Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W31: Focus Session: Magnetic Nanoparticles and Nanowires |
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Sponsoring Units: DMP GMAG Chair: Amanda Petford-Long, Argonne National Laboratory Room: 335 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W31.00001: Comparative studies of Co nanowires of different diameters electroplated into porous aluminum oxide membranes Zuxin Ye, Haidong Liu, Zhiping Luo, Han-Gil Lee, Wenhao Wu, D. G. Naugle, I. Lyuksyutov The correlation between the structural and magnetic properties of template-electroplated Co nanowires has been investigated. Co nanowires of diameters either 65 or 200 nm were fabricated by electroplating Co into the pores of anodic aluminum oxide membranes. Strikingly different structures were observed in these two types of Co nanowires. The 65 nm-thick Co nanowires are composed of long Co single crystal segments with a hexagonal close-packed major phase, while the 200 nm-thick Co nanowires are composed of hexagonal close-packed and face center cubic Co single crystal segments. Correspondingly, different magnetic properties were revealed in these two types of Co nanowires. The 65 nm-thick Co nanowires have a magnetic hysteresis that is significantly larger than that of the 200 nm-thick Co nanowires. Spontaneous nanowire magnetic moments are parallel to the nanowires in the 65 nm-thick Co nanowires but are transverse to the nanowires in the 200 nm-thick Co nanowires, as observed by the magnetic force microscopy. The correlation between their different magnetic properties and microstructures is discussed. This work was supported by DOE No. DE-FG02-07ER46450, NSF No. DMR-0606529, and the Robert A. Welch Foundation A-0514. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W31.00002: Self-Assembled Superparamagnetic Binary Nanoparticle Superlattices J. Chen, X. Ye, Y. Zhang, J.M. Kikkawa, C.B. Murray We report binary nanoparticle superlattices (BNSLs) composed of two different types of superparamagnetic nanoparticles (NPs). Since the magnetic properties of these NPs depend both on size and composition, two strategies are used to form BNSLs. First, we use different sizes of the same material (e.g.-10.5 nm and 5.6 nm diameter Fe$_{3}$O$_{4}$ NPs). Second, we use different materials, such as 14.2 nm Fe$_{3}$O$_{4}$ NPs and 6 nm FePt NPs, or 14.2 nm Fe$_{3}$O$_{4}$ NPs and 7.1 nm CoPt$_{3}$ NPs. We observe the formation of large scale BNSLs (up to several $\mu $m) due to the high uniformity of these nanoparticles. Using a serial tilting capability of our TEM tomography holder we confirm that the BNSLs are icosohedral NaZn$_{13}$ and AlB$_{2}$ type structures, which are thermodynamically stable due to their high packing density. We further measured the magnetic properties of these BNSLs samples, and single component samples, by SQUID magnetometry. Dipolar and/or exchange coupling between two components is studied. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W31.00003: Magnetic domain formation in monolayer nanoparticle films Brian Maranville, Kathryn Krycka, Julie Borchers, Charles Hogg, Sara Majetich, Yumi Ijiri Self-assembled magnetic nanoparticle films offer promise as data storage media, but an understanding of the interactions is missing. Modified Langmuir-Blodgett methods were used to prepare monolayer films of 7 and 11 nm diameter Fe$_3$O$_4$ nanoparticles with large structural domains. Small-angle neutron scattering (SANS) shows a peak at a wavevector $Q$ corresponding to the particle size and spacing, and scattering at intermediate $Q$ indicating possible long-range correlations. We extend to lower $Q$ with off-specular neutron reflectivity, achieving high intensity by sacrificing resolution along one in-plane direction $y$ while retaining high resolution in the other in-plane direction $x$ and the normal direction $z$. We measure in saturation and zero field to extract magnetic scattering. In high fields, the specular scattering ($Q_x=0$) is increased, consistent with aligned moments. Preliminary results show weak magnetic scattering for nonzero $Q_x$ . Since the maximal $Q_x$ roughly corresponds to the lowest $Q$ in SANS, the combination of these techniques allows us to quantify field-dependent magnetic domain size. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W31.00004: Finite size effects and long wavelength magnetic structures in Mn$_{3}$O$_{4}$ nanoparticles R. Regmi, R. Tackett, G. Lawes Mn$_{3}$O$_{4}$ (Hausmannite) having normal spinel structure with Mn$^{2+}$ ion at tetrahedral A site and Mn$^{3+}$ ion at octahedral B site orders ferrimagnetically to Yafet-Kittel phase at 42K. The interplay between the different magnetic ions leads to additional magnetic transitions in bulk, including incommensurate and commensurate phases developing at 40K and 34K respectively. We have investigated the magnetic properties of Mn$_{3}$O$_{4}$ nanoparticles through both thermodynamic and magnetic studies. Both of these measurements observe only a single magnetic transition at 42K; the transitions at 40K and 34K appear to be completely suppressed. We motivate this suppression by comparing the long wavelength of the magnetic structure in the lower temperature phases with the particle size. These nanoparticles also exhibited superparamagnetic blocking near 40K and frequency dependent magnetic loss at 30K, which we attribute to surface spin effects. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W31.00005: Fresnel Lorentz Microscopy Imaging of Domains in Fe3O4 Nanoparticle Arrays S. A. Majetich, E. R. Evarts, C. Hogg, K. Yamamoto, T. Hirayama Fresnel Lorentz microscopy was used to study the magnetic domain structures of self-assembled nanoparticle arrays as a function of temperature, from 24 to 605 \r{ }C. 11 nm diameter Fe3O4 nanoparticles with an edge-to-edge spacing of 2.5 nm form magnetic domains through magnetostatic interactions alone. At room temperature stripe domains were evident in monolayer arrays. The average domain size in monolayer regions is larger than that in bilayers. Mean field theories predict a reduced stabilization energy for bilayers, relative to that for monolayers. The domain wall positions were fairly stable up to 500 \r{ }C, though the contrast in the walls diminished, indicating reduced magnetic order. Above 500 \r{ }C there were large temperature-dependent changes. The walls surrounding the smaller domains disappeared at lower temperatures than those of the larger domains. Some magnetic contrast was visible up to 575 \r{ }C, close to the Curie temperature of Fe3O4 (585 \r{ }C). Transmission electron microscopy after cooling showed that the particle shape and position in the ordered arrays had been preserved during the high temperature imaging experiments. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W31.00006: Magnetic imaging of individual nanomagnets B. Kalisky, J. R. Kirtley, L. Qian, N. Koshnick, M. E. Huber, K. A. Moler \newline Characterization of nanomagnets is usually done in ensembles, which is problematic because their magnetic properties are inherently sensitive to small variations in volume, shape and structure. Our aim is to detect and characterize \textit{individual} nanomagnets using scanning microscopy, which allows gathering statistics about the behavior of many individual particles under the same conditions. Scanning SQUID is a suitable tool for this challenge because it has sensitivity of $\sim $800 spins. We built a scanning microscope for this purpose, which is intended to measure the nanomagnets up to their superparamagetic state while keeping the SQUID superconducting. We will present on our preliminary efforts to measure FePt particles. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W31.00007: FePt nanoparticles as high resolution magnetic force microscope (MFM) probes Lisa Qian, Jaemin Kim, John Kirtley, Beena Kalisky, Shouheng Sun, Kathryn Moler Current MFM probes are often fabricated by sputtering a magnetic thin film across the entirety of an atomic force microscope (AFM) cantilever, limiting their spatial imaging resolution to about 30nm. We report our progress on improving this resolution by using single crystal, high-coercivity ferromagnetic FePt nanoparticles as magnetic sensors for MFM. By attaching nanomagnets 5-10 nm in diameter to the end of a functionalized AFM tip, this technique has potential for an image resolution of under 10nm. We are attempting to characterize the magnetic properties of a single nanomagnet using a novel scanning SQUID susceptometer capable of raising the sample temperature well above the SQUID temperature, with a SQUID pickup loop diameter and sensor-sample spacing well below a micron. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W31.00008: Mechanical Measurement of Magnetization Reversal in a Single Iron Filled Carbon Nanotube Palash Banerjee, M. Herman, K.C. Fong, D.V. Pelekhov, Yu. Obukhov, P. Chris Hammel, F. Wolny, U. Weissker, T. M\"{u}hl, A. Leonhardt, Bernd B\"{u}chner The hysteresis loop and switching behavior of an {\em individual} Fe-filled carbon nanotube (FeCNT) has been measured at low temperatures using cantilever magnetometry. From the magnetometry data, we are able to extract the total moment of the nanotube and the effective anisotropy field arising from the extreme aspect ratio of the nanotube (length $\sim 13~\mu$m, diameter $\sim$ 25 nm). We find the magnetization reversal in the FeCNT occurs at a well defined switching field $H_{sw}$ and in a single step. These switching fields ($H_{sw}$ = 2245 G at 4.2 K) are characterized by a narrow distribution ($\sigma_{sw} \leq$ 1 G) and their measured temperature dependence is consistent with a thermally activated process of magnetization reversal. This work was supported by the NSF Materials World Network grant DMR-0807093 and a NSF I2CAM Grant DMR-0645461. P.B. acknowledges support of the ICAM Branches Cost Sharing Fund for a postdoctoral fellowship. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W31.00009: Nanowire-Based Magnetorheological Elastomers Richard C. Bell, Jacob L. Planinsek, Joseph A. Filer II, Hyun J. Song, Norman M. Wereley Magnetorheological elastomers (MREs) are composite materials consisting of ferromagnetic particles aligned within an elastomer matrix. The stiffness of the elastomer can be controlled by varying the magnitude of an applied magnetic field. In this study, we present the static and dynamic characteristics of nanowire-based MREs and compare their response to those containing conventional particles. The MRE samples were fabricated using various ferromagnetic materials (iron, cobalt, and nickel) and particle loadings in a silicone rubber matrix and their characteristics evaluated using a material test machine. The static and dynamic properties of the MREs were evaluated under a compressive load for the various compositions. The equivalent damping coefficient of the MRE samples was measured and compared under various magnetic field intensities. The dynamic characteristics, including the dynamic stiffness and loss factor, were measured under sinusoidal excitation in the frequency domain. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W31.00010: Functional nanocomposite polymer films with uniform magnetic nanoparticle dispersions K. Stojak, S. Pal, M.J. Miner, H. Srikanth, S. Skidmore, J. Wang, T. Weller Magnetic nanoparticles embedded in polymer matrices are good examples of functional nanostructures with excellent potential in applications such as tunable microwave devices, EMI shielding, and flexible electronics. The challenge comes with evenly dispersing the nanoparticles once they are embedded in the polymer matrix. To avoid clustering of particles in the polymer nanocomposites and achieve excellent dispersion, competition between polymer-polymer and polymer-particle interactions must be balanced. In earlier work, we demonstrated the synthesis of 2$\mu $m thick, spin-coated nanocomposite PMMA films with Fe$_{3}$O$_{4}$ (mean size 15nm) nanoparticles embedded that displayed superparamagnetic behavior. In this work we will report on the successful extension of this strategy to 20 $\mu $m thick films that are needed for microwave applications. In addition to Fe$_{3}$O$_{4}$, we have also functionalized the films with other ferrite nanoparticles. Magnetic characterization and microstructural studies of the polymer nanocomposites will be presented and discussed. Microwave response of these films using a coplanar waveguide fixture will also be reported. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W31.00011: Effects of varying surfactant chain lengths on the magnetic, optical and hyperthermia properties of ferrofluids Corneliu Rablau, Prem Vaishnava, Rajesh Regmi, Chandran Sudakar, Correy Black, Gavin Lawes, Ratna Naik, Melissa Lavoie, David Kahn We report studies of the structural, magnetic, magneto-thermal and magneto-optic properties of dextran, oleic acid, lauric acid and myristic acid surfacted Fe$_{3}$O$_{4}$ nanoparticles of hydrodynamic sizes ranging from 32 nm to 92 nm. All the samples showed saturation magnetization of $\sim $50 emu/g, significantly smaller than the bulk value for Fe$_{3}$O$_{4}$, together with superparamagnetic behavior. The ac magnetization measurements on the dextran coated nanoparticles showed frequency dependent blocking temperature, consistent with superparamgnetic blocking. The ferrofluid heating rates in a 250 Gauss, 100 kHz ac magnetic field varied with the chain lengths of the surfactants, with higher heating rates for longer chains. DC-magnetic-field-induced light scattering patterns produced by two orthogonal He-Ne laser beams passing through the ferrofluid sample revealed different optical signatures for different surfactants. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W31.00012: Metallic Iron Nanoparticles for MRI Contrast Enhancement Hafsa Khurshid, Michael Bonder, Srinivasan Balakrishnan, Costas Hadjipanayis, George Hadjipanayis This study is focused on our chemically synthesized iron nanoparticles, coated with carboxyl-methyl terminated polyethylene glycol to make them biocompatible and water dispersible. The particles have an average size of 14 nm and a magnetization of 110 emu/g. TEM studies revealed their core shell structure with iron in the core and iron oxide in the shell. The effects of these nanoparticles on MRI contrast enhancement were studied in vitro using a clinical MRI scanner at a magnetic field of 1.5 T. Both the r$_{2}$ (1/T$_{2})$ and r$_{2}^{\ast }$(1/T$_{2}^{\ast })$ were found to be significantly higher than those of iron oxide nanoparticles with a similar size. This behavior is attributed to their stronger magnetic susceptibility, leading to spin dephasing and shortening of T2 effects and thus darkening of the MRI contrast. These results suggested that the iron nanoparticles are expected to be more useful for MRI contrast enhancement and other biomedical applications than the currently used iron oxide nanoparticles. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W31.00013: RIE-based Pattern Transfer Using Nanoparticle Arrays as Etch Masks Chip Hogg, Sara A. Majetich, James A. Bain Nanomasking is used to transfer the pattern of a self-assembled array of nanoparticles into an underlying thin film, for potential use as bit-patterned media. We have used this process to investigate the limits of pattern transfer, as a function of gap size in the pattern. Reactive Ion Etching (RIE) is our chosen process, since the gaseous reaction products and high chemical selectivity are ideal features for etching very small gaps. Interstitial surfactant is removed with an O$_2$ plasma, allowing the etchants to penetrate between the particles. Their pattern is transferred into an intermediate SiO$_2$ mask using a CH$_4$-based RIE. This patterned SiO$_2$ layer is finally used as a mask for the MeOH-based RIE which patterns the magnetic film. We present cross-sectional TEM characterization of the etch profiles, as well as magnetic characterization of the film before and after patterning. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W31.00014: Magneto-Transport in quantum dot films Alexandre Pourret, Philippe Guyot-Sionnest Colloidal semiconductor quantum dots are being studied intensely because of their tunable optical properties. Quantum dot solids (e.g. films) present further interesting possibilities for designing novel materials with control of the electronic properties at the nanometer scale. While neutral CdSe or CdSe/CdS nanocrystal films are quite insulating, photo excitation or doping the film electrochemically leads to higher conductivity. In this talk I will present photoconductivity and conductivity measurements of charged CdSe and CdSe/CdS nanocrystal films at low temperature under a magnetic field. The combination of electrochemistry and spectroscopy enables the precise control and detection of electrons injected into the quantum dot films. The temperature and electric field dependent conductivity is analyzed with the variable range hopping model of Efros and Shklovskii and the magneto-conductivity is discussed in terms of a spin-blockade. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W31.00015: Surface and Size Manipulation of the Magnetic Properties of CdSe Quantum Dots. Robert Meulenberg, Jonathan Lee, Scott McCall, Louis Terminello, Tony van Buuren The appearance of magnetism in otherwise non-magnetic materials has recently been reported for a number of nanoscale materials. Coupled with the size-dependent optical and electronic properties of the nanocrystalline materials, this magnetic behavior opens the possibility for an extended range of technological applications. As such, identifying the origin of the magnetism is an extremely important goal, yet this remains the subject of some controversy in the literature. We report evidence that paramagnetism in CdSe QDs can be induced via manipulation of the particle size and surface ligands. Using SQUID magnetometry and x-ray absorption spectroscopy, we demonstrate that the paramagnetic behavior of the CdSe QDs can be varied by changing the ligand endgroup functionality of the passivating layer. Contrary to previous reports, no evidence for ferromagnetism was observed. [Preview Abstract] |
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