Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y37: Focus Session: Nanomagnetism -- Nanoparticles I |
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Sponsoring Units: DMP GMAG Chair: Paolo Vavassori, CIC Nanogune Room: E147-E148 |
Friday, March 19, 2010 8:00AM - 8:36AM |
Y37.00001: Charge-controlled magnetism in colloidal doped semiconductor nanocrystals Invited Speaker: Electrical control over the magnetic states of doped semiconductor nanostructures could enable new spin-based information processing technologies, but the relatively weak interactions between dopants and charge carriers have so far suggested that such gated magnetism will be limited to cryogenic temperatures. This talk will describe the observation of a large, reversible, room-temperature magnetic response to charge injection in free-standing colloidal ZnO nanocrystals doped with Mn(II) ions. Injected electrons are found to delocalize throughout the entire nanocrystal, and to activate new ferromagnetic Mn(II)-Mn(II) exchange interactions that are strong enough to overcome antiferromagnetic coupling between nearest-neighbor Mn(II) ions, making the full magnetic moments of all dopants observable upon charging. Removal of the electron causes the system to revert to its original form, allowing reversible charge-controlled manipulation of room-temperature nanocrystal magnetism. The physical properties of these charged, doped nanocrystals are directly analogous to those of bound magnetic poltroons (BMPs) postulated to underlie high-temperature ferromagnetic ordering in the bulk forms of this and related diluted magnetic oxides. This discovery of charge-controlled magnetism in free-standing colloidal nanocrystals that is large, reversible, and stable at room temperature presents new opportunities for fundamental studies and raises interesting possibilities for the development of spin-based information processing technologies from solution-processable semiconductor nanostructures. \textbf{Related references:} Ochsenbein, S. T.; Feng, Y.; Whitaker, K. M.; Badaeva, E.; Liu, W. K.; Li, X.; Gamelin, D. R., \textit{Nature Nanotechnology}, $4$, 681 \textbf{(2009)}; Liu, W. K.; Whitaker, K. M.; Kittilstved, K. R.; Gamelin, D. R., \textit{J. Am. Chem. Soc.}, $128$, 3910 \textbf{(2006)}. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y37.00002: ABSTRACT WITHDRAWN |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y37.00003: Magnetoresistance sign reversal in La1-xSrxMnO3 Nanoparticle samples Tao Lin, Zhiyong Wang, Jing Shi La1-xSrxMnO3 (LSMO) exhibits interesting properties such as half-metallic ferromagnetism which is desirable for spintronics. Here we report a study of electrical transport properties of LSMO nanoparticles. The LSMO nanoparticle samples are prepared by the liquid phase codeposition method. The temperature dependence of the resistance shows a peak at about 120 K which is the characteristic of the ferromagnetic transition. The magnetoresistance is measured over a wide range of temperatures, and its magnitude is $\sim $30{\%} at 1T at 20K. The bias dependence of the magnetoresistance is studied at low temperatures. We observe a sign change in the magnetoresistance at large currents, i.e. the magnetoresistance switches from negative to positive as the current exceeds a threshold. In the meantime, the resistance of LSMO undergoes an abrupt change from high to low. The sign reversal of the magnetoresistance is found to be correlated with the current-induced resistance state switching, which has not be found in LSMO films. We will discuss possible mechanisms of these effects. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y37.00004: Local Structures and Magnetic Properties of Co-doped Y2O3 Nanocrystals Yun-Liang Soo, C.S. Wang, S.L. Chang, P.P. Chu, J.F. Lee Local environments surrounding Co and Y atoms in Co-doped nanocrystalline Y2O3 powders were investigated by using extended x-ray absorption fine structure (EXAFS) technique. Thermal annealing at different temperatures was employed to control the particle size from 5nm to 134nm. Superconducting quantum interference device (SQUID) measurements revealed that these materials are ferromagnetic with various Curie temperatures and saturation magnetization. As the annealing temperature increases, saturation magnetization increases with decreasing coordination number of the nearest O shell around Co impurity atoms. However, the coordination number of O shell around Y atoms remains the same. From our experimental data, we propose that Co