Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z31: Focus Session: Magnetic Nanoparticles and Nanocomposites: Characterization |
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Sponsoring Units: DMP GMAG Chair: Manh-Huong Phan, University of South Florida Room: 335 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z31.00001: Crystal Structure and Magnetic properties of Fe-substituted nanoscale Hydroxyapatite Andreas Kyriacou, Riccardo Venturelli, Korey Sorge, Theodora Leventouri Magnetic nanoscale hydroxyapatite (HAp) of chemical formula Ca$_{(5-x)}$Fe$_{x}$(PO$_{4})_{3}$OH has been prepared by a chemical precipitation method where $x$ varies from 0 to 1.26. Single phase HAp is identified in XRD patterns of samples with $x \quad \le $ 0.30 while maghemite (Fe$_{2}$O$_{3})_{ }$is formed as a secondary phase for $x \quad \ge $ 0.60. The average crystallite size as calculated by the Scherrer equation varies from 16 nm to 28 nm. Rietveld refinement reveals a decrease of the unit cell for $x \quad \le $ 0.15. Magnetic moment measurements as a function of temperature at applied field \textit{$\mu $}$_{0}H$ = 1.5 T shows a two component system: a temperature-dependent paramagnet (PM) or superparamagnet (SPM) and a roughly temperature-independent ferromagnetic (FM) component. No FM activity is shown for low $x$, followed by increased activity for higher $x$. Increasing SPM activity is observed for $x\le $0.60. Hysteresis measurements show irreversible loops for $x \quad \ge $ 0.22. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z31.00002: Structure determination of CoPt nanoparticles: Chemical ordering and its effect on magnetic properties Nils Blanc, Laurent Bardotti, Matthias Hillenkamp, Alexandre Tamion, Florent Tournus, Juliette Tuaillon-Combes, Veronique Dupuis, Edgar Bonet, Helio Tolentino, Aline Ramos, Maurizio De Santis, Philippe Ohresser, Thierry Epicier Due to the huge magnetocrystalline anisotropy of bulk CoPt crystallized in the L1$_{0}$ phase, CoPt nanoparticles have been widely studied during the last decade. In order to determine the intrinsic magnetic properties of CoPt clusters, we synthesize benchmark samples: 3 nm diameter CoPt clusters, pre-formed in the gas phase, are embedded in an amorphous carbon matrix under UHV conditions. The transition from the chemically disordered A1 to the ordered L1$_{0}$ phase is then obtained by annealing. Chemical ordering has clearly been evidenced by different techniques (HRTEM, GIXRD). In the case of nanoparticles, this phase transition goes with a magnetic anisotropy increase much lower than for the bulk. Besides, XMCD measurements have revealed a $\mu $L/$\mu $S increase for Co and Pt atoms and a strong $\mu $S enhancement for Co upon L1$_{0}$ ordering. F. Tournus et al. Phys. Rev. B 77, 144411 (2008) Thanks are due to the CLYM (Centre Lyonnais de Microscope) for the access to the tranmission electron microscope [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z31.00003: Origin of magnetic anomalies and relaxation mechanisms in ferrofluids M. B. Morales, M. H. Phan, N. A. Frey, S. Pal, H. Srikanth From a fundamental physics perspective, it is proposed that blocking of magnetic nanoparticles and freezing of a carrier fluid would affect the magnetization and relaxation processes in ferrofluids. To verify this hypothesis, we have conducted systematic DC magnetization and AC susceptibility studies in different ferrofluids composed of Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles suspended in hexane and dodecane, which respectively have freezing temperatures below (178K) and above (264K) the blocking temperature of magnetic nanoparticles ($\sim $200K). Experimental results reveal that the particle blocking and carrier fluid freezing effects play key roles in the formation of glass-like relaxation peaks in ferrofluids, which remained largely unexplained in previous studies. It is also shown that the nature of these peaks is strongly affected by varying particle size and carrier fluid medium. Quantitative fits of the frequency dependent AC susceptibility to the Vogel-Fulcher model, $\tau =\tau _{o}$exp[E$_{a}$/k(T-T$_{o})$], clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics. A clear correlation between the blocking and freezing temperatures emerges from our studies for the first time. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z31.00004: Competing effect of blocking and spin frustration in nanostructured gadolinium iron garnets M.H. Phan, M.B. Morales, H. Srikanth, C.N. Chinnasamy, V.G. Harris The ground state magnetic properties and relaxation mechanism in magnetically frustrated system of Gd$_{3}$Fe$_{5}$O$_{12}$ is of topical interest due to its complex magnetic structure. As a consequence of geometric and magnetic frustrations, the Gd$_{3}$Fe$_{5}$O$_{12}$ system is expected to show glassy magnetic behavior. Through a comprehensive study of DC magnetization, AC susceptibility, transverse susceptibility, and magnetocaloric effect in Gd$_{3}$Fe$_{5}$O$_{12}$ bulk and nanostructured materials, we provide physical insights into the glassy nature and magnetic relaxation mechanisms in the gadolinium iron garnet system. It is shown that bulk Gd$_{3}$Fe$_{5}$O$_{12}$ undergoes two different glassy states at temperatures below its compensation temperature with the low temperature glass properties strongly influenced by Gd ordering. However, the glassy nature is largely suppressed in Gd$_{3}$Fe$_{5}$O$_{12}$ nanoparticles in which the blocking phenomenon competes with the spin frustration effect. As particle size is decreased, the blocking effect is dominant over the spin frustration effect. As a result, the nanostructured system shows magnetic relaxation features arising mainly from superparamagnetism. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z31.00005: Thermoinduced Magnetization in NiO Nanoparticles Gregory Brown The low-temperature magnetic susceptibility for model NiO nanoparticles is calculated using the Monte Carlo method, and three different behaviors are seen. With uncompensated spins present, the susceptibility diverges as $T$$\rightarrow$$0$. For cube- shaped nanoparticles, a temperature-dependent thermoinduced magnetization is observed. For spherical and octahedral nanoparticles, a temperature-independent susceptibility associated with the spin-flop configuration is observed. Calculations for arbitrary values of the uniaxial anisotropy indicate that thermoinduced magnetization can be observed for all geometries in materials with strong enough anisotropy. This work was sponsored by the LDRD program of ORNL, by the DOE-OS through the Offices of BES, Division of MSE and ASCR, MICS Division. The $\Psi$--Mag tool set was developed as part of a BES sponsored Computational Material Science Network project. ORNL is managed by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z31.00006: Magnetic and EPR Characterization of Ni(core)/NiO(shell) Nanoparticles Saritha Nellutla, Alex Smirnov, Junwei Wang, Joseph B. Tracy Core/shell nanoparticles have interesting applications in various fields [1-4]. Among these systems, ferromagnet(core)/antiferromagnet(shell) are of particular importance because of their potential use as MRI contrasting agents, high density magnetic recording devices, etc. [3, 4]. Here, Ni(core)/NiO(shell) nanoparticles of different core sizes ranging from 8 nm to 22 nm have been synthesized and characterized by TEM, magnetic susceptibility and electron paramagnetic resonance (EPR) spectroscopy, as ``free'' (non-agglomerated) particles and agglomerated clusters. Using EPR at 9.1 GHz it is shown that the temperature dependence of the g-value and the EPR linewidth are similar for both the free particles and the agglomerated clusters. This suggests that at this magnetic field ($\sim $0.3 T) the EPR signal arises mostly from the saturated magnetic moment. EPR measurements at multiple fields/frequencies provide further insight on the microscopic magnetic structure in the free particles and the agglomeration effects. [1]. M. A. Hines, P. Guyot-Sionnest, \textit{J. Phys. Chem.}, $100$, 468 (\textbf{1996}). [2]. Z.C. Xu, Y.L. Hou, S.H. Sun, \textit{J. Am. Chem. Soc.