Fept And Fe Nano Composite By Intermediate Annealing Temperature
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View Fept And Fe Nano Composite By Intermediate Annealing Temperature as PDF for free.
More details
- Words: 1,971
- Pages: 3
JOURNAL OF APPLIED PHYSICS
VOLUME 95, NUMBER 11
Magnetic Nanoparticles: FePt, CoPt, Etc.
1 JUNE 2004
Dieter Weller, Chairman
FePt and Fe nanocomposite by annealing self-assembled FePt nanoparticles M. H. Lu,a) T. Song, and T. J. Zhou Data Storage Institute, 5 Engineering Drive 1, NUS, Singapore 117608, Singapore
J. P. Wang MINT and Electrical and Computer Engineering Department, University of Minnesota, Minneapolis, Minnesota 55455
S. N. Piramanayagam and W. W. Ma Data Storage Institute, 5 Engineering Drive 1, NUS, Singapore 117608, Singapore
H. Gongb) Department of Materials Science, National University of Singapore, Kent Ridge, Singapore 119260, Singapore
共Presented on 6 January 2004兲 We present a simple fabrication of FePt and Fe nanocomposite by annealing self-assembled FePt nanoparticles. 5 nm FePt nanoparticles were prepared by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in the presence of oleic acid and oleylamine as stabilizers. The as-synthesized FePt nanoparticles can self-organize well on a clean SiO/Si substrate and, then, were annealed in a vacuum using a rapid thermal process at different temperatures for 0.5 h. It is found that the annealing temperature plays a key role in determining the final products. The samples annealed at 550 °C and 700 °C consist of a FePt-face-centered-tetragonal 共fct兲 phase with a coercivity of around 1000 and 8800 Oe, respectively, whereas, the samples annealed at 580 °C are composed of two phases: one is the FePt-fct phase and the other is a Fe-body-centered-cubic phase. The hysteresis loop of the sample annealed at 580 °C is also two-phase like with a kink at low field and its coercivity is around 8000 Oe. That indicates that Fe can be segregated from FePt by proper annealing, forming FePt and Fe nanocomposites. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1652412兴
I. INTRODUCTION
cursors to previously synthesized 3 nm particles. Threedimensional 共3D兲 superlattices could be generated when the solvent was evaporated slowly. The interparticle spacing could be tuned by controlling the chain length of the surfactants. Recently, Zeng et al.3 and Sun et al.7 fabricated FePt and Fe3 Pt exchange-coupled nanocomposites using nanoparticle self-assembly. They mixed hexane dispersions of FePt and Fe3 O4 nanoparticles with selected concentration, volume, and sizes under ultrasonic agitation. The FePt and Fe3 O4 nanoparticles form binary assemblies, which were converted into FePt–Fe3 Pt nanocomposites by annealing under a flow of Ar⫹5% H2 at 650 °C for 1 h. By optimizing the exchange coupling, they produced exchange-coupled isotropic FePt–Fe3 Pt nanocomposites with an energy product of 20.1 MG Oe.3 Although the work of Zeng et al.3 is an exciting development for making strong magnets for practical applications, there are still many challenges to be faced and the fabricating processes are also a little complicated. Here, we report an approach to fabricate FePt and Fe nanocomposites by simply annealing self-assembled FePt nanoparticles at certain temperature using a rapid thermal process.
The synthesis and assembly of small hard magnetic nanoparticles have attracted more and more attention because of their potential applications in ultrahigh-density magnetic recording,1 ferrofluids,2 advanced nanocomposite permanent magnets,3,4 and even biological microsystems. The chemically ordered FePt face-centered-tetragonal 共fct兲 phase is an excellent candidate for those applications due to their good chemical stability and high magnetocrystalline anisotropy, K u (108 erg/cm3 ). 5,6 Sun et al.7 synthesized monodisperse FePt nanoparticles by the simultaneous reduction of platinum acetylacetonate (Pt (acac) 2 ) and thermal decomposition of iron pentacarbonyl in the presence of oleic acid and oleylamine as stabilizers. The composition was adjusted by changing the molar ratio of the two precursors. The particle size could be varied from 3 to 10 nm by adding more pre-
a兲
Also at: Department of Materials Science, National University of Singapore, Kent Ridge, Singapore 119260. b兲 Author to whom correspondence should be addressed; electronic mail: [email protected] 0021-8979/2004/95(11)/6735/3/$22.00
6735
© 2004 American Institute of Physics
Downloaded 07 Sep 2004 to 152.1.149.160. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
6736
Lu et al.
