5 times
intermittent
Hc (kOe)
Ms (emu/g)
0.75 2.1 2.9 0.75 1.9 2.7
49 61 64 49 52 62
particles were treated at 280 °C, the (111) position was shifted to a higher diffraction angle (41.0°) than that of the assynthesized nanoparticles (40.3°). After heat treatment at 290 °C, the ordering peaks were weakly observed, which was also confirmed by synchrotron XRD (S-XRD) measurements (Supporting Information Figure S3). The clear ordering peaks assigned to the (001), (110), (210), and (112) planes appeared at temperatures above 290 °C, both in the XRD and in the S-XRD patterns. These results indicate that hydrogen atoms promote the diffusion of Fe and Pt atoms in the FePt nanoparticles around 300 °C under high-pressure conditions, resulting in the effective transformation from fcc to L10. In order to investigate the time-dependent ordering process under high-pressure hydrogen, FePt nanoparticles were treated with 10 MPa hydrogen for 5, 10, and 30 min at 300 °C and monitored using S-XRD measurements (Supporting Information Figure S4). After 5 min, the (111) peak position was already shifted to a higher angle from 12.86° (as-synthesized) to 13.05°, and this peak shift proceeded as the treatment time increased, occurring with a slight evolution of the ordering peaks. Consequently, treatment of FePt nanoparticles with hydrogen at 10 MPa and ca. 300 °C, with a treatment time longer than 60 min, is effective for transforming the crystal structure from fcc to L10. However, as indicated by the coherent domain size change from 7.4 to 10 nm resulting from the rise in temperature from 280 to 320 °C in Figure 2, the coalescence of FePt nanoparticles was inevitable, although not as much as in the case at 400 °C. With regard to the effect of hydrogen pressure on the crystal structural transformation of fcc FePt nanoparticles and on the coalescence of the particles, the nanoparticles were treated with hydrogen at an applied pressure of 0.1 MPa at 300 °C for 1-5 h. Figure 3 shows the XRD patterns and room-temperature magnetic properties of the FePt nanoparticles treated with 0.1 MPa hydrogen at 300 °C for different times. After 1 h, the positional shift of the (111) peak to a higher angle (40.8°) than that of the as-synthesized particles (40.3°) and the appearance
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Figure 3. XRD patterns (left) and room-temperature magnetic properties (right) of FePt nanoparticles (a) before and after hydrogen treatment at 0.1 MPa at 300 °C for (b) 1 h, (c) 3 h, and (d) 5 h.
Figure 4. XRD patterns (left) and room-temperature magnetic properties (right) of FePt nanoparticles (a) before and after intermittent hydrogen treatment at 0.1 MPa at 300 °C (b) once, (c) 3 times, and (d) 5 times.
of small ordering peaks were observed (Supporting Information Figure S5). Elongating the treatment time led to the growth of ordering peaks and a further shift of the (111) peak, which proceeded slower in comparison with the case of 10 MPa hydrogen treatment. After 5 h, the (111) peak position finally reached 41.0° (d111 ) 0.220 nm), comparable to a nearly ordered value (d111 ) 0.2197 nm), and the ordering peaks (001), (110), (210), and (112) were clearly observed. The time-dependent crystal structural transformation of the FePt nanoparticles was also confirmed by SQUID at room temperature. Both the Hc and saturation magnetization (Ms) values increased with increasing treatment time, indicating that the ordering of the nanoparticles proceeds over time (Table 1). Hydrogen treatment for 5 h endowed the nanoparticles with Hc ) 2.9 kOe and Ms ) 64 emu/g. In addition, a fwhm width for the (111) peak of 1.4° implies a comparative suppression of the particle coalescence. Here, the question as to which of the continuous or intermittent (repetition of storage and release of hydrogen) hydrogen treatment methods is more effective in crystal structural transformation arises, as there is a possibility that the process of hydrogen release from the nanoparticles could promote the diffusion of Fe and Pt atoms. Therefore, the cycle involving 0.1 MPa hydrogen treatment at 300 °C for 1 h, followed by evacuation for 30 min, was repeated once, three, and five times. Figure 4 shows the XRD patterns and room-temperature magnetic properties of FePt nanoparticles after the intermittent hydrogen treatment. The XRD patterns and hysteresis loops of the FePt nanoparticles treated with 0.1 MPa hydrogen once, three, and five times are consistent with those of the nanopar-
ticles treated with 0.1 MPa hydrogen for 1, 3, and 5 h, respectively (Table 1). This result leads to the conclusion that the crystal structural transformation, Hc, and Ms are obviously dependent on the overall hydrogen treatment time and that the crystal structural transformation takes place during the process of hydrogen storage. In addition, in situ S-XRD measurements also confirmed that the crystal structural transformation proceeds at the hydrogen storage stage (Supporting Information Figure S6). Finally, in order to justify our claim that the crystal structural transformation is promoted only by hydrogen without the aid of particle growth, the in situ S-XRD measurements of FePt nanoparticles were conducted under various hydrogen pressures at 240 °C (Supporting Information Figure S7), at which the fcc FePt nanoparticles were chemically synthesized. The (111) peak position was not shifted in the range of hydrogen pressure from 0 to 13 kPa, whereas the (111) peak shift toward the higher diffraction angle was observed without coalescence at pressures of more than 40 kPa. This clearly demonstrates that the hydrogen atoms stored in the FePt nanoparticles promote the diffusion of both metal atoms to remove any distortion of the fcc structure and transform the crystal structure from fcc to L10 via the hydrogen storage process. Therefore, the hydrogen-induced crystal structural transformation without particle coalescence at low temperature would be possible by further modification of the experimental conditions. In conclusion, we found that the FePt nanoparticles have a hydrogen storage ability, and their crystal structure was transformed from fcc to the L10 phase via the hydrogen storage
7234 J. Phys. Chem. C, Vol. 111, No. 20, 2007 process. The ordering degree was controlled by the treatment temperature, hydrogen pressure, and treatment time. This ordering technique provides a simple procedure for the preparation of L10-FePt nanoparticles and enables us to use the relatively low-heat-resistant substrates for ultrahigh-density magnetic recording media. Now, the development of the ordering technique without particle coalescence, as well as the application of this technique to other bimetallic nanoparticle systems, is in progress. Acknowledgment. This work was supported by Industry Technology Research Grant Program in 2004 from NEDO of Japan, (T.T.), and Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (M.N.). Supporting Information Available: TEM image of assynthesized 5.1 nm Fe49Pt51 nanoparticles, the characteristics of hydrogen pressure, content of storage and temperature (PCT) of the FePt nanoparticles at 30 °C, S-XRD patterns of FePt nanoparticles before and after hydrogen treatment at 10 and 0.1 MPa, in situ S-XRD patterns of FePt nanoparticles after hydrogen treatment at 0.1 MPa at 300 °C for 0-180 min, and S-XRD patterns of FePt nanoparticles at 240 °C under a hydrogen pressure of 0-101 kPa. This material is available free of charge via the Internet at http://pubs.acs.org.
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