Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B16: Focus Session: Magnetic Nanostructures II |
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Sponsoring Units: DMP GMAG Chair: Stephane Mangin, Nancy-Universite Room: D173 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B16.00001: Electronic structure and electron spectroscopy of magnetic iron oxide nanoparticles J. Gazquez, J. Salafranca, M. Varela, S. Pennycook, S.T. Pantelides, P. Morales, N. Perez, A. Labarta, X. Batlle Magnetic iron oxide nanoparticles are good candidates for biomedical applications due to their low toxicity and easy functionalization. We synthesized magnetite (Fe3O4) nanoparticles by a high temperature decomposition method. They present some very desirable properties for applications: very high saturation magnetization, and excellent degree of crystallinity. Transmission electron microscopy images, and electron energy loss spectroscopy with atomic resolution allow a composition map that shows small variations in relative composition between the core and the surface, and subtle changes in the absorption spectra. Our density functional (DFT) calculations address different factors contributing to the magnetic properties. Changes in the electronic structure correlate with different features in the experimental absorption spectra, yielding a better understanding of the magnetic order. We study the role of structural defects, the organic surfactant, stoichiometry and the nominal oxidation state of iron, and their effect in determining the equilibrium magnetic state. This work is supported by DOE Materials Sciences and Engineering Division and the European Research Council Starting Investigator Award. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B16.00002: Magnetic properties of Fe/Fe$_{3}$O$_{4}$ core/shell nanostructure Vivek Singh, Mohindar Seehra, S. Bali, E. Eyring, N. Shah, F. Huggins, G. Huffman Magnetic properties of a core/shell nanostructure with spherical core of Fe/FeB and a shell of Fe$_{3}$O$_{4}$/$\gamma $-Fe$_{2}$O$_{3}$ are reported employing magnetometry, electron magnetic resonance (EMR) and M\"{o}ssbauer spectroscopy. This nanostructure was produced by reducing FeCl$_{3}$$\cdot$6H$_{2}$O with NaBH$_{4}$. Combining the results from XRD, TEM and M\"{o}ssbauer spectroscopy showed the nanostructure to consist of a core of diameter D$\simeq $20 nm containing both $\alpha $-Fe with D$\simeq $7 nm and amorphous Fe-B alloy and a shell of thickness 5 nm containing Fe$_{3}$O$_{4}$/$\gamma $-Fe$_{2}$O$_{3}$. Measurements of the magnetization M vs. temperature (2 K to 370 K) and in H upto 65 kOe show a blocking temperature T$_{B}\simeq $30 K associated with the oxide shell and ferromagnetism upto 370 K with nearly temperature-independent saturation M$_{S}\simeq $70 emu/g and coercivity H$_{C}\simeq $100 Oe. In EMR studies at 9.28 GHz, two lines are observed: a narrower line with linewidth $\Delta $H$\simeq $600 Oe and g$\simeq $2 and a broader line with $\Delta $H$\simeq $4200 Oe and g$\simeq $2.2. These parameters of the narrower line combined with its disappearance below 50 K suggests its origin to be the oxide shell whereas the broader line is due to Fe/FeB core. Research supported by U. S. Dept. of Energy, Contract {\#}DE-FC26-05NT42456. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B16.00003: Fe and Fe oxide nanoparticles ensembles with macroscopic anisotropy Miguel Angel Garcia We report here the fabrication Fe and Fe oxide nanoparticles over glass substrates exhibiting macroscopic anisotropy. Fe thin films were deposited onto glassy substrates by thermal evaporation and were subsequently annealed in air and argon atmosphere. The difference of thermal expansion coefficient between the substrate and the metallic film induces stresses in the substrate-metal interface leading to hole nucleation, growing and percolation, and finally to the formation of a metallic a nanoparticles layer. Anisotropic nanoparticles can be obtained by applying mechanical stress during the thin the film deposition or the annealing process. The applied stress induce anisotropy axis for the NPs shape that lead to the formation of elongated nanoparticles with macroscopic texture. Anisotropy can be increased by applying a magnetic field during thermal annealing. We analyze here the magnetization and anisotropy of individual nanoparticles and nanoparticles interactions and their relationship with the processing parameters. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B16.00004: Magnetic Core-Shell Morphology of Structurally Uniform Magnetite Nanoparticles Invited Speaker: Magnetic nanoscale structures are intriguing, in part, because of the exotic properties that emerge compared with bulk. The reduction of magnetic moment per atom in magnetite with decreasing nanoparticle size, for example, has been hypothesized to originate from surface disordering to anisotropy-induced radial canting, which are difficult to distinguish using conventional magnetometry. Small-angle neutron scattering (SANS) is ideal for probing structure, both chemical and magnetic, from nm to microns across an ensemble of particles. Adding polarization analysis (PASANS) of the neutron spin orientation before and after interaction with the scattering particles allows the magnetic structure to be separated into its vector components. Application of this novel technique to 9 nm magnetite nanoparticles closed-packed into face-centered crystallites with order of a micron revealed that at nominal saturation the missing magnetic moments unexpectedly interacted to form well-ordered shells 1.0 to 1.5 nm thick canted perpendicular to their ferrimagnetic cores between 160 to 320 K [1]. These shells additionally displayed intra-particle ``cross-talk'', selecting a common orientation over clusters of tens of nanoparticles. However, the shells disappeared when the external field was removed and interparticle magnetic interactions were negligible (300 K), confirming their magnetic origin. This work has been carried out in collaboration with Ryan Booth, Julie Borchers, Wangchun Chen, Liv Dedon, Thomas Gentile, Charles Hogg, Yumi Ijiri, Mark Laver, Sara Majetich, James Rhyne, and Shannon Watson.\\[4pt] [1] K.L. Krycka \textit{et al}., Phys. Rev. Lett. 104, 207203 (2010) [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B16.00005: Magnetic Characterization of Ferrite Nanoparticles Matthew Bryan, Paul Sokol, Greg Gumina, Lyudmila Bronstein, Bogdan Dragnea Magnetic nanoparticles (NPs) of different compositions (FeO/Fe$_{3}$O$_{4}$, g-Fe$_{2}$O$_{3}$, FePt, and CoFe$_{2}$O$_{4})$ have been synthesized using high temperature organometallic routes described elsewhere. NPs (16.6 nm in diameter) of a mixed FeO/Fe$_{3}$O$_{4}$ (wuestite/magnetite) composition were prepared by thermal decomposition or iron oleate in the presence of oleic acid as a surfactant in dodocane at 370C in argon atmosphere. After the thermal treatment of the reaction solution at 200 C under air for 2 hours these NPs are transformed into maghemite (g-Fe$_{2}$O$_{3})$, the magnetization of which is significantly enhanced. NPs of CoFe$_{2}$O$_{4}$ (8 nm) have been prepared by simultaneous decomposition of Co(II) and Fe(III) acetylacetonates in the presence of oleic acid and oleylamine. The X-ray diffraction profile of these NPs is characteristic of cobalt ferrite. Alternatively, alloyed 1.8 nm FePt NPs prepared by simultaneous decomposition of Fe and Pt acetylacetonates in the reductive environment demonstrate a completely disordered structure, which is reflected in their magnetic properties. SQUID magnetometry was used to measure the magnetization of NPs at high and low temperatures. Zero-field cooling and field-cooling measurements were taken to demonstrate superparamagnetic behavior and an associated blocking temperature. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B16.00006: Magnetically Tunable Polymer Nanocomposites for RF and Microwave Device Applications K. Stojak, S. Pal, H. Srikanth, C. Morales, J. Dewdney, J. Wang, T. Weller There has been much interest in polymer nanocomposites (PNC) recently due to potential applications for EMI shielding, tunable electromagnetic devices and flexible electronics. We report synthesis, structural, magnetic and RF characterization on PNCs ranging from 20-80 wt-{\%} loadings of Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles ($\sim $8nm) in a thermosetting resin from the Rogers Corporation. Nanoparticles were synthesized by thermal decomposition and characterized by XRD and TEM. Magnetic properties were studied using a Quantum Design PPMS. PNCs displayed characteristic features of superparamagnetism at room temperature and blocking at low temperature. Microwave transmission/reflection studies were done using a microstrip resonator. Strong tunability in the microwave absorption was observed. We extend our study to include nanoparticle-filled multi-walled carbon nanotubes synthesized by CVD. These high-aspect ratio magnetic nanostructures, with tunable anisotropy, are of particular interest in enhancing magnetic and microwave responses in existing PNCs. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B16.00007: Field dependence of T{\_}B in NiO and (Ni, Zn)O Nanoclusters Yung Huh, M. Peck, R. Skomski, R. Zhang, P. Kharel, M. Allison, D. Sellmyer, M. Langell Size dependence of magnetic properties of rocksalt NiO and Zn substituted NiO nanoparticles are investigated. Nanoparticle diameters are determined from 8 to 30 nm by XRD and AFM. Uncompensated spins at the nanoparticle surface contribute to superparametism at low temperatures and their blocking temperatures increase with stronger applied field. The field induced spin canting of the antiferromagnetic sublattices is a bulk effect and studied by the substitution of Zn with transition metal. Nanoparticles start exhibiting bulk magnetic behavior with size greater than 18 nm. Magnetization rotation of uncompensated spins under the magnetic field is mainly due to nanoscale size effect. The anisotropy of the nanoparticle is about four times larger than that of the bulk NiO. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B16.00008: Interacting Superparamagnetic Brownian Particles in an Array of 2D Asymmetric Magnetic Traps Gregory Vieira, Aaron Chen, R. Sooryakumar We report on the ordering and fluctuation of multiple superparamagnetic particles confined by a thin liquid layer in a two-dimensional array of asymmetric magnetic trapping potentials. The repulsive dipolar interaction between magnetic particles and their confinement by the trapping potential cause the particles to form a cluster with characteristic inter-particle spacing within each trapping site, while the particles undergo thermal fluctuations. Applying an external magnetic field offers a convenient way to control the strength of the dipolar interactions and change the trapping potential landscape. Results on (a) Brownian motion of individual particles in the cluster, (b) re-distribution of particles into new clusters driven by a change in the external field, and (c) hopping of particles between clusters under fluid flow will be presented. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B16.00009: Realization of a Bowl-like Potential and Its Confinement of Magnetic Microspheres Aaron Chen, Thomas Henighan, Gregory Vieira, Ratnasingham Sooryakumar Field-induced self-assembly of fluid-borne superparamagnetic microspheres not only has its importance in nanotechnology, but it also serves as a model for studies of phase transitions at the nano- to micro-meter scale. In this report, we experimentally demonstrate and theoretically account for the dynamics and structural order of a two-dimensional cluster of microspheres in the presence of a bowl-like potential. The potential is derived from magnetic patterns imprinted on the surface together with externally applied magnetic fields. Due to competition between the repulsive dipolar interaction amongst the microspheres and the confining force provided by the bowl-like potential, a cluster of microspheres with characteristic inter-sphere spacing is stabilized within the potential. The role of external magnetic fields which provide a convenient means to tune the strength of the dipolar interactions, and thereby control the relative importance of the two competing interactions, will be presented. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B16.00010: Tailored Magnetostructural Transitions Radhika Barua, Felix Jimenez-Villacorta, Donald Heiman, Laura H. Lewis Dominance of the surface atoms over the bulk atoms in nanoscaled magnetostructural systems may alter the ground state of the system and thereby change the transition character. Creation of a nanostructured magnetostuctural system was carried out via rapid solidification of (FeRh)$_{5}$Cu$_{95}$ to precipitate nanoscaled isolated FeRh precipitates in a Cu matrix upon annealing. Bulk FeRh has an abrupt antiferromagnetic - ferromagnetic transition around T =370 K. X-ray diffraction performed on the quenched (FeRh)$_{5}$Cu$_{95}$ alloy indicates only the presence of Cu of slightly expanded lattice parameter a=3.62 {\AA}, with ferromagnetism confirmed at room temperature by SQUID magnetometry. Vacuum annealing at 200 $^{\circ}$C causes a secondary phase to appear with an abrupt magnetic phase transition at T$_{t}$ = 130 K. Details of the magnetic behavior of this nanostuctured phase will be discussed. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B16.00011: Synthesis of Ferrimagnetic Fe$_{3}$Se$_{4}$ Nanostructures with Giant Coercivity Hongwang Zhang, Gen Long, Da Li, Hao Zeng In this study, we present the synthesis of Fe$_{3}$Se$_{4}$ nanostructures by a one-pot high temperature organic solution-phase method. The size of these nanostructures can be tuned from 50 to 500 nm and their shapes can be varied from nanosheets and nano-cactus to faceted nanoparticles by changing the precursors and reaction conditions. These nanostructures exhibit hard magnetic properties, with giant coercivity values reaching 40 kOe at 10 K, and 4 kOe at room temperature. The estimated lower bound of the magnetocrystalline anisotropy constant is 6$\times $10$^{6}$ erg/cm$^{3}$, comparable to that of hcp Cobalt. The large coercivity/anisotropy is rare for compounds without noble metal or rare-earth elements. If Fe$_{3}$Se$_{4}$ based phases can be doped to enhance their Curie temperature and magnetization, they can be a low cost, non-toxic alternative to noble metal or rare earth based advanced magnets. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B16.00012: Synthesis and Magnetic Properties of FePt@MnO Nano-\textit{hetero}-Particles Thomas Schladt, Tanja Graf, Oskar Koehler, Kerstin Schneider, Wolfgang Tremel Monodisperse FePt@MnO nano-\textit{hetero}-particles with different sizes and morphologies were prepared by a seed-mediated nucleation and growth technique. Both, size and morphology of the individual domains could be controlled by adjustment of the synthetic parameters. As a consequence, different particle constructs, including dimers, dumbbells and flowers, could be obtained by changing the polarity of the solvent. The FePt@MnO nano-\textit{hetero}-particles were thoroughly characterized by (HR-)TEM- and XRD analysis and SQUID magnetometry. Due to a sufficient lattice match, the MnO NPs preferentially grow on the (111) surfaces of the \textit{fcc}-FePt seeds. Furthermore, the surface spins of the antiferromagnetic MnO domains pin the magnetic moments of the ferromagnetic FePt NPs which leads to an exchanged biased magnetic hysteresis. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B16.00013: Electron-electron correlations and magnetic properties of small FePt clusters Alamgir Kabir, Neha Nayyar, Volodymyr Turkowski, Talat S. Rahman We have applied the DFT+U and the Nanoscale Dynamical Mean-Field Theory (NDMFT) [1] approaches to study the magnetic properties of small FePt clusters. The role of correlation effects in determining the geometry and the magnetic properties of the clusters as a function of chemical composition and the effective local Coulomb repulsion energy U is examined. We find that the magnetization to decrease with increasing number of Pt atoms. Interestingly, contrary to the bulk case, Pt clusters have nonzero magnetization. The magnetic properties are found to be very sensitive to the value of U, as a result of the dependence of the single particle energy levels on this parameter. Dynamical correlation effects, which are taken into account in the DFT+DMFT approach and which lead to an increase of the average in time double occupancy of the d-orbitals, results in a significant decrease of magnetization as compared to the results from the DFT+U case. \\[4pt] [1] V. Turkowski et al, J. Phys.: Condens. Matt. 22, 462202 (2010) [Preview Abstract] |
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