Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y14: Focus Session: Magnetic Nanostructures II |
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Sponsoring Units: DMP GMAG Chair: Alejandra Lukaszew, University of Toledo Room: Colorado Convention Center Korbel 4E |
Friday, March 9, 2007 11:15AM - 11:51AM |
Y14.00001: InAs quantum well Hall devices for room-temperature detection of magnetic biomolecular labels Invited Speaker: The integration of micro- and nanoscale magnetics with molecular biology promises novel applications in fundamental studies of molecular interactions as well as in bioanalysis and biomedical functions. The implementation of this concept requires detection of biomolecular labels in the form of superparamagnetic micro/nano beads, ideally with single bead sensitivity. In this talk we will present our work on the development of miniaturized Hall sensors for detection of such beads. The devices, with Hall cross widths of $\sim $1 $\mu $m and $\sim $250 nm, were fabricated from InAs/AlSb quantum well semiconductor heterostructures. Their room-temperature characteristics were examined by Hall effect and electronic noise measurements. In the low frequency range, from 20 Hz to 1.6 kHz, devices have the noise-equivalent magnetic moment sensitivities of order 10$^{6}\mu _B /\sqrt {\mbox{Hz}} $ and 10$^{5}\mu _B /\sqrt {\mbox{Hz}} $ respectively. The sensitivity of the latter reaches the 10$^{4}\mu _B /\sqrt {\mbox{Hz}} $ range above $\sim $ 1 kHz. By using a phase-sensitive measurement technique and micron-sized Hall crosses we achieved detection of a single 1.2 $\mu $m diameter bead with a signal to noise ratio (S/N) of $\sim $ 33.3 dB, as well as detection of six 250 nm beads with S/N of $\sim $ 2.3 dB per bead. Our results from the micro-Hall susceptibility measurement on a single microbead can be explained quantitatively as due to the magnetic response of an ensemble of non-interacting magnetic nanoparticles with broad distribution of magnetic moments. The work demonstrates the efficacy of InAs quantum well Hall devices for applications in high sensitivity magnetic biomolecular detection. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y14.00002: Superparamagnetic bead assembly via biomolecular recognition and detection using micro Hall sensor Pradeep Manandhar, Goran Mihajlovic, Stephen von Molnar, Peng Xiong, Keita Ohtani, Hideo Ohno, Mark Field, Gerard J. Sullivan Specific binding of biological molecules onto a selectively functionalized area is a necessary key step in biological sensing with a solid state device. Here we present our results on the directed self-assembly of streptavidin coated superparamagnetic micron and submicron sized beads onto selectively biotinylated solid-state surfaces with organic molecular templates. Large-scale high-yield assembly of 130nm strepdavidin-coated beads onto biotinylated micro-patterns with little nonspecific binding is realized using molecular template generated by micro-contact printing. Specific binding of similar beads onto a semiconductor micro-Hall cross can be realized using high-spatial registry functionalization with dip-pen nanolithography, and the binding can be detected using phase sensitive Hall magnetometry with InAs quantum well micro-Hall sensors.\footnote{G. Mihajlovi{\ae} et al., APL 87, 112502 (2005).} [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y14.00003: In-vitro heating with Polyethylene Glycol Coated Magnetic Nanoparticles Srinivasan Balakrishnan, M.J. Bonder, D. Gallo, G. C. Hadjipanayis Magnetic nanoparticles synthesized with a biocompatible polymer coating are under investigation for future detection and treatment of cancer. In this study we investigate the heating characteristics of Fe based nanoparticles coated with polyethylene glycol. Structural characterization indicates a variation in the composition with polymer concentration. The x-ray analyses show that samples become increasingly amorphous as evidenced by the broad amorphous-like peak superimposed on the alpha iron (110) peak. The magnetization curves indicate that all samples are soft ferromagnets with the coercivity dependent on mean particle size. When the nanoparticle suspension is subjected to a 4Oe, 500 kHz AC magnetic field there is a steep rise in