Phase Transition In Fept

JOURNAL OF APPLIED PHYSICS

VOLUME 93, NUMBER 10

Magnetic Nanostructures and Arrays

15 MAY 2003

K. V. Rao, Chairman

Phase transformation and magnetic moment in FePt nanoparticles Y. Ding,a) S. Yamamuro, D. Farrell, and S. A. Majetichb) Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-3890

共Presented on 13 November 2002兲 The phase transformation from fcc to L1 0 in FePt nanoparticles was investigated in both thick film samples and self-assembled arrays as a function of the annealing temperature, using transmission electron microscopy, x-ray diffraction, differential scanning calorimetry, and magnetometry. A significant fraction of the surfactant decomposes into gaseous products below 500 °C, removing the steric barrier between particle cores. This causes the particles to coalesce at the same annealing temperatures where the transformation to the high anisotropy phase occurs. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1544495兴

I. INTRODUCTION

sufficient signal. A previous DSC study found a large, sharp endothermic peak at 378 °C for the decomposition of oleic acid on magnetite nanoparticles,3 so the FePt samples were first annealed at 400 °C in H2 for 30 min. Hydrogen was passed through the sample tube prior to transfer to the preset furnace, during annealing, and after removal while it cooled. The sample mass was measured before and after annealing. A Perkin Elmer DSC7 differential scanning calorimeter was used with an empty copper holder as a reference. The sample pretreated at 400 °C in H2 was equilibrated at 300 °C in flowing argon, and then the temperature was raised to 700 °C at a rate of 40 °C/min while the heat flow was measured. X-ray diffraction 共XRD兲 measurements were done in a Rigaku x-ray diffractometer using Cu K ␣ radiation (␭ ⫽0.154 nm), to determine the FePt phase after annealing samples from the same batch at different temperatures. Thick films 共⬃10 mg, or ⬃104 layers兲 were used to obtain a reasonable signal-to-noise ratio. The magnetic properties of FePt of thick films were measured at room temperature by a Quantum Design superconducting quantum interference device 共SQUID兲 magnetometer.

Self-assembled FePt nanoparticle arrays have been the subject of much interest due to their potential use as ultrahigh density magnetic storage media.1 The advantage of the self-assembly approach is the high degree of uniformity in the grain size and position. However, as-made low anisotropy fcc phase must be transformed into the chemically ordered, high magnetocrystalline anisotropy L1 0 phase2 by annealing, and at the same time the particles sinter together, destroying the key advantage over existing media. Here we compare the crystalline structure and morphology with the magnetic properties following different annealing conditions to identify conditions favoring phase transformation without grain growth. II. EXPERIMENT

FePt nanoparticles were synthesized by standard high temperature chemical methods.1 The surfactant coating was a 1:1 mixture of oleic acid and oleylamine. The particles were washed with ethanol, centrifuged, and redispersed in hexane several times, with additional surfactant to retain a good surface coating. To form nanoparticle arrays, a droplet of the hexane solution was evaporated on a transmission electron microscope 共TEM兲 grid. The arrays were examined with a Philips EM420 TEM operating at 120 keV to determine the particle diameter, the interparticle spacing, and the degree of ordering. To prepare samples for differential scanning calorimetry 共DSC兲, the washed and redispersed particle solution was added to a small copper cup, and the hexane was evaporated. Larger amounts 共⬃10 mg, or ⬃104 layers兲 of particles were used than in the arrays 共several layers兲, in order to obtain

III. RESULTS AND DISCUSSION

During pretreatment at 400 °C, there was a mass loss of ⬃25%. Further heating did not change the mass significantly.

a兲

FIG. 1. Heat flow vs temperature for a thick film of FePt nanoparticles annealed in argon with a heating rate of 40 °C/min. The sample was preannealed in H2 at 400 °C for 30 min.

Author to whom correspondence should be addressed; electronic mail: [email protected] b兲 Electronic mail: [email protected] 0021-8979/2003/93(10)/7411/3/$20.00

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© 2003 American Institute of Physics

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J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003

FIG. 2. X-ray diffraction intensities as a function of angle, for various annealing temperatures. The peaks at 69° and 34° are from the Si sample holder, and the broad peak at 24° in the 700 °C pattern is from the glass substrate.

DSC shows two endothermic peaks, one from 450 to 550 °C and the other from 550 to 650 °C 共Fig. 1兲. The peak starting at 450 °C may be due to reaction of the remaining surfactant with the particles. Surprisingly, there is no exothermic peak associated with the FePt phase transformation from the chemically disordered fcc to the L1 0 phase, as has been reported in FePt thin films.4 The phase transformation temperature for bulk FePt alloy and FePt thin films are 1300 and ⬃400 °C respectively,4 but could be different for nanoparticles, due to their increased surface area and surfactant coating. Figure 2 shows the XRD intensity as a function of scattering angle for the unannealed FePt sample and samples annealed at 550, 650, and 700 °C. The as-made FePt sample shows broad peaks consistent with an fcc structure. In the sample annealed at 550 °C, there are initial signs of the 共002兲, 共201兲, and 共112兲 L1 0 peaks. This suggests that the endothermic DSC peak may mask the phase transformation. The L1 0 peaks become sharper as the annealing temperature increases, indicating significant grain growth. From the 共111兲 diffraction peak ( ␪ ⫽41°), the average particle sizes found by the Scherrer method are 3.3, 11, 27, and 30 nm for the unannealed, 550, 650, and 700 °C samples, respectively, correcting for an instrumental broadening of 3.1 mrad. While Scherrer analysis neglects the contributions of structural disorder, the suggested particle growth was qualitatively confirmed by TEM images of the arrays, as shown in Fig. 3.