J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004
FIG. 2. XRD patterns of FePt particle assemblies: 共A兲 As-synthesized, 共B兲 annealed at 550 °C, 共C兲 annealed at 580 °C, and 共D兲 annealed at 700 °C.
III. RESULTS AND DISCUSSION FIG. 1. TEM micrographs of 5 nm FePt nanoparticles: 共A兲 One layer assembly, 共B兲 3D hexagonal assembly, 共C兲 3D cubic assembly, and 共D兲 selected-area electron diffraction.
II. EXPERIMENT
The synthesis of Fe48Pt52 in principle followed the method reported by Sun et al.1 Pt (acac) 2 共197 mg, 0.5 mmol兲, 1,2-hexadecanediol 共390 mg, 1.5 mmol兲, and octyl ether 共20 ml兲 were added to a three-necked flask equipped with an Ar in/outlet, septa rubber, and thermometer in glove box. The flask was transferred out and connected to an Ar atmosphere system. The mixture was heated to 100 °C. Oleic acid 共0.16 ml, 0.5 mmol兲, oleyl amine 共0.24 ml, 0.5 mmol兲, and Fe (CO) 5 共0.098 ml, 0.75 mmol兲 were added, and the solution was continuously heated to reflux at 297 °C for 15 min. The heating source was removed and the black mixture was cooled to room temperature. 40 ml ethanol was added to the mixture and the mixture was centrifugated. The yellowbrown supernatant was discarded. The black precipitate was then dispersed in hexane 共25 ml兲 in the presence of oleic acid 共0.05 ml兲 and oleyl amine 共0.05 ml兲 and precipitated out by adding ethanol 共20 ml兲, followed by the centrifugation of the mixture. The black precipitate was dispersed in hexane 共20 ml兲 and centrifugated again to remove unsolved precipitate 共almost no precipitate兲. 15 ml ethanol was further added to precipitate out the product. The black precipitate was finally redispersed in hexane and stored in glove box. The composition of as-synthesized FePt nanoparticles was analyzed using inductively coupled plasma-optic emission spectrometry. Transmission electron microscopy 关共TEM兲 Hitachi H-8100兴 was used to observe the selfassembled-particle pattern and determine the particle size. X-ray diffraction 共XRD兲 was applied to check the structure of as-synthesized and annealed FePt particles. Magnetic properties were measured using a vibrating sample magnetometer 共produced by DMS兲 and an alternating gradient magnetometer.
Nanoparticle assembly was performed on a naturally oxidized silicon substrate. One or more drops of hexanedispersed FePt nanoparticles were deposited on a SiO2 /Si substrate. The solvent was allowed to evaporate slowly at room temperature, and FePt nanoparticles were selfassembled into 3D superlattices or a two-dimensional regular array at different evaporate rates with different concentrations. 5 nm FePt particles were synthesized 关Figs. 1共A兲–1共C兲兴. The FePt nanoparticles can self-organize into a regular one layer array 关hexagonal pattern with larger intergrain distance, Fig. 1共A兲兴, close-packed 3D hexagonal superlattice 关Fig. 1共B兲兴, or cubic multilayer 关Fig. 1共C兲兴. The intergrain distance is 4 nm for a one-dimensional self-assembled array 关Fig. 1共A兲兴, whereas, it is about 2 nm for a 3D hexagonal and cubic superlattice 关Figs. 1共B兲 and 1共C兲兴, which is maintained by oleic acid/oleylamine stabilizers. Figure 1共D兲 shows the selected-area electron diffraction pattern of the assynthesized FePt nanoparticles, which reveals that the particles are structurally face-centered cubic 共fcc兲. The as-synthesized FePt nanoparticles were annealed at 550 °C, 580 °C, and 700 °C for 30 min in a vacuum better than 5⫻10⫺6 Torr using a rapid thermal process. Figures 2共A兲–2共D兲 show the XRD patterns of as-synthesized, annealed at 550 °C, 580 °C, and 700 °C particle assemblies, respectively. The as-synthesized particle assembly exhibits very broad peaks, which can be indexed by using the structure of fcc FePt, further confirming that the as-synthesized particles are structurally fcc. When annealed at 550 °C, the peaks become narrow and shift to a high-angle side, indicating the growth of grain and ordering of Fe and Pt atoms. As the peak positions are between those of fcc FePt and fct FePt, the particles are partially chemically ordered. By increasing the annealing temperature to 580 °C, a few peaks appear and the existing peaks are narrowed further and shift to a higher-angle side. Most of peaks can be indexed by using the structure of fct FePt except one positioned at 2 ⫽44.8 °C, which can be attributed to body-centered-cubic 共bcc兲 iron, indicating that those films are composed of two
Downloaded 07 Sep 2004 to 152.1.149.160. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
Lu et al.
