Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P14: Focus Session: Magnetic Nanostructures I |
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Sponsoring Units: GMAG DMP Chair: Richard Kodama, University of Illinois at Chicago Room: Colorado Convention Center Korbel 4D |
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P14.00001: Coercivity of nanometer size Ni granular films as a function of temperature, grain size and dipolar interaction R. Das, A.F. Hebard, A. Gupta, D. Kumar The influence of temperature, grain size and dipolar interaction (DI) on coercive field $H_{c}$ determined from hysteretic magnetization loops has been studied in nanometer size Ni granular films embedded in an insulating AlOx host matrix. Single layer (SL) and multilayer (ML) samples were grown using pulsed laser deposition by sequential deposition from AlOx and Ni targets. The Ni film thickness $d$, and hence the average grain size, is varied over the range of 3nm to 60nm. In the ML samples, the Ni layers are separated by 3nm-thick AlOx. At low temperatures $H_{c}(d)$ exhibits a peak at a crossover thickness $d_{x}$ delineating \textit{single domain} (SD) from \textit{multi domain} (MD) behavior. The ML sample has a smaller $d_{x}$ because of the increase in magneto static energy due to an increased DI associated with a greater number of nearest neighbors. In the SD region common to both samples, the $H_{c}$`s are considerably higher for ML samples compared to those for SL samples. This effect can be understood in terms of collective dynamics of the interacting particles [1]. Surprisingly, $H_{c}(T)$ shows the well known Stoner-Wohlfarth square root temperature dependence in the MD region for both SL and ML samples. Even more surprising is the unexpected oscillatory dependence of $H_{c}(d)$ in the MD region for the SL samples. [1] C. Djurberg \textit{et al.}, Phys. Rev. Lett. \textbf{79}, 5154 (1997). [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P14.00002: Dynamic Hysteresis of Fe$_{10}$Co$_{90}$ Nanoparticle Compacts K.M. Chowdary, S.A. Majetich The time-dependent magnetic response of composites made of consolidated Fe$_{10}$Co$_{90 }$nanoparticles was measured and modeled. 200 nm particles with average grain size 20 nm synthesized by the polyol method were consolidated to 95{\%} theoretical density by plasma pressure compaction. Power loss, complex permeability, and coercivity were extracted from dynamic minor hysteresis loops measured over a range of temperatures (77 K -- 873 K) and frequencies (100 Hz -- 100 kHz) for toroidal samples. When the data were scaled relative to the peak frequency of the imaginary permeability, universal behavior was observed, with two distinct components. This behavior is explained through simulations of the N\'{e}el-Brown thermal aftereffect in which a time-dependent energy barrier in an Arrhenius-N\'{e}el law gives a rate equation for magnetization reversal. Quantitative attempts to match model and experiment indicate a distribution of energy barriers along with coupled and uncoupled regions in the compacted sample. The uncoupled regions limit the useful frequency range of the sample. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P14.00003: Cluster Beam Synthesis of Magnetic Nanoparticles R.H. Kodama, J.J. Kavich, M. Vedpathak, M.C. Peterson Highly mono-dispersed Ni and Fe nanoparticles are produced using a cluster beam source. The source chamber is isolated from a deposition chamber using a small orifice. By balancing process gas flow, orifice size, and pumping speed we can create a high-pressure sputtering environment, suitable for nanoparticle condensation. The average beam flux and a spatial beam profile are acquired using a quartz crystal monitor with linear motion control. We find that the time stability of the nanoparticle flux is very sensitive to sputtering power and temperature gradients in the cluster source. AFM and TEM measurements have shown a correlation of particle size with position in the beam. Both size distribution and time stability seem to be sensitive to small perturbations in the gas flow near the sputtering source. High-Resolution TEM images indicate that the particles are randomly oriented and nano-crystalline in nature. The magnetic properties of Ni nanoparticles are measured using a SQUID magnetometer. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P14.00004: Detection of exchange interaction in diatomic molecules by Fano resonance Jonas Fransson, Alexander Balatsky We propose a mechanism to use STM for direct measurements of the two-electron singlet-triplet exchange splitting $J$ in diatomic molecular systems, based on the coupling between the molecule and the substrate electrons. The different pathways for electrons lead to interference effects and generate kinks in the differential conductance at the energies for the singlet and triplet. These features are related to Fano resonance due to the branched electron wave functions. Since the ratio between the tunnelling through the two atoms can be modulated by spatial movements of the tip along the surface this suggests a technique for detection of the singlet-triplet exchange splitting with STM. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P14.00005: Measurement of Magnetic Anisotropy for Individual Atomic Spins on Surfaces Cyrus F. Hirjibehedin, Alexander F. Otte, Markus Ternes, Christopher P. Lutz, Andreas J. Heinrich We measure the effects of magnetic anisotropy on individual magnetic atoms on a thin-insulating surface. Using the inelastic electron tunneling spectroscopy capabilities of a scanning tunneling microscope, we probe the spin excitation spectra of Mn and Fe atoms adsorbed on a single copper nitride layer. Magnetic anisotropy is directly manifested as finite-energy spin excitations that exist even in the absence of a magnetic field. The effects of anisotropy are found to be relatively weak for Mn atoms but are substantially larger for Fe atoms, in which spin-orbit coupling is prominent. When a magnetic field is applied to the Fe atoms, the spin excitations shift in a manner that is strongly dependent on the direction of the applied field. These shifts in energy can be understood both qualitatively and quantitatively with a Hamiltonian containing in-plane and out-of-plane magnetic anisotropies. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P14.00006: Ferromagnetic multipods fabricated by solution phase synthesis and hydrogen reduction Yucheng Sui, Yao Zhao, Jun Zhang, Sitaram Jaswal, Xingzhong Li, David Sellmyer New functional materials might emerge if nanocrystals of higher complexity than those with simple geometries (spheres, rod, discs) could be produced. Branched nanostructures (called multipods) have attracted much attention owing to their potential as building blocks in the fabrication of complex, multi-terminal devices through self assembly. In this work, we demonstrate that ferromagnetic Co multi-branched nanostructures can be produced through the combination of solution-phase synthesis and hydrogen reduction. The CoO multipods were produced through the pyrolysis of cobalt-oleate in octadecane at 280$^{o}$C in the presence of oleic acid under the protection of pure nitrogen. Arm lengths and diameters of the CoO multipods are about 30 and 10 nm respectively, and the angles between the nearest arms are 90 degrees. The multipods were assembled onto Si substrates, and after reduction in flowing hydrogen gas at 290$^{o}$C, pure cobalt with hexagonal crystal structure and multi-branched structures were created. Anisotropic magnetic properties were found for cobalt multipods. The growth mechanism of CoO multipods will be presented in this work. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P14.00007: Magnetic Exchange Force Microscopy Invited Speaker: Magnetic Exchange Force Microscopy (MExFM) is a new technique that was proposed [1] to perform magnetic imaging with atomic resolution. It is based on conventional atomic force microscopy, but uses a magnetic tip, which is approached very closely to a magnetic sample in order to detect the magnetic exchange interaction. Unlike Spin-Polarized Scanning Tunneling Microscopy (SP-STM) [2], it is not limited to well conducting materials. Although theoretical calculations indicate the feasibility of MExFM and several attempts have been made to perform such an experiment, no clear evidence for successful MExFM imaging has been reported so far. To prove the detection of the magnetic exchange interaction between magnetic moments (spins) of tip and sample, we investigated the (001) surface of the prototypical antiferromagnetic insulator nickel oxide. Imaging with atomic resolution was performed in the non-contact attractive force regime using the dynamic mode with frequency modulation. Apart from the chemical contrast between nickel and oxygen atoms an additional modulation originating from the row-wise antiferromagnetic arrangement of the spins at the nickel atoms could be observed. We discuss experimental prerequisites to perform MExFM and present different tests to unambiguously assign the additional modulation to the magnetic exchange force.\newline \newline [1] R. Wiesendanger et al., J. Vac. Sci. Technol. B \bf{9} \rm, 519 (1990).\newline [2] S. Heinze et al., Science \bf{288} \rm, 1805 (2000). [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P14.00008: Micromagnetic Modeling of Ferromagnetic Resonance in Nonuniform Magnetic Field D.V. Pelekhov, I. Martin, Yu. Obukhov, J. Kim, E. Nazaretski, T. Mewes, P.E. Wigen, R. Movshovich, P.C. Hammel We compare micromagnetic modeling of Ferromagnetic Resonance (FMR) excitations in thin ferromagnetic samples in the presence of a nonuniform magnetic field to our FMR data obtained with Magnetic Resonance Force Microscopy (MRFM). MRFM is a novel scanned probe technique based on mechanical detection of magnetic resonance. Its extreme sensitivity originates partially from the high magnetic field gradient of the MRFM probe micromagnet. The presence of the high field gradient imposes unusual conditions on the FMR resonance in the sample under investigation. We will discuss their manifestations in both simulations and experimental data. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P14.00009: Local investigations of 2 micrometer permalloy dot array using Magnetic resonance force microscopy. J. Kim, Yu. Obukhov, D. Pelekhov, T. Mewes, S. Batra, P.E. Wigen, S. An, T. Gramila, P.C. Hammel Ferromagnetic resonance images of 2 micrometer diameter permalloy dots in an array with a center to center distance of 2.2 micrometer have been microscopically investigated at 4K using magnetic resonance force microscopy. Both local and global ferromagnetic resonance properties of the sample are observed due to the influence of the strong field immediately beneath the micromagnetic probe. Localized spectral changes reveal the dynamics of ferromagnetic resonance of a dot just underneath the tip and neighboring dots in proximity to the tip. The combination of spatial and spectral information is a promising new way to investigate magnetization dynamics using magnetic resonance force microscopy. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P14.00010: Self-assembly of magnetic nanoparticles JiYeon Ku, Phillip Geissler When a solution containing nanocrystals dries, the solute deposits onto the underlying substrate. The nonequilibrium nature of such a process, together with anisotropic interactions between nanoparticles, can drive the formation of intricate transitory patterns. In particular, we are investigating how magnetic nanocrystals can coalesce into faceted, mesoscopic domains that have been observed in experiments. We model the nanoparticles as dipolar spheres and use Monte Carlo methods to advance their arrangements in time from an initially dispersed configuration. Competition between short-ranged, isotropic van der Waals forces and long-ranged, anisotropic electrostatic forces generates diverse hybrid structures, which exhibit both imperfect close-packing and incomplete dipole alignment. We explore the structures obtained under various conditions and speculate on dynamical mechanisms of aggregation and pattern formation. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P14.00011: FeCo Nanoparticles by Salt-Matrix Annealing Narayan Poudyal, Girija S. Chaubey, Chuan-bing Rong, J. Ping Liu Preparation of monodisperse FeCo nanoparticles remains a challenge due to poor chemical stability of the nanoparticles during heat treatments. We report a novel route of preparation of monodisperse FeCo nanoparticles with controllable particle size and size distribution. CoFe$_{2}$O$_{4}$ nanoparticles were first prepared by chemical solution method via reduction of iron acetylacetonate and cobalt acetylacetonate. The as-synthesized CoFe$_{2}$O$_{4}$ nanoparticles were then mixed with NaCl powder particles and the mixtures were annealed in forming gas to form FeCo nanoparticles. Structural characterization showed that the FeCo nanoparticles obtained by salt-matrix annealing have been transformed to body-centered cubic (bcc) structure without sintering and agglomeration. The particle size can be well controlled by adjusting the synthetic parameters for CoFe$_{2}$O$_{4}$ nanoparticles. It is also found that the recovered bcc FeCo nanoparticles are stable under ambient condition. The magnetization of the FeCo nanoparticles is found to be size dependent. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P14.00012: Static and dynamic magnetic properties of ``dumbbell'' and ``flower'' shaped Au-Fe$_{3}$O$_{4}$ nanoparticles N.A. Frey, S. Srinath, H. Srikanth, Chao Wang, Shouheng Sun We report studies of the static (DC) and dynamic (AC, RF) magnetization of chemically synthesized Au-Fe$_{3}$O$_{4}$ nanoparticles with dumbbell and flower shaped configurations. Dumbbell particles form with Fe$_{3}$O$_{4}$ (18 nm) growing epitaxially on Au seed particles (4 -- 8 nm). Multiple Fe$_{3}$O$_{4}$ particles also can be made to grow on Au particles with flower-like cluster geometry. While measurements on dumbbell particles revealed standard signatures of superparamagnetism, the flower-like nanoparticles exhibited remarkable novel features. Two magnetic transitions are observed --one representing the blocking temperature ($\sim $88K) and the other ($\sim $48K) likely associated with freezing of surface spins. Our experiments revealed the presence of exchange bias (EB), high field irreversibility as well as training and memory effects. EB was also confirmed through RF transverse susceptibility measurements that directly probe the effective magnetic anisotropy and switching fields. Our studies demonstrate how engineering the configuration of nanoparticle clusters in a controlled manner can result in dramatically different magnetic properties. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P14.00013: Magnetite nanoparticles with almost bulk magnetic properties: the role of the surfactant Xavier Batlle, Pablo Guardia, Oscar Iglesias, Am\'ilcar Labarta, Alejandro G. Roca, M. Puerto Morales, Carlos J. Serna Uniform magnetite nanoparticles of 6, 10 and 17 nm were synthesised by thermal decomposition of an iron precursor. Oleic acid was used as surfactant. Saturation magnetization M$_{s}$ reaches the expected value for bulk magnetite at low temperature, in contrast to results in small particle systems for which M$_{s}$ is usually much smaller due to surface spin disorder. The coercive field for the 6 nm particles is also in agreement with that of bulk magnetite. Both results suggest that the oleic acid molecules covalently bonded to the nanoparticle surface yield a strong reduction in the surface spin disorder, such that the new O$^{2-}$ surface ligands partially reconstruct the crystal field of the surface Fe cations, as suggested by XPS. This may be of relevance in biomedical applications to reduce the strength of the magnetic field required to obtain a high M$_{s}$ and opens the question of whether M$_{s}$ above the bulk value may be obtained by taking advantage of the orbital contribution. Work funded by Spanish NAN2004-08805-CO4-02 and NAN2004-08805-CO4-01, and CONSOLIDER CSD2006-12 [Preview Abstract] |
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