temperature reaching an equilibrium temperature for all cases. Correlating the equilibrium temperature with the static magnetic properties shows that the temperature is linearly dependent on the saturation magnetization and inversely proportional to the coercivity of the particle. Work supported by NSF DMR-0302544 . [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y14.00004: Ferromagnetic Gd$_{100-x}$Fe$_{x}$ (x = 4 - 40) Nanostructures D. Schmitter, J. Goertzen, G. Shelburne, T. M. Pekarek, J. E. Shield, P. M. Shand, D. Haskel, D. L. Leslie-Pelecky Iron in concentrations as small as 4 at. {\%} (where the distance between Fe atoms is more than three lattice spacings) produces ferromagnetic behavior at temperatures up to 50 K above the Gd Curie temperature T$_{C}$ in Gd$_{100-x}$Fe$_{x}$ nanostructures. X-ray diffraction and XAFS show that Gd$_{100-x}$Fe$_{x}$ nanostructures made by inert-gas-condensation and melt-spinning have nanoscale hcp Gd grains with Gd-Fe grain boundaries. Magnetization and XMCD measurements indicate that, above the bulk Gd T$_{C, }$Fe atoms polarize Gd atoms and produce ferromagnetic behavior with coercivities on the order of 50-100 Oe. The coercivity decreases as the temperature decreases toward the Gd T$_{C, }$which we attribute to random anisotropy averaging produced by ordering of the hcp-Gd grains. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y14.00005: Geometrically frustrated honeycomb and ladder lattices of nanoscale ferromagnetic islands Jie Li, Xianglin Ke, Ruifang Wang, William McConville, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied arrays of interacting single-domain ferromagnetic islands which are arranged on lattices such that the interactions between the islands are frustrated by the geometry of the arrays. While previous studies in our group [1] have focused on a frustrated square lattice, we now report results on lattices with the honeycomb geometry and with a topologically equivalent ladder geometry in which the islands meet in vertices of three islands. Each permalloy island measures approximately 80nm by 220nm with a thickness 25nm, and is evenly spaced with lattice spacing ranging from 225nm to 425nm for honeycomb lattice and from 320nm to 880nm for ladder lattice. Magnetic force microscopy measurements of the arrays after demagnetization demonstrate that the interactions between the islands are frustrated and that the correlations between islands decrease with increasing spacing of the islands. A detailed analysis of the correlations between the islands will be presented. This research has been supported by the Army Research Office. [1] R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J. Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Crespi, and P. Schiffer, Nature 439, 303 (2006). [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y14.00006: Field annealing study of a frustrated interacting nanomagnet array X. Ke, J. Li, W. McConville, R. Wang, C. Nisoli, P. Lammert, V. Crespi, P. Schiffer Lithographically patterned ferromagnetic nano-islands provide an ideal model to explore the physics of frustrated `spin ice' materials due to the competition of dipole interaction between elements [1]. Since the energy scales are large compared to thermal energies, field annealing is crucial to obtaining a low-energy demagnetized state among the interacting islands. We have studied various field annealing protocols to demagnetize the array by rotating the sample in a time-varying magnetic field. We find that reversing the field direction while stepping down the field magnitude is needed to successfully demagnetize the array. The annealing can also be tuned by varying the field step size, especially for field magnitudes near the coercive field of the array. The competition of dipole interaction with external field and dipole field of neighboring elements will be discussed. \newline [1] R. Wang, et al, Nature \textbf{439}, 303 (2006). [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y14.00007: Magnetic domain wall phases in perpendicularly magnetized ultrathin films. Nidal Abu-Libdeh, David Venus At low temperature, the ground state of a perpendicularly magnetized ultrathin film is the stripe domain state with an equilibrium density (n) of domain walls that