J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004
FIG. 3. Hysteresis loops of FePt nanoparticles annealed at different temperatures: 共A兲 550 °C, 共B兲 580 °C, and 共C兲 700 °C.
phases: One is fct FePt and the other is bcc Fe 共Fe-rich FePt alloy兲 关Fig. 2共C兲兴. Further increasing the annealing temperature to 700 °C, no peaks of bcc Fe are found and all peaks can be fully indexed by using the structure of fct FePt, indicating that the films become single fct-FePt phase. The in-plane and out-of-plane hysteresis loops of annealed films are shown in Figs. 3共A兲–3共C兲. For films annealed at 550 °C, the in-plane and out-of-plane coercivity are 954.6 Oe and 1021 Oe, respectively 关Fig. 3共A兲兴, further confirming that the particles are partially chemically ordered due to a relatively low annealing temperature. For the films annealed at 580 °C, the in-plane and out-of-plane hysteresis
6737
loops are two phaselike with a kink at low field 关Fig. 3共B兲兴, revealing that the films consist of one magnetically hard phase and one magnetically soft phase. The in-plane coecivity is more than 8000 Oe and out-of-plane coecivity is around 7500 Oe 关Fig. 3共B兲兴. Combined with the XRD results, it is believed that the magnetically hard phase is fctFePt and the magnetically soft phase is bcc-Fe 共Fe-rich FePt alloy兲, indicating that Fe can be segregated from FePt by proper annealing and FePt and Fe nanocomposite are formed. Increasing the annealing temperature to 700 °C, the hysteresis loops become single phaselike with in-plane and out-of-plane coercivity of 8676 Oe and 8813 Oe, respectively 关Fig. 3共C兲兴, which agrees well with the XRD results. The observed results could be explained as follows. When the films were annealed in a vacuum using rapid thermal processes, the phase transition for fcc to fct takes place. There possibly exists one intermediate phase, which is an Fe-rich bcc Fe phase, at a special annealing temperature. Once cooled down very fast at this temperature, this intermediate phase could be maintained. When increasing the annealing temperature, the intermediate phase may not appear, therefore, only one phase is observed. In summary, a simple method in the fabrication of fctFePt and Fe 共Pt兲 nanocomposite was presented. It is found that the annealing temperature plays a key role in determining the final products using a rapid thermal process. When annealed at 550 °C, the particles consist of a single phase partially chemically ordered fct FePt. When annealed at 580 °C, two phases appear, which are magnetically hard fctFePt and magnetically soft Fe-rich bcc iron. By increasing the temperature to 700 °C, the particles are composed of a chemically ordered fct-FePt single phase. 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 B. M. Berkovsky, V. F. Medvedev, and M. S. Krakov, Magnetic Fluids: Engineering Applications 共Oxford University Press, Oxford, 1993兲. 3 H. Zeng, J. Li, J. P. Liu, Z. L. Wang, and S. H. Sun, Nature 共London兲 420, 395 共2002兲. 4 M. Watanabe, T. Masumoto, D. H. Ping, and K. Hono, Appl. Phys. Lett. 76, 3971 共2000兲. 5 A. Cebolleda, D. Weller, J. Sticht, G. R. Harp, R. F. C. Farrow, R. F. Marks, R. Savoy, and J. C. Scott, Phys. Rev. B 50, 3419 共1994兲. 6 R. F. C. Farrow, D. Weller, R. F. Marks, M. F. Toney, A. Cebollada, and G. R. Harp, J. Appl. Phys. 79, 5967 共1996兲. 7 S. H. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287, 1989 共2000兲.
