Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D34: Focus Session: Nanomagnetism -- Nanowires & Thermal Effects |
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Sponsoring Units: DMP GMAG Chair: Dmitri Litvinov, University of Houston Room: E144 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D34.00001: Fabrication and magnetic properties of carbon nanotubes filled with Fe$_{3}$O$_{4}$ nanoparticles S. Pal, K. Stojak, S. Chandra, M.H. Phan, P. Mukherjee, H. Srikanth Carbon nanotubes (CNTs) filled with magnetic nanoparticles (NPs) are interesting systems for potential applications in electromagnetic sensing and nanomedicine. We report on the synthesis, structural and magnetic properties of hollow, straw-like CNTs (5-6 $\mu $m length, $\sim $ 300 nm diameter, 30 nm wall thickness) filled with the Fe$_{3}$O$_{4}$ NPs ($\sim $ 6 nm). The Fe$_{3}$O$_{4}$ NPs were synthesized by chemical co-precipitation and multi-walled CNTs grown by CVD in porous alumina templates. A novel magnetically assisted capillary action method was used to obtain uniform dispersion of Fe$_{3}$O$_{4}$ NPs inside the hollow portion of the nanotubes. XRD and TEM images of the samples were analyzed. DC and AC magnetization measurements were conducted using a Physical Property Measurement System (PPMS). Saturation magnetization (M$_{s \sim }$65emu/g) was found to be enhanced in Fe$_{3}$O$_{4}$-CNTs compared to Fe$_{3}$O$_{4}$ (M$_{s \sim }$60 emu/g). The M-H curves displayed superparamagnetic behavior at room temperature and conventional blocking at low temperature. We will also present the measurements and analysis of the temperature and field dependent AC susceptibility that points to strong inter-particle interactions within the confined nanotube structures. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D34.00002: Studies of Magnetization Reversal and Switching Characteristics of Individual Iron Filled Carbon Nanotubes using Cantilever Magnetometry Palash Banerjee, M.R. Herman, D.V. Pelekhov, Yu. Obukhov, P. Chris Hammel, F. Wolny, S. Philippi, T. M\"{u}hl, A. Leonhardt, B. B\"{u}chner Iron-filled carbon nanotubes (FeCNTs) form a class of novel materials in which a single-crystal iron nanowire is encapsulated within a carbon nanotube. The iron filled core can be grown to significant lengths (10--15 $\mu$m). Their narrow diameters (15--25 nm) and high crystalline quality make them promising candidates for studying magnetism in thin nanowires. We present a detailed characterization of the magnetic properties of a single FeCNT. These were measured by attaching individual FeCNTs to silicon cantilevers and performing ultrasensitive cantilever magnetometry. We find that the magnetization reversal in the FeCNT occurs at an exceptionally well defined and reproducible switching field and in a single step. These switching fields are characterized by a narrow distribution ($\sim 1$ Gauss) and their measured temperature dependence reveals that their magnetization reversal is thermally activated. We also compare and contrast these results with the low temperature switching behavior of high anisotropy individual micron sized SmCo particles and FePt nanoparticles. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D34.00003: Magnetic properties of cobalt nanowires and nanotubes Jia Grace Lu, Dongdong Li, Richard Thompson, Gerd Bergmann Ferromagnetic Co nanowire and nanotube arrays have been synthesized via low voltage electrodeposition method. High resolution TEM and XRD results show that the nanostructures are uniform in size, and consist predominantly \textit{hcp }structure with the magnetocrystalline easy axis ($c$-axis) perpendicular to the wire axis. For solid wires, SQUID measurement demonstrates the dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the wire axis. MFM shows a strong dipole at the ends and a spatial magnetization modulation along the wire with a period around 700 nm. Based on theoretical modeling, such intrinsic modulation originates from the competition between the magnetocrystalline along the easy axis and the shape anisotropy along the wire axis. In contrast, for Co nanotube with wall thickness $\sim $15 nm and outer diameter $\sim $80 nm, SQUID shows a sheared hysteresis dependence for field applied along the tube axis, and MFM yields weak magnetic signal. They manifest that the magnetization follows a circumferential direction around the tube. This has been confirmed by theoretical model taking into account the magnetocrystalline, shape demagnetization and magnetic exchange energies. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D34.00004: Magneto-transport properties of boron-incorporated Si-based nanostructures Sungmu Kang, Jugdersuren Battogtokh, Andrew C. Buechele, David A. McKeown, Robert DiPietro, Donald Heiman, Ian L. Pegg, John Philip Boron-incorporated Mn$_{5}$SiC nanowires were grown using chemical vapor deposition. Cluster of these nanowires exhibit magnetic hysteresis loops at room temperature and the strength of the magnetic behavior depend on the concentration of the boron incorporation. In the presence of an external magnetic field, Mn$_{5}$SiC nanowire-based devices exhibit tunable spin dependent transport properties. Huge magnetoresistance and metallic behavior were observed in lightly boron-incorporated nanowires, on the other hand, low magnetoresistance and semiconducting behavior were observed with higher boron content in Mn$_{5}$SiC nanostructures. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D34.00005: Defect-Induced Magnetism in Wide-gap Semiconductor Nanowires: Role of Surface Effects and Quantum Confinement Pratibha Dev, Peihong Zhang Observation of unconventional magnetism in undoped wide gap semiconductors, most notably in nanostructures and thin films, has attracted much research attention. While the fundamental mechanism remains under debate, there is increasing evidence that surface/interface and/or localized defect states are responsible for the observed magnetism. Nanowires offer a unique 1D-environment where many phenomena -- confinement and surface effects--play a crucial role. We have studied defect-induced magnetism in wide-gap semiconductor nanowires using density functional theory based {\it ab-initio} methods. Quantum confinement and the surface effects are shown to play a crucial role in the enhancement of the defect-induced magnetism. [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D34.00006: Nano-Torsional Resonator Torque Magnetometry John Davis, Doug Vick, Dave Fortin, Jacob Burgess, Wayne Hiebert, Mark Freeman Torque magnetometry has been used for many years for a variety of magnetic measurements. Moving to nanoscale torsional resonators can push the limits of sensitivity for mechanical torque measurements. We report the use of magnetic torque to drive multiple torsion modes of nanoresonators that have been integrated with nanomagnetic elements. The interferometric response can be used to sensitively measure the magnetic behavior of single nanoscale magnetic objects with excellent sensitivity ($\approx 10^8 \mu_B$ for a single hysteresis curve). [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D34.00007: Spin dynamics simulations for a nanoscale Heisenberg antiferromagnet Zhuofei Hou, D.P. Landau, G. Brown, G.M. Stocks Thermoinduced magnetization(TiM) is a novel response which was predicted to occur in nanoscale antiferromagnetic materials. Extensive Monte Carlo simulations \footnote{G. Brown, A. Janotti, M. Eisenbach, and G. M. Stocks, Phys.Rev.B {\bf 72}, 140405(2005)} have shown that TiM is an intrinsic property of the antiferromagnetic classical Heisenberg model below the Neel temperature. To obtain a fundamental understanding of TiM, spin dynamics(SD) simulations are performed to study the spin wave behavior, which seems to be the cause of TiM. A classical Heisenberg model with an antiferromagnetic nearest-neighbor exchange interaction and uniaxial single-site anisotropy is studied. Simple-cubic lattices with free boundary conditions are used. We employed the fast spin dynamics algorithms with fourth-order Suzuki-Trotter decompositions of the exponential operator. Additional small excitation peaks due to surface effects are found in transverse S(q,w). [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D34.00008: Giant Orbitals Currents in Nanostructures Ralph Skomski, D.J. Sellmyer The possibility and origin of giant orbital currents [1] in nanostructures is investigated by model calculations. We compare two models: (i) a model where electrons are confined to a ``racetrack'' around the dot and (ii) a tight-binding