Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D27: Focus Session: Magnetic Nanowires and Nanodots I |
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Sponsoring Units: GMAG DMP Chair: Paul Crowell, University of Minnesota Room: Morial Convention Center 219 |
Monday, March 10, 2008 2:30PM - 2:42PM |
D27.00001: The anomalous exchange bias effect in core-shell Co/CoO nanoparticles Mikhail Feygenson, Yiu Yuen, KiSub Kim, Meigan Aronson We study the anomalous exchange bias effect in Co/CoO nanoparticles by means of neutron and x-ray scattering and magnetic experiments. The Co nanoparticles were prepared in oleic acid by thermal decomposition of Co$_{2}$(CO)$_{8}$ and were subsequently oxidized. Co core- CoO shell nanoparticles with differing core and shell dimensions were obtained. The magnetic measurements indicated that there is an optimal ratio of the core and shell dimensions which maximizes the exchange bias field. Anomalous small angle x-ray scattering experiments using core-shell contrast and energy analysis provide high accuracy measurements of the core and shell, and their respective size distributions. Neutron diffraction measurements find that oxidation introduces a new modulation wave vector for the magnetization, leading to the increasing magnetic decompensation of the core-shell interface. It is our proposal that this interface moment enhances the exchange coupling of the core and shell, and leads to the extraordinarily large exchange bias effect. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D27.00002: Artificial Nanomagnet with Lateral Confinement Lifeng Yin, Noppi Widjaja, Jian Shen For artificially low dimensional nanodots, the exchange interaction and dipole-dipole magnetostatic interaction can no longer stabilize the long range magnetic order at finite temperature. However, the electron-mediated indirect exchange interaction could be enhanced at surfaces due to the symmetry broken. A collective ferromagnetic behavior in two-dimensional Fe dot assemblies grown on a single crystal Cu(111) surface has been reported$^{[1]}$. These Fe nanodots were grown using a novel method called buffer-layer assisted growth. The ferromagnetic ordering temperature appears to depend not only sensitively on the average spacing between the dots, but also strongly associated with the presence of surfaces states. The vicinal surfaces have been found a rich variety of novel behavior that results from broken translational symmetry by surface atomic steps. The presence of a free-electron-like Shockley surface state on the corresponding flat Cu(111) surface will be interrupted on vicinal surface. More interestingly, a switch between two qualitatively different regimes at a miscut of 7$^{o}$ takes place$^{[2]}$. In this work, a curve-polished Cu(111) (0$\sim $8$^{o}$ miscut) substrate is used to tune the surface electronic states, and in turn influences the electron-mediated indirect exchange interaction of Fe nanodots. [1] J. P. Pierce et al., Phys. Rev. Lett. 92, 237201 (2004). [2] J.E. Ortega et al., Phys. Rev. Lett. 84, 6110 (2000). [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D27.00003: Magnetic Correlations In A Magnetite Nanoparticle Assembly Investigated Using Polarized SANS Kathryn Krycka, Charles Hogg, Yumi Ijiri, Ryan Booth, Julie Borchers, Wangchun Chen, Mark Laver, Thomas Gentile, Brian Maranville, Benjamin Breslauer, Sara Majetich Using small angle neutron scattering (SANS) with polarization analysis, we have studied ferromagnetic magnetite monodisperse nanospheres in order to determine the field (0 and 1.3 Tesla) and temperature (50, 100, and 200 K) dependence of the magnetic interparticle correlations. These particles were 7 nm in diameter with an average edge-to-edge separation of 2.5 nm. Preparation techniques are described elsewhere [1]. An FeSi supermirror polarized the incident neutrons, and a polarized 3He cell was used as a spin analyzer. While a typical magnetic SANS experiment observes the convolution of the nuclear and magnetic terms, we have implemented and further developed an algorithm to separate the four spin dependent cross sections. This provides an unambiguous separation and measurement of magnetic and nuclear contributions. At low temperatures, magnetic correlation lengths have been found to be significantly larger than at high temperatures.\newline [1] J. Am. Chem. Soc. 2002, 124, 8204-8205. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D27.00004: Magnetic Nanocheckerboards with Tunable Sizes in the Mn-Doped CoFe$_{2}$O$_{4}$ Spinel Chenglin Zhang, C.M. Tseng, C.H. Chen, S. Yeo, Y.J. Choi, S.