Downloaded 07 Sep 2004 to 152.1.149.160. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
VOLUME 95, NUMBER 11
Magnetic Nanoparticles: FePt, CoPt, Etc.
1 JUNE 2004
Dieter Weller, Chairman
FePt and Fe nanocomposite by annealing self-assembled FePt nanoparticles M. H. Lu,a) T. Song, and T. J. Zhou Data Storage Institute, 5 Engineering Drive 1, NUS, Singapore 117608, Singapore
J. P. Wang MINT and Electrical and Computer Engineering Department, University of Minnesota, Minneapolis, Minnesota 55455
S. N. Piramanayagam and W. W. Ma Data Storage Institute, 5 Engineering Drive 1, NUS, Singapore 117608, Singapore
H. Gongb) Department of Materials Science, National University of Singapore, Kent Ridge, Singapore 119260, Singapore
共Presented on 6 January 2004兲 We present a simple fabrication of FePt and Fe nanocomposite by annealing self-assembled FePt nanoparticles. 5 nm FePt nanoparticles were prepared by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in the presence of oleic acid and oleylamine as stabilizers. The as-synthesized FePt nanoparticles can self-organize well on a clean SiO/Si substrate and, then, were annealed in a vacuum using a rapid thermal process at different temperatures for 0.5 h. It is found that the annealing temperature plays a key role in determining the final products. The samples annealed at 550 °C and 700 °C consist of a FePt-face-centered-tetragonal 共fct兲 phase with a coercivity of around 1000 and 8800 Oe, respectively, whereas, the samples annealed at 580 °C are composed of two phases: one is the FePt-fct phase and the other is a Fe-body-centered-cubic phase. The hysteresis loop of the sample annealed at 580 °C is also two-phase like with a kink at low field and its coercivity is around 8000 Oe. That indicates that Fe can be segregated from FePt by proper annealing, forming FePt and Fe nanocomposites. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1652412兴
I. INTRODUCTION
cursors to previously synthesized 3 nm particles. Threedimensional 共3D兲 superlattices could be generated when the solvent was evaporated slowly. The interparticle spacing could be tuned by controlling the chain length of the surfactants. Recently, Zeng et al.3 and Sun et al.7 fabricated FePt and Fe3 Pt exchange-coupled nanocomposites using nanoparticle self-assembly. They mixed hexane dispersions of FePt and Fe3 O4 nanoparticles with selected concentration, volume, and sizes under ultrasonic agitation. The FePt and Fe3 O4 nanoparticles form binary assemblies, which were converted into FePt–Fe3 Pt nanocomposites by annealing under a flow of Ar⫹5% H2 at 650 °C for 1 h. By optimizing the exchange coupling, they produced exchange-coupled isotropic FePt–Fe3 Pt nanocomposites with an energy product of 20.1 MG Oe.3 Although the work of Zeng et al.3 is an exciting development for making strong magnets for practical applications, there are still many challenges to be faced and the fabricating processes are also a little complicated. Here, we report an approach to fabricate FePt and Fe nanocomposites by simply annealing self-assembled FePt nanoparticles at certain temperature using a rapid thermal process.
The synthesis and assembly of small hard magnetic nanoparticles have attracted more and more attention because of their potential applications in ultrahigh-density magnetic recording,1 ferrofluids,2 advanced nanocomposite permanent magnets,3,4 and even biological microsystems. The chemically ordered FePt face-centered-tetragonal 共fct兲 phase is an excellent candidate for those applications due to their good chemical stability and high magnetocrystalline anisotropy, K u (108 erg/cm3 ). 5,6 Sun et al.7 synthesized monodisperse FePt nanoparticles by the simultaneous reduction of platinum acetylacetonate (Pt (acac) 2 ) and thermal decomposition of iron pentacarbonyl in the presence of oleic acid and oleylamine as stabilizers. The composition was adjusted by changing the molar ratio of the two precursors. The particle size could be varied from 3 to 10 nm by adding more pre-
a兲
Also at: Department of Materials Science, National University of Singapore, Kent Ridge, Singapore 119260. b兲 Author to whom correspondence should be addressed; electronic mail: [email protected] 0021-8979/2004/95(11)/6735/3/$22.00
6735
© 2004 American Institute of Physics
Downloaded 07 Sep 2004 to 152.1.149.160. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
6736
Lu et al.