model where atomic spin-orbit coupling creates macroscopic currents at the periphery of the dots. The first model yields expressions very similar to Ref. 1, but the corresponding spin-orbit coupling [2] is negligibly small, because it strongly decreases with increasing orbital radius. Furthermore, the orbital moment rapidly collapses due to a redistribution of electron with wave vectors of opposite sense of rotation. In the second model, the relatively strong intra-atomic spin-orbit interaction yields orbital currents that add [3] between neighboring atoms and create a macroscopic current at the periphery of the dot. This current corresponds to a magnetic Berry phase and cannot dissipate, because the underlying atomic orbital moments are quantized. References: [1] A. Hernando, P. Crespo, and M. A. Garc\'ia, Phys. Rev. Lett. \textbf{96}, 057206 (2006). [2] R. Skomski, IEEE Trans. Magn. \textbf{32}, 4794 (1996). [3] J. Zhang, R. Skomski, Y. F. Lu, and D. J. Sellmyer, Phys. Rev. B \textbf{75}, 214417 (2007). [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D34.00009: Origin and Properties of Uncompensated Magnetization in FeF$_{2}$ Karie Badgley, M. Erekhinsky, Ivan K. Schuller, Mikhail Zhernenkov, Michael R. Fitzsimmons, Casey W. Miller, Igor V. Roshchin Recent studies\footnote{M.R. Fitzsimmons et al. Phys. Rev. B \textbf{75}, 214412 (2007).} of exchange bias demonstrated uncompensated magnetization in an antiferromagnet. To further investigate the properties and the origin of this magnetization, two types of 36nm thick samples of FeF$_{2}$ on MgF$_{2}$ were prepared: with and without a 3nm top layer of Al used to prevent possible oxidation. SQUID VSM measurements on these samples showed uncompensated magnetization. Samples coated with Al were found to have larger magnetic susceptibility. These findings are in agreement with neutron scattering measurements, which also showed the uncompensated magnetization primarily at the top surface of FeF$_{2}$ for the Al-coated sample. Comparative analysis of the field and temperature-dependent magnetization of the Al-coated and uncoated samples provides insight into the origin of the uncompensated magnetization in FeF$_{2}$. Funded by Texas A{\&}M University, Texas A{\&}M University -- CONACyT Collaborative Research Grant Program, DOE, and NSF-9976899. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:30PM |
D34.00010: In-Plane Magnetic Structure of Exchange Biased Co/FeF2 X. Lu, S. Roy, E. Blackburn, Mikhail Erekhinsky, Ivan K. Schuller, J.B. Kortright, S.K. Sinha We report on measurements of magnetic X-Ray specular and diffuse scattering on an exchange biased Co/FeF2 bilayer. The data has been analyzed using the full theory of resonant x-ray scattering in the distorted wave born approximation, and various structural and magnetic roughness parameters have been extracted. We find that the length associated with magnetic corrections in the antiferromagnet increases as the temperature decreases, but remains unchanged for the ferromagnet. The diffuse scattering together with the specular reflectivity provide a more complete picture of the magnetic interfacial phenomena that give rise to exchange bias. [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 4:42PM |
D34.00011: Magnetocaloritronic nanomachines Alexey A. Kovalev, Yaroslav Tserkovnyak We introduce and study a magnetocaloritronic circuit element based on a domain wall that can move under applied voltage, magnetic field and temperature gradient. We draw analogies between the Carnot machines and possible devices employing such a circuit element. We propose a realization of magnetocaloritronic cooling and point out the parallels between the operational principles of magnetocaloritronic and thermoelectric cooling and power generation. Following this analogy, we introduce a magnetocaloritronic figure of merit that encodes information about the maximum efficiency of such devices. Even though the magnetocaloritronic figure of merit turns out to be very small for transition-metal based magnets, we speculate that larger numbers may be expected in ferromagnetic insulators. [Preview Abstract] |
Monday, March 15, 2010 4:42PM - 4:54PM |
