Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B14: Focus Session: Magnetic Nanoparticles I |
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Sponsoring Units: GMAG DMP Chair: Dario Arena, Brookhaven National Laboratory Room: 316 |
Monday, March 18, 2013 11:15AM - 11:51AM |
B14.00001: Recent advances in magnetic nanoparticles with bulk-like properties Invited Speaker: Xavier Batlle Magnetic nanoparticles (NP) are an excellent example of nanostructured materials and exhibit fascinating properties with applications in high-density recording and biomedicine. Controlling the effects of the nanostructure and surface chemistry and magnetism at the monolayer level have become relevant issues. As the size is reduced below 100 nm, deviations from bulk behavior have been attributed to finite-size effects and changes in the magnetic ordering at the surface, thus giving rise to a significant decrease in the magnetization and increase in the magnetic anisotropy. The existence of a surface spin glass-like state due to magnetic frustration has been widely suggested in ferrimagnetic NP [1]. However, in this talk, we will show that high crystal quality magnetite Fe$_{\mathrm{3-x}}$O$_4$ NP of about a few nanometers in diameter and coated with different organic surfactants [2] display bulk-like structural, magnetic and electronic properties. Magnetic measurements, transmission electron microscopy, X-ray absorption and magnetic circular dichroism and Monte Carlo simulations, evidenced that none of the usual particle-like behavior is observed in high quality NP of a few nm [3]. Consequently, the magnetic and electronic disorder phenomena typically observed in those single-phase ferrimagnetic NP should not be considered as an intrinsic effect. We also performed a real-space characterization at the sub-nanometer scale, combining scanning transmission electron microscopy, electron energy loss spectroscopy and electron magnetic chiral dichroism. For the first time, we found that the surface magnetization is as high as about 70{\%} of that of the core [4]. The comparison to density functional theory suggested the relevance of the strong surface bond between the Fe ions and the organic surfactant. All the foregoing demonstrates the key role of both the crystal quality and surface bond on the physical properties of ferrimagnetic NP and paves the way to the fabrication of the next generation of NP with optimal magnetic properties [5]. Some bio-applications will also be discussed [6]. \\[4pt] In collaboration with A Labarta, N Perez, O Iglesias, A Fraile, C Moya(U Barcelona); A Roca, MP Morales, CJ Serna (ICMM-CSIC); F Bartolome, LM Garcia, J. Bartolome (CSIC-U Zaragoza); R Mejias, DF Barber (CNB-CSIC); M Varela, J Gazquez, J Salafranca, SJ Pennycook (ORNL), ST Pantelides (Vanderbilt U).\\[4pt] [1] X. Batlle, A. Labarta, J.Phys.D 35,R15 (2002) [2] P. Guardia, Langmuir 26,5843 (2010) [3] N. Perez, Appl.Phys.Lett. 94,093108(2009) [4] J. Salafranca, NanoLetters 12,2499 (2012) [5] X. Batlle, J.Appl.Phys. 109,07B524 (2011) [6] R. Mejias, Nanomedic. 5,397 (2010) [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B14.00002: Ligand effects on the electronic structure and magnetism of magnetite surfaces Katarzyna Brymora, Florent Calvayrac We address the effect of functionalization on the electronic and magnetic properties of magnetite surface as an indicator of the same properties in nanoparticles too big for a direct ab-initio approach. Using well-established methods and references (namely LDA+U on magnetite surfaces) we could verify the validity of our approach, and using two typical ligands, dopamine and citrate, namely $\pi$ and $\sigma$ electron donors, we could predict that those ligands would induce a different change in the electronic properties of the systems, but in both cases an enhancement of magnetization. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B14.00003: Synthesis, characterization, and fabrication of magnetic nanoparticles for low energy loss applications Hongseok Yun, Jun Chen, Vicky Doan-Nguyen, James Kikkawa, Christopher Murray It is important to increase operating frequency of power electronics for miniaturization of components. Magnetic materials are used as inductor cores to increase inductance proportional to their magnetic permeability. However, traditional magnetic materials are not used at high frequency (\textgreater 100MHz) because of large hysteresis and eddy current loss. Superparamagnetic nanoparticles are good candidates to resolve these problems because they have zero hysteresis loss. In addition, eddy currents can be reduced due to their high electric resistivity originating from the organic ligands on the surface. Magnetic nanoparticles such as NiFe$_{2}$O$_{4}$, Ni$_{1-x}$Zn$_{x}$Fe$_{2}$O$_{4}$, MnFe$_{3}$O$_{4}$ and ZnFe$_{2}$O$_{4}$ have been synthesized~via~high temperature thermal decomposition method and can be tuned to desired size, shape and chemical composition. To understand structural and magnetic properties of nanoparticles, the nanoparticles have been characterized by TEM, SQUID, PPMS, and Network Analyzer. UV-induced polymerization and pressing method have been implemented for film deposition. Finally, AC susceptibility of the nanoparticle film have been measured and discussed for low energy-loss applications. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B14.00004: Size-dependent optical properties of $\alpha$-Fe$_{2}$O$_{3}$ nanoparticles K.R. O'Neal, B.S. Holinsworth, P. Chen, J.L. Musfeldt, J.M. Patete, S.S. Wong, S.A. McGill We investigated the variable temperature optical properties of nanoscale hematite ($\alpha$-Fe$_{2}$O$_{3}$) with special attention to the parity-forbidden Fe$^{3+}$ $\textit{d-d}$ excitation that is activated by hybridization and symmetry-breaking phonons. An oscillator strength analysis of the rhombohedra, cubes, and rice reveals that the energy of the coupling phonon scales as (size)$^{-1}$. Moreover, preliminary work in high magnetic fields shows a field-induced color change. These findings are important for more deeply understanding finite length scale effects in this iconic material and other nanoscale transition metal oxides. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B14.00005: Chemical Synthesis and Physical Characterization of Hexagonal Ni Nanoparticles John Klodnicki, Brian Kelly, Karl Unruh Elemental Ni nanoparticles with a hexagonal close packed (HCP) crystal structure have been prepared by the reduction of nickel acetate in diethylene glycol (DEG) without the addition of any other reactants. No metallic Ni was formed at a reaction temperature of 195 $^{\circ}$C. At a reaction temperature of 210 $^{\circ}$C a two phase mixture of face centered cubic (FCC) and HCP Ni was obtained. With increasing temperature, the ratio of HCP to FCC Ni increased until at 245 $^{\circ}$CC (i.e. the boiling temperature of DEG) the reaction product was entirely HCP. The structural and magnetic properties of the HCP Ni were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and vibrating sample magnetometry (VSM) measurements. The SEM measurements revealed the presence of approximately spherical particles about 500 nm in diameter, as well as a number of rod-like structures. Based on a Rietveld-type analysis of the HCP Ni, best fit lattice parameters of a$=$0.26473(6) and c$=$0.43348(10) nm were obtained. Room temperature VSM measurements revealed a small magnetic moment of about 2 emu/g. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B14.00006: Synthesis and Characterization of CoNi and FeCo Nanowires With High Coercivity J. Ping Liu, Narayan Poudyal, Kinjal Gandha Ferromagnetic nanocrystals with shape anisotropy have drawn a great attention in the past decade because of their unique magnetic properties and potential applications in magnetic recording media and high performance nanocomposite magnets. CoNi and FeCo nanocrystals with different size, shape and composition were successfully synthesized via catalytic and non-catalytic chemical solution methods. It was found that the structure and morphology of the nanocrystals can be controlled by varying synthetic parameters such as solvent amount, catalyst and surfactant concentration, and heating rate. The length of the nanowires can be adjusted by changing the catalyst concentration. It has also been observed that the growth mechanisms for FeCo and CoNi nanowires are different. Magnetic properties of the FeCo and CoNi nanocrystals including coercivity and magnetization are found to be dependent on size, shape and composition of the nanowires. By optimizing the synthesis conditions, the FeCo and CoNi nanowires with enhanced magnetization and coercivity can be obtained. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B14.00007: Structural origin of low temperature glassy relaxation in magnetic nanoparticles Suvra Laha, Rajesh Regmi, Gavin Lawes Magnetic nanoparticles often exhibit glass-like relaxation features at low temperatures. Here we discuss the effects of doping boron, cobalt, gadolinium and lanthanum on the low temperature magnetic properties of Fe$_3$O$_4$ nanoparticles. We investigated the structure of the nanoparticles using both X-ray diffraction and Raman studies, and find evidence for secondary phase formation in certain samples. We acquired Transmission Electron Microscopic images to give direct information on the morphology and microstructure of these doped nanoparticles. We measured the ac out-of-phase susceptibility ($\chi^{//}$) vs temperature (T) to parameterize the low temperature glassy magnetic relaxation. All samples show low temperature magnetic relaxation, but the amplitude of the signal increases dramatically for certain dopants. We attribute these low temperature frequency-dependent magnetic relaxation features to structural defects, which are enhanced in some of the doped Fe$_3$O$_4$~nanoparticles. These studies also confirm that the low temperature relaxation in nanoparticles arises from single particle effects and are not associated with interparticle interactions. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B14.00008: Magnetic relaxation in dipolar magnetic nanoparticle clusters Ondrej Hovorka, Joe Barker, Roy Chantrell, Gary Friedman Understanding the role of dipolar interactions on thermal relaxation in magnetic nanoparticle (MNP) systems is of fundamental importance in magnetic recording, for optimizing the hysteresis heating contribution in the hyperthermia cancer treatment in biomedicine, or for biological and chemical sensing, for example. In this talk, we discuss our related efforts to quantify the influence of dipolar interactions on thermal relaxation in small clusters of MNPs. Setting up the master equation and solving the associated eigenvalue problem, we identify the observable relaxation time scale spectra for various types of MNP clusters, and demonstrate qualitatively different spectral characteristics depending on the point group of symmetries of the particle arrangement within the cluster -- being solely a dipolar interaction effect. Our findings provide insight into open questions related to magnetic relaxation in bulk MNP systems, and may prove to be also of practical relevance, e.g., for improving robustness of methodologies in biological and chemical sensing. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B14.00009: Reentrant superparamagnetism induced by spin glass behavior at the surfaces of magnetic nanoparticles Wei Qin, Xiaoguang Li, Yi Xie, Zhenyu Zhang Superparamagnetism appears when the Neel relaxation time of magnetic nanoparticles is shorter than the measurement time. Recent experimental studies of different types of magnetic nanoparticles revealed that superparamagnetic transitions could also take place below the blocking temperatures [1-3], an intriguing phenomenon tentatively termed as quantum superparamagnetism. Here we elucidate the microscopic origin of the reentrant superparamagnetism in such systems using a phenomenological model, which emphasizes the dynamical coupling between the ferromagnetic core and the spin glass surface layer of a given nanoparticle [4]. We first obtain expressions for the thermal relaxation of the total magnetization of the particle upon finite-field and zero-field cooling, then carry out numerical simulations using physically realistic materials parameters. Our findings provide a more plausible interpretation of the observed reentrant superparamagnetism beyond the previous macroscopic quantum tunneling picture.\\[4pt] [1] C. T. Hsieh, J. T. Lue, Phys. Lett. A 316 329 (2003)\\[0pt] [2] W. W. Zheng, P. Kumar, A. Washington, Z. X. Wang, et al, J. Am. Chem. Soc. 134 2172 (2012).\\[0pt] [3] C. Xiao, J. J. Zhang, Y. Xie, Sci. Rep. 2 755 (2012).