J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004
FIG. 2. XRD patterns of FePt particle assemblies: 共A兲 As-synthesized, 共B兲 annealed at 550 °C, 共C兲 annealed at 580 °C, and 共D兲 annealed at 700 °C.
III. RESULTS AND DISCUSSION FIG. 1. TEM micrographs of 5 nm FePt nanoparticles: 共A兲 One layer assembly, 共B兲 3D hexagonal assembly, 共C兲 3D cubic assembly, and 共D兲 selected-area electron diffraction.
II. EXPERIMENT
The synthesis of Fe48Pt52 in principle followed the method reported by Sun et al.1 Pt (acac) 2 共197 mg, 0.5 mmol兲, 1,2-hexadecanediol 共390 mg, 1.5 mmol兲, and octyl ether 共20 ml兲 were added to a three-necked flask equipped with an Ar in/outlet, septa rubber, and thermometer in glove box. The flask was transferred out and connected to an Ar atmosphere system. The mixture was heated to 100 °C. Oleic acid 共0.16 ml, 0.5 mmol兲, oleyl amine 共0.24 ml, 0.5 mmol兲, and Fe (CO) 5 共0.098 ml, 0.75 mmol兲 were added, and the solution was continuously heated to reflux at 297 °C for 15 min. The heating source was removed and the black mixture was cooled to room temperature. 40 ml ethanol was added to the mixture and the mixture was centrifugated. The yellowbrown supernatant was discarded. The black precipitate was then dispersed in hexane 共25 ml兲 in the presence of oleic acid 共0.05 ml兲 and oleyl amine 共0.05 ml兲 and precipitated out by adding ethanol 共20 ml兲, followed by the centrifugation of the mixture. The black precipitate was dispersed in hexane 共20 ml兲 and centrifugated again to remove unsolved precipitate 共almost no precipitate兲. 15 ml ethanol was further added to precipitate out the product. The black precipitate was finally redispersed in hexane and stored in glove box. The composition of as-synthesized FePt nanoparticles was analyzed using inductively coupled plasma-optic emission spectrometry. Transmission electron microscopy 关共TEM兲 Hitachi H-8100兴 was used to observe the selfassembled-particle pattern and determine the particle size. X-ray diffraction 共XRD兲 was applied to check the structure of as-synthesized and annealed FePt particles. Magnetic properties were measured using a vibrating sample magnetometer 共produced by DMS兲 and an alternating gradient magnetometer.
Nanoparticle assembly was performed on a naturally oxidized silicon substrate. One or more drops of hexanedispersed FePt nanoparticles were deposited on a SiO2 /Si substrate. The solvent was allowed to evaporate slowly at room temperature, and FePt nanoparticles were selfassembled into 3D superlattices or a two-dimensional regular array at different evaporate rates with different concentrations. 5 nm FePt particles were synthesized 关Figs. 1共A兲–1共C兲兴. The FePt nanoparticles can self-organize into a regular one layer array 关hexagonal pattern with larger intergrain distance, Fig. 1共A兲兴, close-packed 3D hexagonal superlattice 关Fig. 1共B兲兴, or cubic multilayer 关Fig. 1共C兲兴. The intergrain distance is 4 nm for a one-dimensional self-assembled array 关Fig. 1共A兲兴, whereas, it is about 2 nm for a 3D hexagonal and cubic superlattice 关Figs. 1共B兲 and 1共C兲兴, which is maintained by oleic acid/oleylamine stabilizers. Figure 1共D兲 shows the selected-area electron diffraction pattern of the assynthesized FePt nanoparticles, which reveals that the particles are structurally face-centered cubic 共fcc兲. The as-synthesized FePt nanoparticles were annealed at 550 °C, 580 °C, and 700 °C for 30 min in a vacuum better than 5⫻10⫺6 Torr using a rapid thermal process. Figures 2共A兲–2共D兲 show the XRD patterns of as-synthesized, annealed at 550 °C, 580 °C, and 700 °C particle assemblies, respectively. The as-synthesized particle assembly exhibits very broad peaks, which can be indexed by using the structure of fcc FePt, further confirming that the as-synthesized particles are structurally fcc. When annealed at 550 °C, the peaks become narrow and shift to a high-angle side, indicating the growth of grain and ordering of Fe and Pt atoms. As the peak positions are between those of fcc FePt and fct FePt, the particles are partially chemically ordered. By increasing the annealing temperature to 580 °C, a few peaks appear and the existing peaks are narrowed further and shift to a higher-angle side. Most of peaks can be indexed by using the structure of fct FePt except one positioned at 2 ⫽44.8 °C, which can be attributed to body-centered-cubic 共bcc兲 iron, indicating that those films are composed of two
Downloaded 07 Sep 2004 to 152.1.149.160. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
Lu et al.
