Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G22: Focus Session: Magnetic Nanoparticles I |
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Sponsoring Units: GMAG DMP Chair: Hariharan Srikanth, University of South Florida Room: Baltimore Convention Center 319 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G22.00001: Monte Carlo study of transverse susceptibility in ordered arrays of magnetic nanoparticles Dimitris Kechrakos, Kalliopi Trohidou We present Monte Carlo simulations of the field-dependent reversible transverse susceptibility (RTS) for a hexagonal array of dipolar interacting magnetic nanoparticles with random anisotropy. The thermal evolution of the RTS curves exhibits three distinct temperatures that indicate the merge of the coercive to the anisotropy peak (T$_{sw})$, the crossover from double-peak to single-peak behavior (T$_{cross})$ and the transition to the superparamagnetic state (T$_{b})$, successively. Above T$_{cross}$ the array exhibits positive spin correlations attributed solely to dipolar interactions. With decreasing interparticle spacing, the anisotropy peak shifts to lower (or higher) values for in-plane (or off-plane) bias-field and the coercive peak merges to the anisotropy peak indicating a transition to collective reversal of the moments. Our results are in agreement with recent RTS measurements in self-assembled Fe nanoparticles. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G22.00002: Railroading cells, microfluidic magnetic cell separation using ferromagnetic stripes David Inglis, Robert Austin, James Sturm Magnetic cell separation has been demonstrated using recessed ferromagnetic stripes. Here we demonstrate precise control of the cell's positions using branched magnetic stripes in a microfluidic device. The technique allows magnetically separated cells to be delivered in precise locations to downstream cellular analysis components. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G22.00003: Directed self-assembly and detection of iron oxide nanoparticles on an InAs quantum well micro-Hall sensor P. Manandhar, G. Mihajlovic, W. Setyawan, S. von Molnar, P. Xiong, S. Hong, D. Magana, G. F. Strouse, K. Ohtani, H. Ohno, M. Field, G. J. Sullivan Biosensors based on magnetic detection of superparamagnetic nanoparticles have potential in many bioanalysis and biomedical applications. An important step towards this goal is to assemble magnetic nanoparticles precisely on a sensing device and detect them. Here we demonstrate directed self-assembly of superparamagnetic iron oxide nanoparticles onto a micron or sub-micron sized semiconductor Hall sensor and their detection at room temperature using Hall magnetometry. Hall devices were fabricated from MBE grown InAs quantum well heterostructures using lithographical methods. Organic molecular templates were created on Au coated active Hall cross regions using dip pen nanolithography (DPN) with 16- mercaptohexadecanoic acid (MHA). Magnetic nanoparticles were then assembled specifically onto the MHA regions and characterized by detecting stray magnetic fields emanating from the nanoparticles in the presence of a magnetizing field by using phase sensitive Hall magnetometry. *This work has been supported by NSF NIRT grant ECS-0210332. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 9:12AM |
G22.00004: Biomedical Applications of Magnetic Nanoparticles and Fluids. Invited Speaker: Nanomaterials play an increasingly important role in the research, diagnosis and treatment of numerous pathologies. Biomedical applications such as drug delivery, magnetic resonance imaging and hyperthermia require magnetic nanoparticles with a large saturation magnetization that are biocompatible, form stable suspensions in water-based fluids, and can be functionalized. We use chemical synthesis and inert-gas condensation into fluids to produce biocompatible magnetic nanoparticle fluids that allow magnetic targeting of drugs and simultaneous magnetic resonance imaging. We have developed a water-dispersible oleic-acid/Pluronic/iron-oxide nanoparticle formulation that can be loaded with high doses of water-insoluble anti-cancer drugs. An external magnetic field is used to attract the nanoparticles to the treatment region and MRI is used to verify their location. A primary limitation on magnetic targeting, however, is the low moment of iron-oxide nanoparticles. Inert-gas-condensation into fluids produces iron and cobalt nanoparticles from 5-45 nm in diameter. Coating or passivation of these materials is required to prevent oxidation; however, the interaction between surface atoms and surfactant or other functionalizing molecules can greatly diminish the magnetic moment. A study of surfactant interactions with iron nanoparticles shows that the physical barrier provided by a concentric shell of polymeric surfactant offers significantly more protection against oxidation than the radial barrier formed by most linear surfactants. The talk will conclude with a brief overview of the opportunities and challenges for condensed matter and materials physicists in biomagnetic materials. This work is done in collaboration with V. Labhasetwar and T. Jain at the University of Nebraska Medical Center, and Marco Morales, Nguyen Hai, Shannon Fritz, Kishore Sreenivasan and David Schmitter at the University of Nebraska -- Lincoln. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G22.00005: Neutron diffraction studies of exchange-biased Co/CoO core-shell nanoparticles Sue Inderhees, Glenn Strycker, Meigan Aronson, Yiming Qiu, Julie Borchers, Yadong Yin We present neutron diffraction data on exchange-biased Co/CoO core-shell nanoparticles. Surfactant-coated Co particles were prepared by thermal decomposition of Co$_{2}$(CO)$_{8}$, then oxidized to form core/shell particles with a poly-crytalline CoO shell. Magnetization data display strong exchange bias below the 200K blocking temperature (T$_{B})$, well below the Neel temperature of bulk CoO (293 K). Neutron diffraction data show a broad peak at the ($\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ antiferromagnetic CoO ordering wave vector. The anti-ferromagnetic order parameter is constant below T$_{B}$, and decreases with increasing temperature above T$_{B}$. These results demonstrate that the diminished T$_{B}$ in small core/shell particles is not due to a reduction of T$_{N}$ of the antiferromagnetic shell. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G22.00006: Characterization of Epsilon-Co Nanoparticles with Thin Oxide Shells Kathryn Krycka, Chi-Chang Kao, Sara Majetich, Madhur Sachan In order to fully understand the magnetism of nanosystems it is often necessary to characterize an unavoidable thin magnetic metal-oxide shell. This is particularly challenging when the system is comprised of self-assembled nanoparticles that can neither be treated as fully ordered nor randomly distributed, as was the case for our sample which coherently close packs in regions on the micron scale. X-ray powder diffraction was used to determine that the phase of the shell was cobalt monoxide while the core remained epsilon-Co. TEM showed that the particles were spherical with an average diameter of 7 nm [1], and using small angle x-ray diffraction the nearest neighbor distance was placed at 10.3 nm. Anomalous small angle scattering taken at several energies about the Co k-edge was used to separate the contributions of the metallic cores, metal-oxide shells, and nanoparticle packing. From this experiment radial sizes were determined for both core and shell, the details of which will be presented. 1. ``Interaction effects within Langmuir Layers and three-dimensional arrays of epsilon-Co Nanoparticles'' (in press to J. Appl. Phys.) [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G22.00007: Inelastic Neutron Scattering on Exchange-Biased Co/CoO Core-Shell Nanoparticles Glenn Strycker, Sue Inderhees, Meigan Aronson, Yiming Qiu, Julie Borchers We report results of inelastic neutron scattering on exchange biased Co/CoO core-shell nanoparticles. Data were taken using time-of-flight techniques at the Disk Chopper Spectrometer (DCS) at the NIST Center for Neutron Research, which allows observation of the dynamics of magnetic spin reversal over a range of energies and length scales. Above the blocking temperature (T$_{B})$ the scattering is quasi-elastic, with an amplitude that peaks at the ($\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ anti-ferromagnetic CoO ordering wave vector. With decreasing temperature the quasi-elastic scattering narrows, consistent with the freezing of longitudinal moment fluctuations, and becomes resolution limited near T$_{B}$. Below T$_{B}$ we observe a spectrum of inelastic excitations arising from a log-normal distribution of energy barriers. We will discuss in detail the length scale and temperature dependences of these features. Work at the University of Michigan performed under the auspices of the Department of Energy. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G22.00008: Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets Marisol Alcantara Ortigoza, Talat S. Rahman, Richard A. Klemm We investigated the effect of intermolecular dipolar interactions on an ensemble of 100 3D-systems of $5\times5\times4$ nanomagnets, each with spin $S = 5$, arranged in a cubic lattice. We employed the Landau-Lifshitz-Gilbert equation to solve for the magnetization curves for several values of the damping constant, the induction sweep rate, the lattice constant, the temperature, and the magnetic anisotropy. The dependencies of the magnetic hysteresis curves on these parameters will be presented. We also reproduce and test a previously reported magnetization curve for a 2D-system [M. Kayali and W. Saslow, Phys. Rev. B {\bf 70}, 174404 (2004)]. Although in 3D systems, dipole-dipole interactions generally diminish the hysteresis, in two- dimensional systems, they strongly enhance it. For both square two- dimensional and rectangular three-dimensional lattices with ${\boldmath B}||(\hat{\boldmath x}+\hat{\boldmath y})$, dipole-dipole interactions can cause large jumps in the magnetization. New results including the low-temperature quantum effects appropriate for single molecule magnets will be presented. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G22.00009: Synthesis and Characterization of Magnetic Alloy Nanowire Arrays M. Padi, S. Talapatra, X. Tang, T. Kim, R. Vajtai, G.V.S. Sastry, M. Shima, P. Ajayan We report the synthesis and characterization of ordered arrays of cobalt-nickel alloy nanowires electrodeposited into the pores of anodic alumina templates (AAO). Controlled diameters, lengths and compositions of these alloys were obtained by varying the pore sizes and electro-deposition conditions. The structural and magnetic properties of the samples were investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). We also present the effect of the compositional ratio of cobalt and nickel on the magnetic properties of the nanowires. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G22.00010: CoPt Nanowires by Electrodeposition Hafsa Khurshid, Y. Huang, G.C. Hadjipanayis CoPt nanoparticles with L1$_{0 }$structure have been studied as a promising material for high density magnetic recording due to the high anisotropy value of the L1$_{0}$ phase (K=2x10$^{7}$ ergs/cc). In this study, we prepared CoPt nanowires (up to 200nm in diameter and 100 micrometer in length) embedded in anodized alumina templates by electrodeposition. The pH values have been varied from 3 to 6 by adding diluted NaOH into the deposition bath. The wires were then annealed in a high vacuum furnace at a temperature range between 650$^{o}$C to 750\r{ }C in order to transform their structure from the disordered fcc phase to the ordered L1$_{0}$ phase. A higher pH value leads to a higher coercivity and smoother loop which can be attributed to a more uniform CoPt composition through out the wires. HRTEM data of as-made samples shows that the long wires consist of both tiny nanoparticles (1-2 nm) and elongated nanorods having a single crystal fcc structure. The microstructural data is consistent with the magnetic measurements, showing a slightly preferred orientation perpendicular to the long axis of the wires with H$_{c }$= 5 kOe and 5.5 kOe along and perpendicular to the direction of the wires, respectively. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G22.00011: Ferromagnetism in colloidal Mn doped ZnO nanocrystals Gil Markovich, Tal Meron, Einat Tirosh, Gabriel Shemer Surfactant coated colloidal Zn$_{1-x}$Mn$_{x}$O (x=0.04$\pm $0.03) nanocrystals of average diameter of 5.5 nm were synthesized using high temperature hydrolysis of Zn(II) and Mn(II) alkoxides in a high boiling point solvent. The magnetic properties of the nanocrystals were measured both for isolated particles diluted in a hydrocarbon matrix and for a nanocrystal powder. Nanocrystals of manganese oxide and ZnO coated with manganese oxide were prepared for comparison to the Zn$_{1-x}$Mn$_{x}$O nanocrystals. We find that the manganese ions primarily substitute zinc ions in the hexagonal ZnO lattice and part of them are ferromagnetically coupled up to room temperature even in isolated non-interacting nanocrystals. The rest of the ions were magnetically disordered or uncoupled. Surprisingly, these small Zn$_{1-x}$Mn$_{x}$O nanocrystals poses relatively large low-temperature magnetic coercivity and relatively high blocking temperature in the isolated form, which indicate large magnetic anisotropy. In the nanocrystal powder the coercive field decreased significantly. This study highlights the advantages of working with non-interacting single domain particles of these intriguing materials. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G22.00012: Superparamagnetic resonance in antiferromagnetic nanoparticles: systematic features in the temperature dependence Prasanta Dutta, Mohindar Seehra Antiferromagnetic (AF) nanoparticles (NP) of dimensions $\approx $ 5 nm usually acquire significant magnetic moment even below T$_{N}$ due to uncompensated surface spins. This moment follows superparamagnetism (SPM) above the blocking temperature T$_{B} \quad <$ T$_{N}$. In this work, we will discuss some unique features in the temperature dependence of the electron magnetic resonance (EMR) spectra which have been reported in NP of ferrihydrite, NiO and $\gamma $Fe$_{2}$O$_{3}$ [1]. As T is lowered towards T$_{B}$ the resonance field H$_{r}$ decreases and the linewidth $\Delta $H increases so that $\delta $H$_{r} \quad \approx $ ($\Delta $H)$^{n}$ with n $\approx $ 3 is observed. This is in line with the variation expected from the demagnetizing fields of non-spherical particles [2]. From the temperature variation of the line intensity I = ($\Delta $H)$^{2}$ h (h = peak-to-peak height), the location of T$_{B}$(m) and T$_{B}$(EMR) has been obtained [1]. For T $<$ T$_{B}$(m), as H$_{r}$ approaches zero, the EMR line becomes highly asymmetrical, which is explained from the combined contributions of resonance at +H$_{r}$ and -H$_{r}$. [1]. Seehra et al, IEEE Trans. Magn; \underline {37}, 2207 (2001); Seehra et al, J. Appl. Phys. \underline {97}, 10J609 (2005); Dutta et al, Phys. Rev. B \underline {70}, 174428 (2004). [2]. Nagata {\&} Ishihara, J. Magn. Magn. Mater. \underline {104-107}, 1571(1992). [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G22.00013: Competition between Thermoinduced Magnetization and Uncompensated Spins Gregory Brown, Markus Eisenbach, G. M. Stocks Thermoinduced magnetization (TiM) is the ferromagnetic response predicted for nanoparticles of normally antiferromagnetic materials. Unambiguous experimental observation of this phenomenon is complicated by the effects of the particle size-distribution within a sample and the uncompensated magnetic moments within a nanoparticle. Monte Carlo calculations of nanoparticles with odd numbers of spins have been employed to resolve the competition between TiM and uncompensated spins. The magnitude of the ferromagnetic response, $\langle|M|^2\rangle$, is easily resolved into the two phenomena. Analysis for the response along the direction of crystalline anisotropy, $\langle M_z^2\rangle$, is complicated by the temperature-dependent relaxation of individual spins away from the anisotropy axis. These results indicate that TiM may be confirmable in nanoparticles with uncompensated spins. However, quantitative estimates of the temperature- and anisotropy-dependence of TiM are significantly affected by uncompensated moments. [Preview Abstract] |
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