\\[0pt] [4] R. H. Kodama, A. E. Berkowitz, Phys. Rev. Lett. 77 2 (1996). [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B14.00010: Voltage-controlled spin transport through a pair of Buckminster fullerene molecules encapsulating cobalt atoms Alireza Saffarzadeh, George Kirczenow Carbon-based nanostructures such as fullerenes, carbon nanotubes, and graphene, are promising candidates for spintronic applications because of their weak spin-orbit coupling and hyperfine interaction which lead to long spin coherence lengths. In particular, a fullerene C$_{60}$ molecule is an interesting carbon nanostructure which can be used as a molecular bridge in magnetic tunnel junctions due to its remarkable structural stability and electronic properties which make the molecule convenient for easier spin injection in magnetic nanojunctions. Here, we show that using cobalt atoms encapsulated in a pair of Buckminster fullerene molecules sandwiched between gold electrodes, density of states spin polarizations as large as 95\% are found by varying the gate and/or bias voltage, due to the spin-splitting of Co $3d$ orbitals. The current-voltage characteristics and strong (up to 100\%) spin polarization of the current indicate that the device can be utilized for highly efficient spin injection into nonmagnetic conductors. These results open the way to voltage-controlled spin filters and magnetic sensors using molecular magnetic junctions. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B14.00011: Magnetization Study of Sulfur-doped Graphitic Nano-platelets and Single Walled Carbon Nanotubes J. Zhu, L. Oliveira, R. Podila, S. Neeleshwar, Y.Y. Chen, J. He, M. Skove, A.M. Rao Recently we investigated the magnetic behavior of as-prepared and sulfur doped chemically exfoliated graphene nano-platelets (GNPs) and single walled carbon nanotubes (SWCNTs). The doping was achieved by annealing desired carbon nanostructures with 0, 1.0, 1.5 and 3 at{\%} sulfur in an evacuated quartz tube at 1000 $^{\circ}$C for 1 day, followed by multiple rinsing in alcohol and drying in vacuum to remove excess sulfur. The isothermal M vs. H as well as the temperature-dependent M vs. T measurements were obtained using a vibrating sample magnetometer. We found that sulfur doping drastically changes the magnetic behavior of the as-prepared samples (both SWCNTs and GNPs). The results of zero-field-cooling (ZFC) and field-cooling (FC) in M vs. T measurements indicated the existence of large amount of coupled super-paramagnetic domains, along with antiferromagnetic domains. The saturation magnetization decreased in S doped GNPs, while a contrasting trend was observed in S doped SWCNTs. The role of edge states and structural defects in carbon nanostructures in the observed magnetic properties will be discussed. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B14.00012: Enhanced Magnetic Properties in Nanoparticle-Filled CNTs K. Stojak, S. Chandra, H. Khurshid, M.H. Phan, H. Srikanth There has been much interest in magnetic polymer nanocomposites (MPNCs) recently due to potential applications for EMI shielding, tunable EM devices and flexible electronics. In past studies, using ferrite fillers, we have shown MPNCs to be magnetically tunable when passing a microwave signal through films under the influence of an external magnetic field. We extend this study to include nanoparticle-filled multi-walled carbon nanotubes (CNTs) synthesized by CVD. These high-aspect ratio magnetic nanostructures, with tunable anisotropy, are of particular interest in enhancing magnetic and microwave responses in existing MPNCs. CNTs have an average diameter and length of 300nm and 6 $\mu$m, respectively and are partially filled with CoFe$_{2}$O$_{4}$ and NiFe$_{2}$O$_{4}$ nanoparticles (NPs) ($\sim$ 7nm). When comparing NPs to NP-filled CNTs, $T_{B}$ increases by $\sim$ 40K and relaxation time, $\tau _{\mathrm{0}}$, increases several orders of magnitude, indicating that enclosing NPs in CNTs enhances interparticle interactions. Structural and magnetic characterization were completed using XRD, TEM and Quantum Design PPMS, using VSM and ACMS options. [Preview Abstract] |
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