respond to the applied magnetic field with an equilibrium magnetic susceptibility$_{ }(\sim $1/n). The change in domain density with temperature involves the creation/annihilation of domain walls. We propose a simple relaxation model in which the domain wall creation/annihilation is an activated process. The model predicts a non-equilibrium domain density and thus an effective susceptibility that depends on the time scale of the measurements. We have measured the ac-magnetic susceptibility of perpendicularly magnetized ultrathin Fe films on a 2 ML Ni/W(110) substrate, as a function of temperature while changing the temperature at different heating rate (R) between 0.03K/s and 1K/s. In the low temperature range, the model calculations provide a consistent explanation of the measured susceptibility. In the high temperature range, the susceptibility measured with low heating rates (0.03K/s -- 0.1K/s), deviates from the calculation due to an increase of the high temperature half-width of the susceptibility. This has been tentatively interpreted as a phase change from the stripe domain phase to the tetragonal phase in which the domains have no preferred direction. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y14.00008: Magnetization Reversal in Europium Sulfide Nanocrystals James Dickerson, Marcela Redigolo, Dmitry Koktysh, Sandra Rosenthal, Zheng Gai, Lan Gao, Jian Shen We report the observation of the reversal in the magnetization hysteresis curve of europium sulfide nanocrystals. This phenomenon was investigated through the temperature-dependent magnetization of two classes of nanomaterials, nanocrystalline (2.0 nm \textit{$\le $ }d$_{NCs}$\textit{ $\le $ }100 nm), and quantum-confined (d$_{NCs}$\textit{ $\le $ }2.0 nm), where d$_{NCs}$ is the diameter of the nanomaterial. The effect of the size of the nanomaterial on the magnetization is attributed to the competition between the magnetic properties of strained surface atoms and unstrained core atoms. Superconducting quantum interference device (SQUID) probed the magnetic response. Electron microscopy and X-ray diffraction spectroscopy revealed the crystallinity and monodispersivity of the nanomaterials. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y14.00009: Magnetism and $^{13}$C NMR relaxation of nanodiamond powder E.M. Levin, S.L. Bud'ko, X.W. Fang, W.E. Straszheim, R.W. McCallum, K. Schmidt-Rohr The magnetization, $^{13}$C NMR relaxation, and composition of commercial nanodiamonds with an average grain diameter of 4 nm have been studied. The magnetization contains several contributions due to (1) the diamagnetic effect of core and valence electrons of carbon, (2) ferromagnetic-like and (3) superparamagnetic contributions from Fe-bearing particles detected in nanodiamonds, and (4) a paramagnetic contribution from unpaired electrons. The spin concentration obtained from the paramagnetic susceptibility is 2.2$\times $10$^{20}$ spins/g. At 300 K, nanodiamond powder shows ferromagnetic magnetization of 0.01 emu/g. $^{13}$C NMR spectra and relaxation times should be unaffected by the ferromagnetic particles with so small magnetization. Thus, a reduction of $^{13}$C NMR $T_{1}$ relaxation times by orders of magnitude compared to microdiamond can be explained by unpaired electrons in the nanodiamond grains. The origins of unpaired electrons and ferromagnetism in nanodiamond powder and other carbon-based materials are discussed in view of our results. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y14.00010: Magnetic Endohedral Metallofullerenes with Floppy Interiors Meichun Qian, Shiv Khanna, Mark Knickelbein Investigations on the electronic structure and magnetic properties of a free Gd$_{3}$N and Gd$_{3}$N@C$_{80}$ have been carried out using to examine the stability and the electronic and magnetic properties of the endohedral species. Using a synergistic approach combining Stern-Gerlach experiments in beams and first principles electronic structure studies, it is demonstrated that an isolated Gd$_{3}$N has a ground state spin moment of 23 $\mu $B followed by a non-collinear state of 17.2 $\mu $B only 88 meV above the ground state. The large moment is largely due to localized f-electrons. As a Gd$_{3}$N is embedded inside a C$_{80}$ cage, the localized f-electrons maintain the magnetic character while the hybridization between the s,d states of isolated Gd$_{3}$N and p-states of C$_{80}$ leads to a strongly bound motif with an interaction energy of 13.63 eV and a large HOMO-LUMO gap of 1.48 eV. Gd$_{3}$N@C$_{80}$ is further shown to possess two isomers corresponding to the location of the N atom on either side of the Gd$_{3}$ triangle with an appreciable electric dipole moment and a low barrier of 91 meV for transition between them offering potential for a fluctuating dipole. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y14.00011: Can Carbon Be Ferromagnetic - X Rays Can Give The Answer Hendrik Ohldag, Tolek Tyliszczak, Roland H{\"o}hne, Daniel Spemann, Pablo Esquinazi, Magda Ungureneau, Tilman Butz While conventional wisdom says that magnetic materials have to contain some metallic atoms [1], the confirmation of intrinsic magnetic order in pure metal free carbon represents an ultimate and general scientific breakthrough because of the fundamental importance of carbon as an elemental building block of organic as well as inorganic matter. The common controversy raised across all disciplines is whether the magnetism of carbon is intrinsic or induced by other elements. We address this controversy by providing clear experimental evidence that metal free carbon can be ferromagnetic at room temperature using dichroism x-ray absoprtion spectro-microscopy. For this purpose we acquired x-ray microsopy images of magnetic structures on a thin carbon film that have been produced by irradiation with a focussed 2.25MeV proton beam [2]. \newline \newline [1] F. Palacio, Nature 413, p. 690 (2001) \newline [2] P. Esquinazi et al., PRL 91, p 227201 (2003) [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y14.00012: Magnetic Properies of Ni Nanoparticles Used for Carbon Nanofiber Synthesis K. D. Sorge, O. Malkina, C. Finkel, J. D. Fowlkes, P. D. Rack, K. L. Klein, A. V. Melechko, M. L. Simpson Magnetic properties of Ni catalyst particles used for vertically-aligned carbon nanofiber (VACNF) synthesis are investigated. Ni thin films are deposited on Si wafers by sputter-depositing to thicknesses of 2--10 nm. The VACNFs are then grown in a Plasma-Enhanced Chemical Vapor Deposition (PECVD) chamber with NH$_3$ and C$_2$H$_2$ at relative flow rates of 80/40 sccm, respectively, a pressure of 3 Torr, and a temperature of 700$^{\circ}$C. The catalyst particles, after nanofiber growth, are 10--150 nm in diameter. Magnetic properties are investigated by SQUID magnetometry in applied magnetic fields of $|H| < 10$ kOe and temperatures $T = 5$--300 K. The catalyst particles are ferromagnetic with low coercivity and remanence. The ferromagnetic properties are thermally stable up to room temperature in all but the smallest particle sizes. Saturation magnetization is much less than would be expected from the deposited quantity of Ni metal. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y14.00013: Moving magnetic nanoparticles through soft-hard magnetic composite system Hemachander Subramanian, Jong Han An important requirement during the design of a nano-electromechanical system is the ability to move a nanoparticle from one point to another in a predictable way. Through simulations, we demonstrate that soft-hard magnetic stuctures can help us move nanoparticles predictably. We simulated a 2-D system, in which the exchange-coupled soft-magnetic magnetization is frustrated with the boundary condition set by a hard magnetic array and rotating external field. We consider a geometry with three-fold degenerate magnetic local minima and show that the hysteretic transitions are manipulated by an external field. Due to the reduced interfacial energy from weak demagnetization energy in the composite magnets and magnetic hysteresis, the energy landscape can be manipulated in a well-defined and predictable manner. We apply this idea to control the movement of a magnetic particle placed on a non-magnetic layer on top of the structure. We are interested in extending this simple, preliminary study to include complex geometries. We expect that complex geometrical constraints would lead to interesting orbits of nanoparticles in these systems. [Preview Abstract] |
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