-W. Cheong In the Mn-doped CoFe$_{2}$O$_{4}$ spinel, a highly ordered array of two types of rectangular nanorods, $\sim $300 nm in length and a few nanometer in size, is achieved through chemical phase separation mediated by cooperative Jahn-Teller distortions. At room temperature, the magnetic nanorods with composition close to CoFe$_{2}$O$_{4}$ interlace with the paramagnetic counterparts and form a highly organized checkerboard pattern in the cross section. The checkerboard size, varying in the range of $\sim $3 nm and $\sim $80 nm, is tunable with composition as well as with the isothermal annealing time This may be of potential significance to the next generation magnetic storage. The magnetic nanocheckerboards exhibit a nearly ideal configuration for perpendicular recording media. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D27.00005: Magnetic interactions among Co nanowires Hafsa Khurshid, Michael Bonder, Gearoge Hadjipanayis Magnetic nanowires have recently gained much attention because of their potential applications in magnetic recording, sensors and other electronic devices. The magnetic properties of nanowires are determined by the competition between magnetocrystalline and shape anisotropy resulting from the reduced dimensionality of the system. In this study we present results on the magnetic interactions among arrays of electrodeposited Co nanowires as a function of inter-wire spacing and nanowire diameter. X-Ray diffraction and electron microscopy reveal that the nanowires exhibit the hexagonal closed packed polycrystalline structure. The direction of the magnetic easy axis is controllable as a function of wire diameter. By increasing the diameter of nanowires from 30 to 188 nm, the magnetic easy axis switches from perpendicular to parallel to the nanowire's major axis. This is further evidenced by a decrease in coercivity from 1200 to 100 Oe and a reduced loop squarness. The magnetic interactions were probed using delta M plots. All samples exhibit a negative delta M curve indicative of magnetostatic interactions. As the inter-wire spacing increases there is a broadening of the dipolar component of delta M plots indicating an increase in the switching field distribution. This work was supported by NSF Grant{\#} DMR-0302544. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D27.00006: Memory Effects and Inter-particle Interactions in Co Nanoparticles Embedded in Carbon Matrix Peng Liu, Michael Bonder, George Hadjipanayis In this work we have studied the magnetic properties of face-centered-cubic (fcc) Co nanoparticles made by the cluster gun. The zero field-cooled (ZFC) and field-cooled (FC) $M(T)$ curves at different fields show that the blocking temperature is shifted to lower temperature when the applied magnetic field is increased. This behavior could be due to a decreased energy barrier at increased filed or to inter-particle dipole-dipole interactions. M vs H/T data above the blocking temperature show that the latter might be responsible for this behavior. The dynamics of the FC magnetization were also studied. The $M(T)$ curves on FC samples obtained with the magnetic field on and off at different temperatures, show that the sample remembered its thermal history and demonstrated a memory effect at temperatures lower than the blocking temperature. However, this memory effects were not observed in the ZFC samples. The magnetic relaxation with a change at low temperature also shows a memory effect at temperature below the blocking temperature. The $M(T)$ curves at different fields and memory effects indicate that the dynamics of nanoparticles are due to the distribution of particle sizes and inter-particle interactions. Work Supported by NSF GRANT {\#} DMR-0302544. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D27.00007: Probing the effect of interparticle interactions in ferrite nanoparticles using the reversible transverse susceptibility method M.B. Morales, P. Poddar, N.A. Frey, H. Srikanth, S.A. Morrison, E.E. Carpenter Spin dynamics in magnetic nanoparticles is an issue of current interest. It is important to understand how interparticle interactions in a 3-dimensional arrangement of nanoparticles as well as their surface functionalization would affect the global magnetic response, in particular, the magnetic anisotropy. We report the influence of surface functionalization and systematic dipolar interactions strength variation on the magnetic properties of surfactant-coated monodispersed manganese zinc ferrite (Mn$_{0.68}$Zn$_{0.25}$Fe$_{2.07}$O$_{3})$ nanoparticles of 15 nm mean particle size using temperature and field-dependent reversible transverse susceptibility measurements at a 12 MHz resonant frequency. Our experiments reveal that the characteristic features in the transverse susceptibility --viz. the position, height and symmetry of the peaks at anisotropy fields -are extremely sensitive to interparticle interactions and surface chemistry of the nanoparticles. In contrast to earlier theoretical suggestions, our experimental results suggest that the transverse susceptibility technique can be used effectively even for strongly interacting magnetic nanoparticle systems. Work at USF supported by NSF through a GOALI grant from DMII. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D27.00008: Effects of magnetic interactions in Ni nanowire arrangements Ovidiu Trusca, Dorin Cimpoesu, Leonard Spinu, John B. Wiley, Jin Hee Lim Systems of magnetic nanowires are considered strong candidates for many technological applications. The main parameter controlling the frequency response of magnetic nanowires assemblies is their aspect ratio (length to diameter ratio) [1], that can be tuned by changing the dimensions of wires. We modified the nanowires aspect ratio by keeping constant the length and changing the diameter. This required designing templates of different diameters with the same average distance between the pores. Two sets of Ni nanowires samples with diameters of 40, 60, 80 nm and constant length of 500 and 1000 nm respectively, obtained by electrodeposition, were studied using X-band ferromagnetic resonance measurements at room temperature. The two series of samples are ideally candidates for verifying the models recently proposed to describe the interactions in such systems [2]. As the nanowire's diameter increases, the peak observed in the angular dependence of the FMR resonant field diminishes. [1] A. Fert, L. Piraux, J. Magn. Magn. Mater, vol. 200, pp. 338-358, 1999. [2] I. Dumitru, et al., IEEE Trans.Mag 42(10), 3225, 2006. Work supported by Louisiana Board of Regents Contract {\#}LEQSF(2007-12)-ENH-PKSFI-PRS-04. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D27.00009: Fabrication of GaMnAs Magnetic Semiconductor Nanodot Arrays S. Bennett, L. Menon, D. Heiman Ordered arrays of GaMnAs ferromagnetic semiconductor nanodots were fabricated using anodic porous alumina templates as etch masks. In this study we used nanochannel porous alumina membranes as masks for thermal evaporation coupled with reactive ion etching for the fabrication of organized hexagonal arrays of both manganese doped semiconductor and metal alloy comprised nanodots. The GaMnAs nanodots have diameters $\sim $40nm and dot periodicity of $\sim $80 nm. Field-cooled and zero-field-cooled magnetization measurements demonstrate that the dots are superparamagnetic at room temperature with a blocking temperature of T=30 K, below which they are ferromagnetic. This illustrates that arrays of uniform and highly-ordered nanodots can be fabricated inexpensively, rapidly and over large length scales for semiconductors which cannot be formed by techniques of self-assembly. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D27.00010: Synthesis of surface functionalized magnetic nanoparticles and their polymer nanocomposites M.J. Miner, M.B. Morales, P. Poddar, H. Srikanth, S.M. Skidmore, T.M. Weller Magnetic nanoparticles embedded in polymer matrices are good examples of functional nanostructures with excellent potential in applications such as tunable microwave devices, EMI shielding, flexible electronics etc. Control over the dispersion of the nanoparticle phase embedded in a polymer matrix is critical and often challenging. To achieve excellent dispersion, competition between polymer-polymer and polymer-particle interactions must be balanced to avoid clustering of particles in polymer nanocomposites. In earlier work, we had demonstrated the successful synthesis of 2$\mu $m thick spin coated nanocomposite PMMA films with Fe$_{3}$O$_{4}$ (mean size 15nm) nanoparticle inclusions exhibiting superparamagnetic behavior. In this work, we will present our attempts to achieve thicker films more suitable for microwave applications and a study of the role of surface functionalization of ferrite nanoparticles synthesized using co-precipitation and hydrothermal routes. Cross-sectional SEM and TEM studies as well as magnetic characterization using a Physical Property Measurement System will be presented and discussed. We will also report on the microwave response of these films using a coplanar waveguide fixture. Work at USF supported by NSF through a GOALI grant from NSF-DMII. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D27.00011: Magnetic behavior of PdNi nanowires and extended thinfilms as a function of the film thickness Juan-Carlos Gonzalez-Ponz, John Henderson, Enrique Del Barco, Barbaros \"Ozyilmaz Recently Pd$_{1-x}$ Ni$_{x}$ alloy has attracted considerable attention as ferromagnetic electrodes in carbon based lateral spin valves. Its wetting properties on carbon nanotubes (CNT) leads to transparent contacts, while its room temperature ferromagnetic behavior provides a means for spin injection. Surprisingly, in the case of CNTs a tunneling barrier between the PdNi and the CNT is unnecessary for spin injection, making PdNi ideal for electrodes in carbon-based electronic devices. Here we report studies of both the anisotropic magneto-resistance (AMR) of PdNi nanowires with varying widths and the ferromagnetic resonance (FMR) behavior of PdNi thin films with varying thickness. The AMR revealed strong angular field dependence with respect to the nanowire, indicating magnetization tilted out of the plane of the wire. The tilt angle decreases with increasing the nanowire width. Room temperature broad-band (5-50GHz) FMR measurements of extended films show in-plane magnetization and out-of-plane uniaxial anisotropy ($K)$, which is not large enough to overcome the demagnetization energy. We speculate that the constriction of a dimension in the film plane modifies the demagnetization factors allowing the out-of-plane anisotropy to push the magnetization out of the plane for small nanowire width. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D27.00012: Synthesis and Characterization of Iron-Nickel (Fe$_{x}$Ni$_{1-x})$ Nanowires Rakesh Shah, Igor Dubenco, Amelia Church, Xianfeng Zhang, Shane Stadler, Saikat Talapatra, Naushad Ali Electrochemical deposition method was used to synthesize ordered arrays of Fe$_{x}$Ni$_{1-x}$ (25 $<$ x $<$ 85) nanowires into porous anodic alumina template. These nanowires were structurally characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). An estimate regarding the elemental composition of the nanowires was obtained by Energy dispersive spectroscopy (EDS). X-ray diffraction studies revealed that the nanowires exhibit a phase transition from face-centered-cubic (FCC) to base-centered-cubic (BCC) crystal structure with increasing iron concentration. The effect of the variation of the compositional ratio of iron and nickel on the magnetic properties of the nanowires will also be presented. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D27.00013: Growth of Co nanoclusters on rutile TiO2 (110) surface En Cai, Xuewen Wang, Jiandi Zhang Fabrication of magnetic nanodots with uniform size and density is one of the key issues of studying the structure and property of a nanomagnet system. We report here our studies on the growth of Co nanoclusters on rutile TiO$_{2}$ (110) surface. Well ordered TiO2 (110) surface is prepared in the UHV chamber via Ar$^{+}$ sputtering and annealing. Co deposition is carried out in situ by molecular beam epitaxy and characterized with STM. Growth parameters are tuned to optimize the uniformity of dot size and density. Co dot coverage, size and density are investigated as functions of deposition rate and time as well as post-annealing temperature. Our results show that uniformity of the dots mainly depends on deposition rate, and the density of the dots primarily depends on the coverage of the dots, while the size of the dots depends mainly on the deposition rate and post-annealing temperature instead of coverage. Growth mechanism will be discussed. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D27.00014: Tuning of the magnetocrystalline anisotropy in Co$_x$Fe$_{3-x}$O$_4$ nanoparticles through cobalt doping Ronald Tackett, Sudakar Chandran, Ratna Naik, Gavin Lawes, Corneliu Rablau, Prem Vaishnava We report on the effect of cobalt doping on the magnetocrystlline anisotropy of Co$_x$Fe$_{3-x}$O$_4$ nanopaticles. The Co$_x$Fe$_{3-x}$O$_4$ ($0 \leq x \leq 0.15$) nanoparticles were synthesized through the coprecipitation of ammounium hydroxide in an environment of Fe$^{2+}$, Fe$^{3+}$, and varying concentrations of Co$^{2+}$. The size and crystallinity were confirmed using transmission electron microscopy, with a mean size of 17 $\pm$ 4 nm which was found to be constant across the different cobalt dopings. The magnetic properties were investigated through the use of dc and ac magnetic susceptibility, with the effective magnetocystalline anistropies being extracted from these data. The effective magnetocrystalline anisotropy found from each method were, within acceptable experimental error, found to agree, as well as increase linearly with cobalt doping. The effective anisotropy values were found to increase in magnitude by 100$\%$ as the cobalt fraction was increased from $x = 0$ to $x = 0.1$. This trend allows for the tuning of the magnetic isotropy of iron oxide nanoparticles through cobalt doping. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D27.00015: Preparation of cobalt-ferrite nanoparticles within a biopolymer template Marco Garza, Virgilio Gonz\'alez, Alejandro Torres-Castro, Mois\'es Hinojosa, Ubaldo Ort\'iz Using an in-situ co-precipitation reaction from solid dissolutions of stoichiometric amounts of Fe (III) and Co (II) inorganic salts, it was prepared highly loaded nanocomposites (as high as 75{\%} w/w) of cobalt-ferrite nanoparticles within a chitosan matrix, with particle size of about 7 nm, narrow particle size distribution and superparamagnetic character. Nanocomposite samples were characterized by high resolution transmission electron microscopy (HRTEM), UV-vis spectrometry and magnetic measurements by SQUID, using magnetization-field dependent, M(H), and magnetization-temperature dependent, M(T), studies. [Preview Abstract] |
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