Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Z22: Focus Session: FePt Nanoparticles for Information Storage |
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Sponsoring Units: GMAG FIAP Chair: Kai Liu, University of California, Davis Room: Baltimore Convention Center 319 |
Friday, March 17, 2006 11:15AM - 11:51AM |
Z22.00001: Media for Extremely High-Density Recording Invited Speaker: |
Friday, March 17, 2006 11:51AM - 12:03PM |
Z22.00002: Novel Chemical Synthesis of FePt and CoPt Magnetic Nanoclusters Yucheng Sui, Yao Zhao, Minglang Yan, David Sellmyer L1$_{0}$ ordered FePt and CoPt clusters are very promising candidates for ultrahigh magnetic recording media applications due to their large magnetocrystalline anisotropies and chemical stability. We report simple and highly efficient approaches for synthesizing FePt and CoPt nanoclusters by the pyrolysis of corresponding Fe, Co and Pt compounds in organic solvents. A mixture of Fe and Pt acetylacetonates was heated in trioctylamine, producing FePt clusters with average size about 4.2 nm and with a narrow size distribution. When the clusters with equal Fe and Pt atomic ratio were dispersed onto a silicon wafer and heated in forming gas, high- anisotropy magnetic FePt clusters with L1$_{0}$ chemical order were obtained. Coercivity values as high as 23 kOe were realized after annealing at 750$^{o}$C for 5 min. When the pyrolysis of Co and Pt acetylacetonates mixture was carried out in trioctylamine, CoPt clusters about 4 nm and narrow size distribution also were produced. The atomic ratio of CoPt clusters can be tuned by the addition of proper reducing agents. The chemical reaction mechanism, the phase transformation and the magnetic properties will be discussed. This research is supported by DOE, NSF-MRSEC, INSIC, NRI and CMRA. [Preview Abstract] |
Friday, March 17, 2006 12:03PM - 12:15PM |
Z22.00003: Size Effect on Chemical Ordering in Face-Centered Tetragonal FePt Nanoparticles Daren Li, Narayan Poudyal, Vikas Nandwana, J.Ping Liu Monodisperse face-centered tetragonal (fct) FePt nanoparticles with high magnetocrystalline anisotropy have been obtained by means of the salt-matrix annealing technique. The as-synthesized face-centred cubic (fcc) FePt nanoparticles of different sizes were mixed with NaCl powder particles and the mixture was annealed at temperatures up to 750 $^{o}$C for several hours. X-ray diffraction (XRD) and transmission electronic microscopy (TEM) analyses showed that the annealed particles transfered to the fct structure without sintering and agglomeration. Magnetization measurements gave the coercivity of the nanoparticles increasing with the particle size. Extended annealing does not change the size dependence of the coercivity. Further XRD analysis revealed that the size dependence of the coercivity is related to the particle size dependance of long-range chemical ordering degree. The long-range order parameter S is 0.78, 0.92, 0.98 and 0.94 for the 4 nm, 6 nm, 8 nm and 15 nm particles, respetively. This trend is consistent with that of the coercivity measured from the nanoparticles. The relatively low ordering parameter of the 15 nm particles may be related to the polycrystalline morphology as shown by the high resolution TEM. [Preview Abstract] |
Friday, March 17, 2006 12:15PM - 12:27PM |
Z22.00004: Chemically Synthesized FePt Binary Alloy Nanoparticles with Different Shapes L. Colak, Y. Huang, M.J. Bonder, G.C. Hadjipanayis, D. Weller Chemically synthesized FePt nanoparticles are promising candidates for future high density magnetic recording media. In this work, FePt and FePt$_{3}$ binary alloy nanoparticles have been synthesized by thermal decomposition of iron pentacarbonyl (Fe(CO)$_{5})$ and reduction of platinum acetylacetonate (Pt (acac)$_{ 2})$ in the presence of oleic acid (OA) and oleyl amine (OY) surfactants at low refluxing temperatures. FePt$_{3}$ and FePt nanoparticles were obtained by varying the Fe:Pt molar ratio in the range of 1.4-1.7. With control of the heating rate to the refluxing temperature, nanoparticles with a size of $\sim $5 nm and with different shapes were obtained for both compositions. The particles showed very little agglomeration to an annealing temperature of 650$^{o}$C, as observed using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). Magnetic measurements show that annealing at 700$^{o}$C partially transforms the FePt and FePt$_{3}$ nanoparticles from the disordered fcc phase to the ordered L1$_{0}$ and L1$_{2}$ phases, respectively. HRTEM and Mossbauer studies will be discussed in terms of the temperature and time dependent evolution of microstructure with annealing. [Preview Abstract] |