D34.00012: Fluctuation-Induced Heat Release from Temperature-Quenched Nuclear Spins near a Quantum Critical Point Y. H. Kim, N. Kaur, B. M. Atkins, N. S. Dalal, Y. Takano The quasi-two-dimensional quantum antiferromagnet Cr(diethylenetriamine)(O$_{2})_{2}\cdot $H$_{2}$O [1] has a magnetic-field-tuned quantum critical point (QCP) at 12.3 T, where a highly polarized antiferromagnetic phase turns into a field-induced ferromagnetic phase. We report a novel relaxation phenomenon near this QCP: quantum-fluctuation-driven annealing of hydrogen nuclear spins frozen in a non-equilibrium high-energy state by temperature quenching. This relaxation phenomenon, with readily detectable heat release from the nuclear spins as they are annealed, reveals the extent of a quantum critical region around the QCP and provides a unique avenue to investigate the dynamics of the divergent quantum fluctuations that underlie quantum criticality. [1] C. M. Ramsey \textit{et al}., Chem. Mater. \textbf{15}, 92 (2003). [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:06PM |
D34.00013: Magnetocaloric Effect in Gd/W multilayers D. Williams, C. Bauer, N. Bingham, H. Srikanth, Casey W. Miller The magnetocaloric effect (MCE) has the potential to lead to advances in magnetic refrigeration. While much is known about the MCE in bulk materials, comparatively little is known about the MCE in thin film nanostructures. To that end, MgO/[W(50{\AA})/Gd(400{\AA})]{\_}8/W(50{\AA}) heterostructures have been grown to study how nanostructuring effects MCE. The entropy change associated with the second order magnetic phase transition was determined from the isothermal magnetization at temperatures. The entropy peak is approximately 2 J/kG-K for a 0-3T field change, and occurs at a temperature of $\sim $280K. The full width at half max of the entropy change peak is about 90K, which is roughly twice that of bulk Gd. Interestingly, the relative cooling power of this nanoscale system is 1.1 J/cm$^3$, which is similar to the 1.4 J/cm$^3$ of bulk Gd. Further analyses of phase transition using the Arrott-Noakes method suggests finite size effects may affect the critical exponents, offering a method by which to tune the MCE. [Preview Abstract] |
Monday, March 15, 2010 5:06PM - 5:18PM |
D34.00014: Thermoelectric Measurements of Magnetic Nanostructures Using Thermal Isolation Platforms A.D. Avery, R. Sultan, D. Bassett, M.R. Pufall, B.L. Zink The effective design of next-generation memory storage and logic devices based on spin necessitates a thorough understanding of transport properties of their potential components. Although electrical transport in magnetic materials is well-understood, thermal transport is historically difficult to measure. Using micromachined thermal isolation structures, we make direct measurements of thermal and electrical transport in these systems. Our technique offers a method for accurately measuring films and other low-dimensional geometries from the microscale down to the nano regime. We will present in-plane thermal conductivity, resistivity, and thermopower results, as well as direct comparisons with the Wiedemann-Franz law for films of various thicknesses and preparation techniques. We will also present the extension of our technique to explore an evaporated multilayer film. Finally, we discuss the application of our method to examining the fundamental physics underlying thermoelectric effects, such as thermally driven spin currents, to further the emerging sub-field of spin caloritronics. [Preview Abstract] |
Monday, March 15, 2010 5:18PM - 5:30PM |
D34.00015: Contribution of electron-magnon scattering to spin-dependent Seebeck effect in a ferromagnet Ashwin A. Tulapurkar, Yoshishige Suzuki Pure spin-current can be generated from a ferromagnet by applying a temperature gradient across it. This spin current can then be used in various devices such as MRAM where it can be used for magnetization switching. The spin-dependent Seebeck coefficients are responsible for the production of spin-current from a ferromagnet in this case. We solve the Boltzmann's transport equations for conduction electrons in a ferromagnet, considering electron-magnon scattering. Such scattering gives rise to spin dependent Seebeck coefficients. We then estimate the temperature gradient required for switching a nano-magnet by using spin-transfer torque. [Preview Abstract] |
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