FIG. 3. TEM bright field images of FePt arrays: 共a兲 unannealed, 共b兲 annealed at 550 °C for 30 min, and 共c兲 annealed at 650 °C for 30 min. The unannealed image shows a typical particle size of 3 nm and a typical interparticle spacing of 4 nm.

Ding et al.

FIG. 4. SQUID hysteresis loops of the FePt thick film samples, as a function of the annealing temperature. The unannealed sample has no coercivity and a very small moment at saturation. The coercivity increases monotonically for samples annealed at 400, 550, and 650 °C.

Figure 3共a兲 shows the morphology of thin, unannealed FePt arrays. The size of the FePt nanoparticles is measured to be 2.7⫾0.6 nm by National Institutes of Health 共NIH兲 image.5 Figures 3共b兲 and 3共c兲 show typical regions of similar arrays annealed at 550 and 650 °C, respectively. While it is possible to find regions where the array ordering has been preserved, most of the particles had sintered 共similar to the results reported before兲.6 The DSC measurements were made with a rapid heating rate, and the TEM results are consistent with assignment of the low temperature peak to surfactant decomposition and the high temperature peak to sintering. Without the surfactant barrier, the particles coalesce due to atomic diffusion and van der Waal’s attractions between particle cores. We observed that thicker arrays had more sintering at a fixed temperature. Since the magnetic behavior of the fcc and L1 0 FePt phases is quite different, SQUID magnetometry was used to monitor the phase transformation. Figure 4 shows room temperature SQUID magnetization curves of the FePt thick film 共⬃10 mg or ⬃104 layers兲 samples. The unannealed FePt nanoparticles were superparamagnetic at 293 K. From a fit to a Langevin function, the unannealed FePt moment is estimated to be on the order of 10⫺18 emu/particle, equivalent to a specific magnetization of 5 emu/g. This estimate assumes that all of the mass following pretreatment is FePt, and FePt nanoparticles have bcc packing.7 After annealing, FePt becomes ferromagnetic, with a magnetization of 54 and 69 emu/g at 50 kOe for samples annealed at 550 and 650 °C, respectively. While XRD results suggest that the phase transformation starts near 550 °C, the sample annealed at 400 °C already shows H c ⬎500 Oe. If even a small fraction of the particles transform, their ferromagnetic contribution will dominate the magnetization. Both H c and the specific magnetization increase at higher annealing temperatures. At the same time the average particle size increases, as seen in TEM and XRD. Since the coercivity increases rapidly with grain or particle size, up to a maximum monodomain size,8,9 共⬃several hundred nm for FePt兲,10 the increase in H c could be due to sintering in combination with the transformation of some of the particles. Preliminary measurements have been made on single TEM grids. These grids have thinner arrays and a reduced degree of sintering. We observe smaller coer-

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Ding et al.

J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003

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civities than in the thick film samples for the same annealing temperature.

temperature, or by increasing the interparticle separation or modifying the thermal stability of the interparticle barrier.

IV. CONCLUSIONS

ACKNOWLEDGMENTS

The structural and morphological properties of FePt nanoparticles were correlated with changes in the magnetic behavior during annealing. TEM studies showed that particle coalescence begins at random locations within nanoparticle arrays by 550 °C, and that once started, grain growth in these regions is rapid. This occurred following the decomposition of the surfactant barriers surrounding the particles, which differential scanning calorimetry showed was completed below 500 °C. A second endothermic DSC peak was noted at 620 °C, most likely due to particle sintering and not to the fcc to L1 0 phase transformation, which is expected to be exothermic. The phase transformation is most likely masked by the other heat flow features, and the peak at 620 °C may be due to a chemical reaction of the residual carbon with FePt or Fe. XRD demonstrated the presence of the L1 0 phase and the beginning of grain growth by 550 °C, and magnetic measurements indicated room temperature coercivity after annealing at temperatures as low as 400 °C. While monolayer films of FePt nanoparticle arrays, oxygen-free annealing atmospheres, and rapid heating reduce the likelihood of particle coalescence during annealing, modifications are needed in order to obtain the benefits of high coercivity and high quality spatial ordering. This could be done by alloying the FePt to reduce the transformation

The authors would like to thank Data Storage System Center 共DSSC兲 at Carnegie Mellon University. They are grateful to Professor Katayun Barmak and Dr. Jihwan Kim for help with DSC measurements. S.A.M. acknowledges support from the National Science Foundation Grant No. CTS-0227645, and the American Chemical Society Petroleum Research Fund Grant No. 37578-AC5. 1

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