}, $129$, 8698 (\textbf{2007}). [3]. V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogues, \textit{Nature} $423$, 850 (\textbf{2003)}. [4]. A. H\"{u}tten, D. Sudfeld, I. Ennen, G. Reiss, W. Hachmann, U. Heinzmann, K. Wojczykowski, P. Jutzi, W. Saikaly, G. Thomas\textit{ J. Biotechnology} $112$, 47-63 (\textbf{2004}). [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z31.00007: Development of Novel Biopolymer/Synthetic-Polymer/Iron Oxide Nanocomposites Marleth Mena Montoya, Sugeheidy Carranza, Mois\'es Hinojosa, Virgilio Gonz\'alez In this work we report the successful development of a family of magnetic nanocomposites based on chitosan or/and polyamide 6 matrix with dispersed iron oxide nanoparticles synthesized by chemical co-precipitation. The iron oxide contents varied from 5 up to 23 wt{\%}, the nanocomposites were studied by FTIR, UV-vis, TGA, XRD, TEM and magnetometry. The FTIR analysis demonstrates an interaction between the amide group of the polyamide 6 and the ceramic material. In formic acid, the nanocomposites absorb in the UV-Vis range, and the magnitude of the band gap (optical), calculated using the band of higher wavelength, is between 2.16 and 2.19 eV. In nanocomposites with chitosan/polyamide 6 matrix the developed morphologies are spherulites of polyamide 6 surrounded by chitosan, with the iron oxide particles presumably in the form of ferrihidryte. The measured magnetic properties revealed a superparamagnetic character on the studied specimens. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z31.00008: Characterization of ultrasonically prepared $\gamma$-Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$ shell-core nanocomposites Matthew Vannette, Joshua Hugen, Daniel Stoecklein, Brett McCarty, Ruslan Prozorov High intensity ultrasonic irradiation (sonication) of slurries of Al$_{2}$O$_{3}$ nanopowder in an Fe(CO)$_{5}$/decane mixture produce superparamagnetic $\gamma$-Fe$_{2}$O$_{3}$ shells on non-magnetic cores. In this contribution we discuss the effect of the various adjustable parameters (sonication time and intensity, powder loading, and Fe(CO)$_{5}$:decane ratio) on the dc and ac magnetic properties of these composite materials. Effects of post production modification such as heat treating powders and cold pressing pellets is also presented for a subset of samples. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z31.00009: Phase Transformation in Silica-Coated FePt Nanoparticles Levent Colak, George Hadjipanayis The A1 to L1$_{0}$ phase transformation has been examined in silica-coated FePt particles. The nanoparticles were synthesized by reduction of platinum acetylacetonate (Pt (acac)$_{ 2})$ followed by thermal decomposition of iron pentacarbonyl (Fe(CO)$_{5})$ in the presence of oleic acid (OA) and oleyl amine (OY) as surfactants at low temperature $^{[1]}$. The monodispersed FePt nanoparticles, with a size of 5.8 nm were then coated with silica (SiO$_{2})$ shells $^{[2]}$ . The thickness of the silica shell could be controlled between 7.5-25 nm. The coated particles were subjected to thermal processing at 800\r{ }C for various amounts of times. No significant sintering was observed up to 2 hours of annealing for the shell thickness of 15.0 nm. In some silica-coated samples an increase in the particle size was observed after annealing. Selected Area Diffraction analysis and magnetic measurements showed the development of ordered L1$_{0}$ structure. Coercivity values up to 15 kOe at 7K are obtained. The phase transformation is currently being examined in other samples annealed at different times and temperatures and the results will be reported.\textbf{1}.Levent Colak and George C. Hadjipanayis, Nanotechnology 19 (2008) 235703.\textbf{2}.M. Aslam, L. Fu, S. Li, Vinayak P. Dravid, Journal of Colloid and Interface Science 290 (2005) 444--449. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z31.00010: Crystallization thermodynamics and kinetics of SmCo$_{5}$/Fe system. Chuanbing Rong, J. Ping Liu High energy ball milling is an effective and economic way to produce the hard/soft nanocomposite permanent magnetic materials which have immense potential to exhibit much higher energy products than the conventional single phase hard magnets. Intermetallic materials undertaken high energy ball milling are usually of amorphous structures. It is therefore necessary to study the grain nucleation and growth behavior of the ball-milled amorphous powders. There has not been a systematical study of thermal dynamic and kinetic behavior of the mechanically milled nanocomposite powders. In this work, powder mixtures of SmCo$_{5} \quad +$ x $\alpha $-Fe (x=0-30 wt{\%} ) were mechanically milled for 2 - 10 hours. The thermal dynamic and kinetic behavior of the powders was studied by measuring the differential scanning calorimetry (DSC) curves with different heating rate and isothermal methods. It was observed that the crystallization process of the SmCo$_{5}$ phase shifted to high temperature while that of Fe phase shifted to low temperature with increasing milling time. Kissinger analysis shows that the activation energy of SmCo$_{5}$ phase significantly decreased with increasing milling time and increasing Fe content. Isothermal analysis showed that the nucleation of SmCo$_{5}$ phase started around 300-350 $^{o}$C which is 100-150 $^{o}$C lower than the crystallization temperature (460 $^{o}$C). [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z31.00011: Magneto-structural study of phase pure $\alpha $ and $\beta $ type MnAs nanoparticles P. Kharel, Keerthi Senevirathne, Ron Tackett, Stephanie Brock, G. Lawes There is extensive interest in understanding the properties of nanoscale materials that exhibit magneto-structural phase transitions because of their possible use in magnetocaloric applications. Bulk MnAs exhibits a ferromagnetic transition in the range 313-317 K, together with a structural transition from a hexagonal (\textit{$\alpha $}-MnAs) to an orthorhombic (\textit{$\beta $}-MnAs) lattice. We have studied the structural and magnetic properties of \textit{$\alpha $}-MnAs and\textit{ $\beta $}-MnAs nanoparticles synthesized using solution-phase arrested precipitation method. XRD and TEM studies show that both the \textit{$\alpha $} and\textit{ $\beta $} phase nanoparticles are crystalline, phase pure, and stable for weeks at room temperature. Magnetic measurements show that both the \textit{$\alpha $}-MnAs and \textit{$\beta $}-MnAs phase nanoparticles undergo ferromagnetic phase transitions near 315K, but we find no evidence for the associated \textit{$\alpha $} to \textit{$\beta $} structural transition seen in bulk MnAs. We will present an experimental investigation on the connection between structural and magnetic properties in \textit{$\alpha $} and\textit{ $\beta $} type MnAs nanoparticles, and discuss relevance to studies on other nanostructured systems. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z31.00012: Magnetic Force Microscopy of Ferromagnetic MnAs Nanoparticles in GaAs. Ben Chaprut, Radhika Barua, Laura Lewis, Don Heiman The switching behavior and anisotropy of ferromagnetic MnAs nanoparticles in GaAs was investigated with variable-temperature magnetic force microscopy. Nanoparticles of MnAs were synthesized by annealing thin layers of GaMnAs, with Mn/Ga=0.1. Annealing at 640 C resulted in thin disc-shaped MnAs particles with diameters $\sim $100 nm embedded in GaAs. Magnetization measurements at room temperature show that the samples are ferromagnetic and exhibit hysteresis with a coercive field $\sim $0.1 T. In MFM images at room temperature, the majority of the nanoparticles have a single-domain magnetic dipole moment which can be oriented in one direction after applying a magnetic field $\sim $0.1 T. The dipole orientation can be reversed after applying a field in the opposite direction. After raising the temperature above the Curie point, Tc=337 K, the sample becomes demagnetized at room temperature, with equal numbers of particles aligned in opposite directions. A sharp phase transition, from the ferromagnetic hexagonal phase to the paramagnetic orthorhombic phase, was found at 340 C, nearly coincident with the Curie temperature. This transition occurs $\sim $30 C higher than in thin MnAs films and is attributed to tensile strain on the nanoparticles from the surrounding GaAs. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z31.00013: Nanoparticulate Alnico Thin Films with High Coercivity Ozan Akdogan, George C. Hadjipanayis Alnico V (Fe--8{\%} Al--14{\%} Ni--24{\%} Co--3{\%} Cu) nanoparticulate thin films have been produced by dc magnetron sputtering. The films were sputtered on Si substrates for magnetic measurements and carbon-coated copper grids for TEM measurements. The as-deposited films have a fine grained microstructure with the bcc crystal structure. The as-made films were subjected to a full heat treatment which consists of heating the sample to 900 $^{\circ}$C, then cooling it to 600 $^{\circ}$C and finally annealing it at 600 $^{\circ}$C for several hours. After the heat treatment, the thin films broke up into large nanoparticles (20-60 nm) surrounded by small nanoparticles (2 nm). Electron diffraction data showed that the annealed samples had an fcc structure. The maximum room temperature coercivity was found to be 2 kOe after 6h of annealing at 600 $^{\circ}$C. The high coercivity could be due to strain that was induced during precipitation. The evolution of crystal structure and microstructure with annealing will be monitored and related to the observed magnetic properties. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z31.00014: Magnetic Properties of As-Prepared and Annealed Nanocrystalline Fe Particles Karl Unruh, Thomas Ekiert Air stable Fe-core/oxide-shell particles with diameters between about 100 and 200 nm have been synthesized by the reduction of a ferrous Fe salt in the presence of citrate ions. Structural, chemical, and magnetic measurements indicate that the oxide shell is 2-3 nm thick and that the core consists of essentially oxide free, $\alpha $-Fe nanocrystals (about 5 nm in diameter) in addition to regions of non-crystalline, disordered Fe. The as-prepared particles evolve into a continuous porous solid structured at about the 100 nm scale after annealing in forming gas at temperatures near 750 K followed by a progressive elimination of the porosity at higher annealing temperatures. Prior to the formation of the porous solid the saturation magnetization, coercivity, and remanence ratio all increase slightly with annealing temperature due to an increase in the size of the core Fe crystallites at the expense of the disordered Fe component. The structural transformation to a porous solid, however, results in an abrupt increase in both the coercivity (by about 50{\%} at 300 K and 100 {\%} at 5 K) and remanence ratio (about 100{\%} at 300 K and 150{\%} at 5 K). [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z31.00015: Anisotropic Sm-Co(Fe) Nanoparticles Produced by Surfactant-Assisted Ball Milling Nilay Gunduz-Akdogan, George C. Hadjipanayis, David J. Sellmyer Magnetically hard SmCo$_{5}$ and Sm$_{2}$(Co$_{0.8}$Fe$_{0.2})_{17}$ nanoparticles have been produced by using surfactant assisted low- and high-energy ball milling. Surfactants prevent the re-welding of the crashed particles during the milling process and thus limit the particle growth. Oleic acid was used as the surfactant and the heptane as the milling medium. High energy ball milling experiments took place in a milling vial with carbon steel balls by using a Spex 8000M high energy ball milling machine. The coercivity was found to increase with milling time with a value of 2.9 kOe for Sm$_{2}$(Co$_{0.8}$Fe$_{0.2})_{17}$ and 19.5 kOe for SmCo$_{5}$ after 12 hrs of milling. TEM data showed that the milled powders have a narrow size distribution. The TEM grid-deposited samples showed self-assembled nanoparticles in the Sm$_{2}$(Co$_{0.8}$Fe$_{0.2})_{17 }$alloy after 4 hours of milling, which could be further aligned when subjected to a magnetic field. The evolution of structural and microstructural properties of the particles will be monitored and compared with their magnetic properties. [Preview Abstract] |
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