Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures II |
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Sponsoring Units: DMP GMAG Chair: David Pappas Room: LACC 150B |
Monday, March 21, 2005 2:30PM - 3:06PM |
D42.00001: Self-Assembled Nanomagnets Invited Speaker: The fabrication of high density and high performance magnetic nanodevices requires arrays of nanomagnets with controlled magnetics. Self-assembly of magnetic nanoparticles offers a versatile process for the formation of nanomagnetic arrays. It uses pre-formed monodisperse magnetic nanoparticles as building blocks that self-organize into 2D and 3D superlattices. Because nanoparticles can be synthesized with controlled dimension and surface coating, their self-assembly could yield a nanomagnet array with characteristic dimensions much smaller than those from the lithographic approaches. In this talk, I will outline our recent efforts in producing nanomagnet arrays using self-assembly. We apply different particle surface coatings or thermal annealing to control interparticle spacing and interface diffusion, and use particle shapes to control crystal orientation of each particle in a self-assembled superlattice. The Co or Fe$_{3}$O$_{4}$ nanoparticle arrays show Coulomb blockade and MR effects, while ferromagnetic FePt nanoparticle and FePt-Fe$_{3}$Pt nanocomposite assemblies are of interest for ultra-high density information storage and high performance permanent magnetic applications. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D42.00002: Direct synthesis of ordered L10 FePt nanoparticles in the gas phase Jian-Ping Wang, Jiao-Ming Qiu, Jianmin Bai Uniform L1$_{0}$ FePt nanoparticle is one of the candidates for future extremely high magnetic recording media. Chemical methods of fabricating FePt nanoparticles require post-annealing process that usually leads to particle agglomeration$^{1}$. We have developed a controllable approach to fabricate ordered FePt nanoparticles with uniform size and free of particle agglomeration based on nanocluster deposition technique$^{2}$. In the approach, FePt nanoparticles were generated through gas-phase aggregation using magnetron sputtering at high argon pressure. Differential pressure forces drove the particles flying through an on-line infrared heater where particles transform from disordered A1 phase into ordered L1$_{0}$ phase. Particle nucleation, growth and ordering happened at separated sequential stages in vacuum. FePt nanoparticle size can be controlled by adjusting various deposition parameters including sputtering power density, argon pressure, aggregation length, etc. Without further treatment, FePt nanoparticles with on-line heating showed high anisotropy that verified the direct deposition of the L1$_{0}$ phase particles. References: \begin{enumerate} \item Z. R. Dai, S. Sun, and Z. L. Wang, Nano Lett. \textbf{1}, 443 (2001) \item H. Haberland, M. Karrais, M. Mall, Y. Thurner, J. Vac. Sci. Technol. A \textbf{10}, 3266 (1992) \end{enumerate} [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D42.00003: Monte Carlo simulation of equilibrium L1$_{0}$ ordering in FePt nanoparticles Roman Chepulskyy, William Butler Mixing potentials for FePt alloys were calculated from first principles. Using the mixing potentials obtained in this manner, the dependency of equilibrium L1$_{0}$ ordering on temperature and concentration was studied for bulk and for spherical nanoparticles of different sizes by use of Monte Carlo simulation and the analytical ring approximation. For nanoparticles of finite size, the (long range) order parameter changed continuously from unity to zero with increasing temperature. Rather than a discontinuity indicative of a phase transition we obtained an inflection point in the order as a function of temperature. This inflection point occurred at a temperature below the bulk phase transition temperature and which decreased as the particle size decreased. Our calculations predict, for example, that 3.5nm diameter particles in configurational equilibrium at 600\r{ }C (a typical annealing temperature for promoting L1$_{0}$ ordering) have an L1$_{0}$ order parameter of 0.83 (compared to a maximum possible value equal to unity). According to our investigations, the experimental absence of (relatively) high L1$_{0}$ order in small nanoparticles (2-6 nm in diameter) annealed around 600\r{ }C or below is primarily a problem of kinetics rather than equilibrium. As FePt nanoparticle size is reduced, the maximum of equilibrium order parameter shifts from equiatomic concentration toward higher iron concentration. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D42.00004: Annealing effects of chemically synthesized FePt nanocrystal films Changbae Hyun, Casey Israel, Alex de Lozanne, Doh C. Lee, Brian A. Korgel Chemically synthesized FePt nanocrystals can exhibit room temperature ferromagnetism after being annealed at temperatures above $\sim$500$^{\circ}$C[1]. The thermal annealing changes the crystal structure from face-centered cubic to the hard magnetic face-centered tetragonal phase. In thick nanocrystal films, the coercivity can be quite large, however, the coercivity of thin films has been found to decrease significantly with decreasing thickness, even losing the room temperature ferromagnetism in some cases[2]. In order to help determine how the microscopic magnetic structure in these thin films evolves with film thickness, we studied using magnetic force microscopy (MFM), under external applied fields, films consisting of 4 to 15 nanocrystal monolayers. We cast smooth films of 4 nm diameter FePt nanocrystals and annealed them at temperatures ranging from 400 to 650$^{\circ}$C, acquiring MFM images as a function of annealing temperature. Thin FePt films showed lower coercivity than thick films. To help interpret the MFM images, complementary magnetic and structural data was obtained using SQUID magnetometry, x-ray diffraction, and transmission electron microscopy (TEM). [1] S. Sun et al., Science 287, 1989 (2000). [2] G. A. Held et al., Journal of Applied Physics 95, 1481 (2004) [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D42.00005: Template-mediated self-assembly of ordered magnetic nanoarrays Yucheng Sui, Wei Liu, Jian Zhou, Lanping Yue, Ralph Skomski, David Sellmyer Creation of magnetic nanostrctures is a very important research topic in nanoscience and nanotechnology. Among the popular bottom-up methods, self-assembly of magnetic nanostructures by chemical synthesis is favorable over others because it represents a low-cost and highly effective approach [1]. In this study, a novel technique template-mediated self-assembly is employed, that is the manipulation of magnetic clusters through both an external magnetic field and an ordered alumina template in order to fabricate ordered magnetic patterns with anisotropic properties [2]. This experiment consists of three parts. First, the synthesis and selection of FePt L1$_{0}$ clusters by hydrogen reduction and their capping by surfactants. Second, the fabrication of ordered alumina template by two-step anodization. Third, the insertion and assembly of clusters in nanopores under external magnetic field. Ordered magnetic dots were created with a coercivity of 13.4 kOe. The interactions between the scanning magnetic tips of MFM and the dots array were studied. This research is supported by DOE, NSF-MRSEC, W.M. Keck Foundation, ARO, and CMRA. 1. Y.C. Sui, R. Skomski, K. D. Sorge, and D. J. Sellmyer. Appl. Phys. Lett. 84, 1525 (2004). 2. Y.C. Sui, W. Liu, L. Yue, X.Z. Li, J. Zhou, R. Skomski and D. J. Sellmyer, J. Appl. Phys. (in press). [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D42.00006: Preparation and orientation of ordered FePt:Ag magnetic nanostructures M. L. Yan, Y. F. Xu, D. J. Sellmyer Magnetic nanostructures are scientifically interesting because of the unique physical characteristics. To take advantage of these properties in applications, it is important to be able to control the properties of particles. For example, in the applications of high-density perpendicular recording media, it is important for films to have decoupled or weakly-interacting, uniformly-sized grains with diameters less than 10 nm as well as perfect perpendicular orientation. The fabrication of magnetic nanostructures with these desirable properties is a major challenge. In this talk, we will present research on fabrication and orientation of FePt:Ag nanostructures. The requirement of narrow size distribution can be met by creating particles with a gas-aggregation cluster deposition. Crystallographic texturing of nanostructures can readily be done by multilayer sputter deposition plus annealing. We discuss preparation, formation of the nanostructures, and mechanism and dynamics of magnetization reversal for FePt:Ag nanostructures with narrow cluster-size distribution. Results on effects of Fe composition, Ag content, total film thickness on ordering and orientation will be reported for oriented FePt:Ag nanostructures. In addition, the lattice parameters, coherence length and exchange interactions will be reported for these FePt:Ag nanostructures. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D42.00007: Surfactant Effects on the Growth of Magnetic Nanocrystals Anna C.S. Samia, John A. Schlueter, J. Samuel Jiang, Samuel D. Bader, Xiao-Min Lin Understanding the role of surface ligands in regulating nanocrystal growth is an important step towards developing synthetic routes for the fabrication of novel nanomagnets. Here we report the surfactant effects on the particle size and growth of FePt and Co nanocrystals. Different concentrations of oleic acid (OA) ligand present during the cobalt carbonyl decomposition result in either the formation of large ferromagnetic nanocrystals or small cluster complexes. More dramatically, by adding or removing free oleic acid ligand from the final product of the carbonyl decomposition, we can turn a large nanocrystal colloid into a cluster complex solution and vice versa. The use of oleic acid as capping material in the organometallic synthesis of FePt leads to the formation of monodispersed 5 nm nanocrystals. On the other hand, changing the surfactant to TOPO results to a bimodal size distribution of large (14 nm) and small (2 nm) FePt nanocrystals. The blocking temperature of the TOPO-capped sample has three times higher blocking temperature than the OA-capped nanocrystals. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D42.00008: Identification and characterization of silica-gold passivated iron nanoparticles Jiunn-Yuan Lin, W. C. Chen, K. H. Wei, J. Y. Tsai, J. Y. Huang, J. Y. Juang, Y. S. Gou, J. M. Lee, J. M. Chen Metal nanoparticles are of current interest owing to the technological applications such as high density storage density and biological labeling, in addition to their novel physical properties. In this paper, Identification and characterization of silica-gold passivated iron nanoparticles are reported. The diameter of the iron nanoparticles is about 3 nm, and the thickness of the gold layer is 0.5 nm. X-ray absorption spectroscopy indicates pure iron cores, in contrast to the previous reported iron oxide formation in cores. Magnetic measurements suggest a superpara- to ferromagnetic transition at low temperatures for the present nanoparticles. This work was supported by National Science Council, Taiwan ROC, under contract NSC93-2112-M-009-015. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D42.00009: Ferromagnetic gamma-Fe Nanoparticles Trapped in Carbon Nanotubes Mutsuhiro Shima, Bingqing Wei, Ranjit Pati, Saroj Nayak, Pulickel Ajayan, Saburo Nasu It is known that iron exists in different allotropic forms, where at ambient conditions the most stable phase is ferromagnetic body-centered cubic, and the face-centered cubic (fcc) phase (gamma-Fe) is thermodynamically unstable and not ferromagnetic. However, theoretical studies have predicted the existence of two different magnetic states in gamma-Fe, i.e. high-spin and low-spin state, depending on the inter-atomic distance in the fcc lattice. We have found that iron catalyst nanoparticles trapped in carbon nanotubes are indeed ferromagnetic gamma-Fe at room temperature. The unusual magnetic moment of gamma-Fe nanoparticles observed using room temperature magnetic measurements and Mossbauer spectroscopy is explained by a lattice expansion due to insertion of carbon atoms into the interstitial sites. According to the Mossbauer data analysis, there exist two different magnetic moments of iron in the gamma-Fe nanoparticles, presumably due to their atomic environments, namely whether they have a carbon atom in the nearest neighbor or not. The First principles calculations corroborate that ferromagnetism observed here is related to both lattice distortion and charge transfer between iron and carbon atoms. Detailed analysis of the structure and magnetic properties of the magnetic clusters will be discussed. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D42.00010: Fe magnetic nanoparticle synthesis by high pressure sputtering Prasanna Shah, Andrew Gavrin We have designed and developed a sputtering magnetron gun to synthesize nanoparticles. The particle gun employs a high pressure environment ($\sim $1 Torr) of argon gas that facilitates the particle formation by collision between the atoms. The particles get captured in the gas flow and they emerge out through an aperture in an inert gas atmosphere of a few millitorrs. Fe nanoparticles produced using the particle gun are characterized using X-ray diffraction, atomic force microscopy and scanning electron microscopy. Scanning electron microscopy and tapping mode AFM results confirm the formation of nanoparticles. The particle size is found to be in the range 10-50 nm; it shows a marked increase as a function of gas pressure. The X-ray diffraction measurements on these particles consist of main Fe peak along with a minor oxide phase. The average particle size measured in the AFM measurements agrees with that of the XRD results. Our goal is to deposit these nanoparticles on a magnetic film in order to study the domains of the magnetic film and hence develop a high resolution bitter microscopy (HRBM) imaging system. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D42.00011: High Magnetization Polyethylene Glycol Coated Nanoparticles M.J. Bonder, Y.H. Huang, Y. Zhang, K. Williams, G.C. Hadjipanayis, V. Papaefthymiou High magnetization nanoparticles coated with a biocompatible polymer or poly-saccharide layer are required for biomedical applications such as targeted drug delivery, MRI contrast enhancement and hyperthermia treatments. This paper discusses the fabrication and characterization of iron nanoparticles coated with carboxyl terminated polyethylene glycol for future biomedical applications. Electron microscopy reveals nanoparticles ranging from 10 to 50 nm in size that have a body centered cubic structure characteristic of alpha Fe. Mossbauer spectroscopy reveals the typical sextet expected for Fe with two different hyperfine fields reminiscent of a core shell morphology indicating that there are two distinct Fe sites. The coated nanoparticles are soft ferromagnets with coercivity below 100 Oe and a saturation magnetization of 50 emu/g as shown by SQUID and vibrating sample magnetometry. Optimization studies are underway and the results will be reported. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D42.00012: Self-Assembled Single Crystal Ferromagnetic Fe and Co Nanowires Formed by Decomposition L. Mohaddes-Ardabili, H. Zheng, S.B. Ogale, F. Zavaliche, L. Salamanca-Riba, D.G. Schlom, R. Ramesh A novel approach to create self-assembled ferromagnetic nanostructures for new magnetic recording media with high storage capacity is reported, which involves spontaneous phase decomposition of a single-phase perovskite oxide during film growth. We are exploring the stability in a nominally single phase LaSrTMO$_{3}$ (TM = Fe, Co, Mn) system as a function of oxygen pressure, using~thin film heteroepitaxy as the processing route.~ We find that the film microstructure depends systematically on the oxygen pressure during deposition. Deposition under reducing environments, leads to the formation of self-assembled arrays of nanowires. In the case of Fe system the deposition under reducing conditions leads to spontaneous formation of an array of single crystalline ferromagnetic \textit{$\alpha $}-Fe nanowires embedded in an antiferromagnetic matrix with nominal compostion of LaSrFeO$_{4}$. The diameter of both \textit{$\alpha $}-Fe and Co nanowires is controlled by growth temperature and the height is controlled by film thickness. The magnetic properties of these nanowires are both dependant on the average diameter and also the height of nanowires. The large remanence and sizable coercivity of the nanowires make them desirable for high-density data storage and other magnetic device applications. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D42.00013: Fabrication of Uniform Magnetic Nanowire Arrays W.J. Yeh, Kun Yang Nickel nanowire arrays with high aspect ratio and large packing densities have been grown in thin nanochannel glass template by an electrochemical deposition method. The template initially was polished and etched to obtain parallel, uniform, hollow channels. One of surfaces of a template was then coated with a copper film layer of around 150 nm in thickness to provide an electrode to drive the electrochemical deposition. The $p$H value of NiSO4 aqueous solution was set to be 1 to 2 and the deposition potential was to be 1.2 V versus the saturated Calomel electrode. Obtained nickel wires were uniform and circular with diameter of down to 80 nm, depending on the size of nanochannel itself. The length of wires was controlled, typically about 140 um, depending on the deposition time. Finally, scanning electron microscopy (SEM) has been used to characterize the structures of nanowires. Magnetic properties of nickel nanowire arrays have been also investigated using a superconducting quantum interference device magnetometer (SQUID). In this presentation, the authors will also present some results of cobalt nanowire arrays grown by superfluid deposition. [Preview Abstract] |
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