atoms in the Y2O3 nanocrystal host migrate from interstitial locations inside the nanoparticle to the particle surface, where more O vacancies are present. The indirect exchange interaction of the bound magnetic polaron model is therefore increased leading to enhanced ferromagnetim in the annealed samples. [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y37.00005: The impact of finite size effects on spin waves in CoO Mikhail Feygenson, Xiaowei Teng, Wenxin Du, Andrey Podlesnyak, Jennifer Niedziela, Mark Hagen, Meigan Aronson We studied the spin waves in nanoscaled CoO using inelastic neutron scattering. The zero-field measurements were carried out on Co/CoO nanoparticles, CoO nanoparticles, and the bulk powder of CoO in the temperature range of 15 -- 300 K. The temperature-dependent inelastic intensity at 2.5 meV, found in all samples, was ascribed to CoO spin waves. We observed an increase at least of factor of 100 in the inelastic intensity for Co/CoO as compared to the CoO bulk, and shift of intensity towards larger scattering vectors. We suggest that new boundary conditions imposed by the nanoparticle surface and the breaking of the symmetry are mainly responsible for this effect. Similar enhancement of the spin wave spectrum was also predicted in thin films [1,2]. [1] S. Reshetnyak et al PMC Physics B 2008 [2] Y.Gorobets et al, Tech. Phys. 1998 [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y37.00006: Antiferromagnetic exchange coupling measurements on single Co clusters deposited on CoO (111) layers Damien Le Roy, Wolfgang Wernsdorfer, Robert Morel, Ariel Brenac, Thierry Crozes, Lucien Notin, David J. Sellmyer We report on single-cluster measurements of exchange anisotropy in cobalt nanoclusters (4 nm) deposited on CoO (111) layers using a microbridge DC superconducting quantum interference device (microSQUID). Recent improvements of microSQUID technique enabled us to perform the first magnetization curves on single nanoclusters. Intrinsic anisotropy is investigated measuring the angular dependence of low-temperature cluster switching field. Co particles exhibit a Stoner-Wohlfarth behaviour. F/AF exchange coupling is investigated on an ideal system in which the ferromagnetic clusters are deposited on single AF domain CoO (111). The coupling induces modification in clusters anisotropy with strong exchange bias. A preferential orientation is observed with a bistable state that is attributed to a strutural anisotropy of CoO (111) layers brought to light by X-Ray diffraction. It is shown that the ferromagnet is strongly coupled with the antiferromagnet with a spin-flop configuration. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y37.00007: Enhancement of surface spin disorder in hollow NiFe$_{2}$O$_{4}$ nanoparticles G. Hassnain Jaffari, Abdullah Ceylan, Chaoying Ni, S. Ismat Shah We present synthesis and magnetic properties of hollow NiFe$_{2}$O$_{4}$ nanoparticles from core (Ni$_{33}$Fe$_{67})$/shell (NiFe$_{2}$O$_{4})$ structure based on Kirkendall effect. Morphology of the particles can be varied from hollow to solid NiFe$_{2}$O$_{4}$ nanoparticles by varying the reaction temperature and time of initial core/shell structure. Particles with hollow morphology are expected to show larger surface spin disordered or spin glass phase due to the presence of additional inner surface in addition to the usual outer surface in nanoparticles. Field cooled hysteresis loops exhibit significantly large shift due to unidirectional anisotropy resulting from the additional inner along with outer surface spin glass interface in particles with hollow morphology compared to particles with non-hollow (or solid) morphology that have only outer spin glass shell coupled with ferromagnetic core. The enhancement in the surface anisotropy is also noticeable which leads to an increase in the blocking temperature of the particles with hollow morphology. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y37.00008: Site Determination of Mn Doping in Protein Encapsulated $\gamma $-Fe$_{2}$O$_{3}$ Nanoparticles V. Pool, M. Klem, C. Jolley, T. Douglas, M. Young, E. Arenholz, Y.U. Idzerda In this study, Mn has been doped (0-33{\%}) into 6 nm, $\gamma $-Fe$_{2}$O$_{3}$ nanoparticles grown inside the horse-spleen ferritin (HSF) protein and compared to similarly protein encapsulated pure $\gamma $-Fe$_{2}$O$_{3}$ and Mn-oxide nanoparticles to determine the Mn doping site. By using soft-X-ray absorption spectroscopy (XAS), soft-X-ray magnetic circular dichroism (XMCD), and frequency dependent Alternating Current Magnetic Susceptibility (ACMS), we have ascertained that the Mn dopant is substituting preferentially as Mn$^{+2}$ and prefers the octahedral site in the $\gamma $-phase Fe$_{2}$O$_{3}$ spinel structure. The measured Mn L$_{23}$ XAS spectra are compared to measured reference powders and molecular-orbital calculations supporting this conclusion of the Mn dopant substitution site. We find that the Mn L$_{23}$ XAS multiplet structure for the nanoparticles is simpler than for our bulk standards, complicating this identification but suggesting that the nanoparticle lattices are relaxed from the distortions present in the bulk. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y37.00009: Probing magnetic anisotropy and exchange bias in coupled Au-Fe$_{3}$O$_{4}$ nanoclusters S. Chandra, N.A. Frey, M.H. Phan, H. Srikanth, C. Wang, S. Sun The study of magnetic anisotropy and exchange bias in coupled nanoparticle systems is of topical interest. We have demonstrated radio frequency (RF) transverse susceptibility (TS) using a sensitive, self-resonant tunnel-diode oscillator (TDO) technique developed by us to be excellent for probing magnetic anisotropy and exchange bias (EB) in Fe$_{3}$O$_{4}$ particles grown epitaxially on one or multiple facets of polyhedral Au seed particles forming dumbbell- or flower-shaped Au-Fe$_{3}$O$_{4}$ nanoclusters. TS experiments reveal a strong increase in magnetic anisotropy in coupled Au-Fe$_{3}$O$_{4}$ nanoclusters compared to pure Fe$_{3}$O$_{4}$ nanoparticles. TS experiments also probe a surface spin glass transition (T$_{F})$, a sharp increase in surface anisotropy at T$_{F}$, and a strong increase in EB with temperature below T$_{F}$ in the flower-shaped nanoclusters. Our RF susceptibility measurements are in good agreement with conventional AC and DC magnetometry. The influence of the Au interface(s) on the surface spin configuration of Fe$_{3}$O$_{4}$ nanoparticles will be discussed. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y37.00010: ABSTRACT WITHDRAWN |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y37.00011: Cluster Growth Mechanism in Sputtering Gas-Aggregation Nanocluster Source M. Tarsem Singh, H. Han, J.A. Sundararajan, Y. Qiang We have studied the influence of some parameters for cluster growth of core shell iron- iron oxide magnetic nanoparticles (MNPs). The nanocluster source which combines a magnetron sputtering gun with a gas aggregation chamber is used to produce MNPs. Nanoclusters of various mean sizes ranging from 1-100 nm can be synthesized by varying the aggregation distance, Ar to He gas ratio, pressure in the aggregation tube, sputter power, and temperature of the aggregation tube. Physical properties -- magnetic measurements by VSM and SQUID and size distribution by SEM and TEM were studied for different MNPs. The significance of this research is to understand the growth mechanism and physical properties as the size of particles grow from few nanometer to hundred of nanometer. Growth of the particles is theoretically explained by the homogenous and heterogeneous growth process. Based on this study, different size of MNPs fits into different category of applications from data storage to biomedical field. [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y37.00012: ABSTRACT WITHDRAWN |
Friday, March 19, 2010 10:48AM - 11:00AM |
Y37.00013: Magnetic properties of small iron clusters: Nanoscale Dynamical Mean-Field Theory analysis Alamgir Kabir, Talat S. Rahman, Volodymyr Turkowski We analyze the role of correlation effects in determining magnetic properties of small iron clusters (Fe$_2$-Fe$_5$) by using the Nanoscale Dynamical Mean-Field Theory (NDMFT) approach. In particular, we study the dependence of the magnetization of the system on temperature, Coulomb repulsion and geometrical structure, including the Jahn-Teller distortion effect. We analyze the dynamical correlation effects in the magnetic behavior of the system by comparing our results with some static approaches, which take into account correlation effects, including the GGA+U approximation. It is shown that the NDMFT approach improves the results obtained by these approaches in the case of moderate Coulomb repulsion energy when the atomic coordination number is large. We discuss possible generalization of the approach to study the magnetic properties of nanoclusters on magnetic and non-magnetic substrates and embedded in matrices. [Preview Abstract] |
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