J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004
FIG. 3. Hysteresis loops of FePt nanoparticles annealed at different temperatures: 共A兲 550 °C, 共B兲 580 °C, and 共C兲 700 °C.
phases: One is fct FePt and the other is bcc Fe 共Fe-rich FePt alloy兲 关Fig. 2共C兲兴. Further increasing the annealing temperature to 700 °C, no peaks of bcc Fe are found and all peaks can be fully indexed by using the structure of fct FePt, indicating that the films become single fct-FePt phase. The in-plane and out-of-plane hysteresis loops of annealed films are shown in Figs. 3共A兲–3共C兲. For films annealed at 550 °C, the in-plane and out-of-plane coercivity are 954.6 Oe and 1021 Oe, respectively 关Fig. 3共A兲兴, further confirming that the particles are partially chemically ordered due to a relatively low annealing temperature. For the films annealed at 580 °C, the in-plane and out-of-plane hysteresis
6737
loops are two phaselike with a kink at low field 关Fig. 3共B兲兴, revealing that the films consist of one magnetically hard phase and one magnetically soft phase. The in-plane coecivity is more than 8000 Oe and out-of-plane coecivity is around 7500 Oe 关Fig. 3共B兲兴. Combined with the XRD results, it is believed that the magnetically hard phase is fctFePt and the magnetically soft phase is bcc-Fe 共Fe-rich FePt alloy兲, indicating that Fe can be segregated from FePt by proper annealing and FePt and Fe nanocomposite are formed. Increasing the annealing temperature to 700 °C, the hysteresis loops become single phaselike with in-plane and out-of-plane coercivity of 8676 Oe and 8813 Oe, respectively 关Fig. 3共C兲兴, which agrees well with the XRD results. The observed results could be explained as follows. When the films were annealed in a vacuum using rapid thermal processes, the phase transition for fcc to fct takes place. There possibly exists one intermediate phase, which is an Fe-rich bcc Fe phase, at a special annealing temperature. Once cooled down very fast at this temperature, this intermediate phase could be maintained. When increasing the annealing temperature, the intermediate phase may not appear, therefore, only one phase is observed. In summary, a simple method in the fabrication of fctFePt and Fe 共Pt兲 nanocomposite was presented. It is found that the annealing temperature plays a key role in determining the final products using a rapid thermal process. When annealed at 550 °C, the particles consist of a single phase partially chemically ordered fct FePt. When annealed at 580 °C, two phases appear, which are magnetically hard fctFePt and magnetically soft Fe-rich bcc iron. By increasing the temperature to 700 °C, the particles are composed of a chemically ordered fct-FePt single phase. 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 B. M. Berkovsky, V. F. Medvedev, and M. S. Krakov, Magnetic Fluids: Engineering Applications 共Oxford University Press, Oxford, 1993兲. 3 H. Zeng, J. Li, J. P. Liu, Z. L. Wang, and S. H. Sun, Nature 共London兲 420, 395 共2002兲. 4 M. Watanabe, T. Masumoto, D. H. Ping, and K. Hono, Appl. Phys. Lett. 76, 3971 共2000兲. 5 A. Cebolleda, D. Weller, J. Sticht, G. R. Harp, R. F. C. Farrow, R. F. Marks, R. Savoy, and J. C. Scott, Phys. Rev. B 50, 3419 共1994兲. 6 R. F. C. Farrow, D. Weller, R. F. Marks, M. F. Toney, A. Cebollada, and G. R. Harp, J. Appl. Phys. 79, 5967 共1996兲. 7 S. H. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287, 1989 共2000兲.
Downloaded 07 Sep 2004 to 152.1.149.160. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