Friday, March 17, 2006 12:27PM - 12:39PM |
Z22.00005: Variability in the extent of ordering for individual FePt L1$_{0}$ nanoparticles Rumyana V. Petrova, Bo Yao, Vu Lam, K.R. Coffey, R.R. Vanfleet L1$_{0 }$ordered phase FePt nanoparticles are of interest as high-density magnetic recording media due to the large uniaxial magnetocrystalline anisotropy,$ K_{u}$ observed in bulk and thin film samples. Similarly high \textit{Ku} values have not yet been achieved for small ($<$10nm) FePt nanoparticles. Mixtures of ordered and disordered nanoparticles are often observed and have been attributed to nucleation barriers for the ordering transformation in individual small nanoparticles. For FePt, $K_{u}$ is known to depend strongly on the extent of long-range chemical order. In this work we report on the variability of the extent of order amongst ordered L1$_{0}$ FePt particles. Well-isolated, small FePt nanoparticles were prepared from discontinuous FePt films, using ultra high vacuum dc magnetron sputtering. A Tecnai F30 analytical transmission electron microscope was used to characterize the FePt nanoparticles. Convergent beam electron diffraction patterns from individual nanoparticles were compared to multislice simulations to determine the extent of order of single nanoparticles. [Preview Abstract] |
Friday, March 17, 2006 12:39PM - 12:51PM |
Z22.00006: Direct Fabrication and Magnetic Characterization of Highly Ordered L1$_{0}$ FePt Nanoparticles Jiao-Ming Qiu, Jian-Ping Wang Future advanced magnetic recording media require the grains to be small and uniform with sufficient anisotropy to sustain thermal fluctuation. L1$_{0}$ phase FePt nanoparticles fit these requirements but their fabrication turns out to be difficult. Conventional methods were reported to be able to make monodispersed FePt nanoparticles with disordered structure$^{1} $. The inevitable phase transformation process is believed to be kinetically limited$^{2}$ and such issues as agglomeration and twinning are to be addressed. Here we report a novel technique that can fabricate monodispersed highly ordered FePt nanoparticles with no need for phase transformation process. In this technique, particle nucleation and growth processes happened in the gas phase and they were separated in space. Modified sputtering plasma made the ordering and particle growth processes proceed simultaneously. Uniform L1$_{0}$ FePt nanoparticles with room temperature coercivity of 8.25 kOe were achieved for particle randomly assembled film. HRTEM analysis shows that these particles have perfect octahedron shape with only {\{}111{\}} surfaces exposed. Ref: 1. Sun, S, et al, Science, 287, 1989, 2000 2. Ding, Y. et al, Appl. Phys. Lett. 87, 022508, 2005 [Preview Abstract] |
Friday, March 17, 2006 12:51PM - 1:03PM |
Z22.00007: Effects of Annealing on the Magnetic and Structural Properties of FePt Nanoparticles by Chemical Synthesis Hongli Wang, Y. Huang, Y. Zhang, G. Hadjipanayis, D. Weller, A. Simopoulos In this paper, we have studied the effects of annealing atmospheres on the particle size, the degree of ordering, and the magnetic properties of FePt nanoparticles made by chemical synthesis. Measurements were made on particle assemblies with controlled dimension and thickness. Annealing was done under forming gas and vacuum atmospheres. Particles did not grow much after annealing at 550 $^{o}$C for 30 min under forming gas, resulting in an average particle size D $\approx $ 5 nm and a coercivity of 4 kOe. After annealing under vacuum using the same annealing temperature and time, the morphology of particles is similar to that of particles annealed under forming gas but with a coercivity less than 1 kOe. We believe that this large difference in coercivity is due to the fact that the forming gas not only assists the transformation from the chemically disordered fcc phase to the chemically ordered L1$_{0}$ phase but also makes the atomic composition of FePt nanoparticles more uniform. M\"{o}ssbauer studies showed a higher percentage of L1$_{0 }$phase in the sample annealed under forming gas than that under vacuum, which further confirms the hypothesis stated above. [Preview Abstract] |
Friday, March 17, 2006 1:03PM - 1:15PM |
Z22.00008: Enhanced orbital magnetism in oxide free Fe50Pt50 nanoparticles probed by x-ray magnetic circular dichroism Michael Farle X-ray absorption spectra at both the Fe and Pt L3,2 edges were measured on wet-chemically synthesized Fe50Pt50 particles with a mean diameter of 6.3 nm. The organic ligands and the oxide shell covering the particles in the as prepared state were removed by soft hydrogen plasma. After thermal treatment under hydrogen atmosphere of 5 Pa, the coercive field increased by a factor of 6. This indicates the formation of the chemically ordered L10 phase and is accompanied by an enhancement of the orbital magnetic moment at the Fe site by 275{\%}. Changes in the frequency of oscillations in the extended x-ray absorption fine structure at the Pt L3,2 edges provide additional crystallographic evidence for the formation of the L10 phase. [Preview Abstract] |
Friday, March 17, 2006 1:15PM - 1:27PM |
Z22.00009: FePt-Si Nanoparticles With Perpendicular Anisotropy J. Wan, Y. Zhang, Y. Huang, M. Bonder, G.C. Hadjipanayis, C. Ni, D. Weller FePt Nanoparticles are attractive for applications in magnetic recording media because of their high magnetocrystalline anisotropy. In this work, FePt-Si nanoparticles were fabricated by sputtering an FePt-Si target onto heated MgO substrates. The deposition temperature varied from 350$^{o}$ to 500$^{o}$C. The ratio of FePt to Si in the films was changed by varying the amount of Si in the target. XRD results show that the lattice parameters of L10 phase changed in the FePt-Si nanoparticles from 3.71 to 3.61 {\AA} for the c parameter and from 3.85 to 3.87 {\AA} for the a parameter. Higher amounts of Si led to a larger change in lattice parameters. The change of lattice parameters in FePt nanoparticles affected the magnetic properties as indicated by the decrease of the coercivity in the FePt-Si samples. TEM images show that the FePt/Si nanoparticles have a better size distribution than the FePt/C nanoparticles. Selected area electron diffraction patterns show that the FePt-Si nanoparticles deposited on MgO have a (001) texture due to the lattice matching between FePt and MgO. The FePt-Si nanoparticles have properties that can be tailored for applications in magnetic recording media. [Preview Abstract] |
Friday, March 17, 2006 1:27PM - 1:39PM |
Z22.00010: Magnetic force microscopy (MFM) study of FePt nanocrystals overcoated with silica Alex de Lozanne, Changbae Hyun, Doh C. Lee, Brian A. Korgel Chemically-synthesized FePt nanocrystals must be annealed at $\sim $550C to induce the hard magnetic L10 phase.~ Sintering of nanocrystal films occurs at these temperatures, resulting in the loss of control over nanocrystal size and separation in the film.~ We have developed a silica overcoating strategy to prevent nanocrystal sintering.~ In this study, 6 nm diameter FePt nanocrystals were coated with 20 nm thick shells of silica using an inverse micelle process.~ Magnetization measurements of the annealed FePt@SiO2 nanocrystals indicate ferromagnetism at room temperature.~ The micromagnetic properties of thin films of these nanocrystals were studied using MFM. [Preview Abstract] |
Friday, March 17, 2006 1:39PM - 1:51PM |
Z22.00011: Enhancement of $L$1$_{0 }$Order in FePt Thin Films and X-ray Rapid Thermal Annealing (XRTA) Rosa Alejandra Lukaszew, Jonathan Skuza, Eric Dufresne, Codrin Cionca, Alfonso Cebollada, Cesar Clavero, Cora Lind Highly ordered $L$1$_{0}$ FePt thin films and nano-structures are important for magneto-recording applications because this ordered phase exhibits very large magnetic anisotropy. One possibility to achieve high degree of chemical order in epitaxial, but somewhat chemically disordered films is to perform annealing treatments. One variation of such treatments is rapid thermal annealing (RTA). Here we describe an innovative application of x-ray undulator radiation to simultaneously perform RTA and probe structural changes that occur \textit{during} annealing. In our studies, we have used XRTA to enhance chemical order in epitaxial (001) FePt thin films. We observed the enhancement of the \textit{fcc-fct} transition with 30 ms temporal resolution in Bragg geometry. The results demonstrate that undulator radiation offers unique possibilities for materials processing, particularly the ability to use the \textit{same }beam for heating and probing. [Preview Abstract] |
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