Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures V |
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Sponsoring Units: DMP GMAG Chair: Jian Shen, Oak Ridge National Lab Room: LACC 150B |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L42.00001: Intergranular Exchange in Magnetic Nanostructures Invited Speaker: Exchange interactions determine not only atomic-scale properties such as the Curie temperature but are also paramount to the realization of mesoscopic magnetism. Nanoscale exchange reflect the relativistic origin of magnetism. On an atomic scale, interatomic exchange tends to be much stronger than magnetic interactions, but the quadratic wave-vector dependence of the exchange energy makes magnetic interactions competitive on a nanoscale. The corresponding characteristic length scale is $a_{o}$/\textit{$\alpha $} = 7.252 nm, where $a_{o}$ is the Bohr radius and \textit{$\alpha $} = 1/137 is Sommerfeld's fine structure constant. In homogeneous solids, the competing relativistic and nonrelativistic interactions determine, for example, the thickness of domain walls. In nanostructures, the situation is more complex, because mesoscopic and atomic exchange effects interfere with structural length scales. This is important in many areas of magnetism, such as permanent magnetism, soft magnetism, spin electronics, and magnetic recording. (For a recent review, see Skomski, J. Phys. CM, vol. 15, 2003, p. R841.) From an atomic point of view, local magnetic moments embedded in an itinerant electron gas are coupled by RKKY-type interactions, whose oscillatory period is determined by the Fermi wave vector $k_{F}$. First, RKKY interaction between embedded clusters or particles do not average to zero but actually \textit{increase} with particle size. Second, the low carrier densities of semimetals and semiconductors yield small Fermi wave vectors and nanoscale oscillation periodicities. From a mesoscopic point of view, traditional random-anisotropy scaling amounts to a dimensionless coupling constant $A$/$K_{1}R^{2}$, but this expression fails to account for important real-structure features. For example, grain boundaries with reduced interatomic exchange give rise to a quasi-discontinuity of the magnetization, create a magnetization perturbation that extends far into the bulk, and modify scaling relations for the coercivity and other quantities. Additional complexity is due to finite-temperature excitations. Nanostructuring has little or no effect on the Curie temperature, but strongly affects the temperature dependence of the coercivity and the magnetization dynamics. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L42.00002: Classifying magnetic interactions in nanoparticle assemblies using Cole-Cole plots Wolfgang Kleemann, Oleg Petracic, Andreas Glatz, Subhankar Bedanta, Xi Chen Measuring the magnetic ac susceptibility, $\chi = \chi' - i \chi''$, of nanoparticle assemblies leads to important conclusions about the role of inter-particle interactions. We demonstrate that the Cole-Cole plot, $\chi''$ vs. $\chi'$, can be used as a tool for classifying the magnetic behavior of interacting or non-interacting nanoparticles. Distinguishing non-interacting superparamagnetic from superspin glass (SSG) or superferromagnetic (SFM) behavior is easily possible. We performed measurements of the magnetization hysteresis and ac susceptibility on both SSG and SFM granular multilayers [Co$_{80}$Fe$_{20}$/Al$_{2}$O$_{3}$] $_n$. The SFM results are successfully compared to simulations of a driven domain wall in a random medium. \noindent Work supported by the DFG, Alexander von Humboldt Foundation and DAAD. [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L42.00003: Transport spectroscopy of Kondo quantum dots coupled by RKKY interaction L.I. Glazman, M.G. Vavilov We develop the theory of conductance of a quantum dot which carries a spin and is coupled via RKKY interaction to another spin-carrying quantum dot. The found dependence of the differential conductance on bias and magnetic field at fixed RKKY interaction strength may allow one to distinguish between the possible ground states of the system. Transitions between the ground states are achieved by tuning the RKKY interaction, and the nature of these transitions can be extracted from the temperature dependence of the linear conductance. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L42.00004: Infrared Reflectivity of Metal Transition Granular Films N.E. Massa, F.P. De La Cruz, J. Denardin, L. Socolosky, M. Knobel, X. X. Zhang We present infrared reflectivity of 550 nm granular films made of transition metals embedded in a SiO$_{2}$ amorphous matrix. TM$_{x}$(SO$_{2}$)$_{1-x}$(TM=Fe, Ni, Co),(0.25$ \leq $x$ \leq $0.85) display giant magnetoresistance and giant Hall effect slightly above the percolation threshold.Our samples yield spectra typical of conducting oxides where carrier localization, depending on concentration, is triggered by the nanoparticles roughness and SiO$_{2}$. They have a distinct Drude component raising in intensity as the concentration and/or size of TM nanocrystallites increases Thus, while granular films of Fe and Ni, x$\sim$ 0.85, above the percolation threshold, have a reflectivity with a tail extending beyond 0.8 eV, characteristic of carrier hopping conductivity (small polaron localization), reducing the size and amount of the transition metal crystallites, as for x$\sim$0.35, the spectra have mid-infrared bands known for a localized polaron scenario with a very strong longitudinal optical-electron interaction. Our results support a model involving transition metal d-orbitals hybridizing oxygen p orbitals, yielding, as the crystallites get closer, the crossover between semiconducting and metal-like behavior within the context of a mixed phase environment. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L42.00005: Extraordinary Hall Effect in (Ni80Fe20)x(SiO2)1-x Thin Films Hui Liu, Fuk Kay Lee, Rong Kun Zheng, X.X. Zhang, Ophelia K.C. Tsui The extraordinary Hall effect (EHE) in ferromagnetic samples is generally attributed to scatterings of iterant electrons in the presence of spin-orbit interactions. In this work, study on the thickness dependence of the EHE in the $(\mbox{Ni}_{\mbox{80}} \mbox{Fe}_{\mbox{20}} )_x (\mbox{SiO}_{\mbox{2}})_{1-x} $ system showed the spontaneous Hall resistivity, $\rho _{sy}^S$, to be a constant while the Hall coefficient, $R_S (\equiv \rho _{sy}^S /M_S$ where $M_S $ is the saturated magnetization) increased monotonically owing to a depression in $M_S $. We propose the constancy of $\rho _{sy}^S $ with reducing thickness to arise from the sample morphological structure becoming two-dimensional with decreasing film thickness, expected from classical percolation theory. We also find in this system with varying $x$ that $\rho _{sy}^S \propto \rho _{xx}^\gamma $, with $\gamma =0.53$ to 1 in disagreement with the value 2 frequently attributed to the side jump effect, but explainable in terms of the more general form $\rho _{sy}^S =\rho _{xx} \Delta y_e /\Lambda _{SO} $, where $\Delta y_e $ is the side jump displacement and $\Lambda _{SO} $ the spin-orbit mean-free-path. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L42.00006: Interaction effects in heterostructures of nanoscale magnetic particles and magnetic thin films Jens Mueller, Stephan von Molnar, Yuzo Ohno, Hideo Ohno Elongated magnetic nanoparticles attract continuing attention both because of potential technological applications such as high-density information storage. Particles of 5 - 15 nm in diameter may be grown by STM assisted CVD, an advantageous technique for exact positioning of individual Fe particles on different substrate materials. A first step towards an ultimate application of these small and local magnetic flux sources to intentionally influence and investigate other materials is to study heterostructures of magnetic particles and an underlying magnetic film. Growing arrays or individual particles onto a magnetic thin film strongly enhances interactions between adjacent particles. Also, the particles alter the magnetic domain structure of the magnetic thin film making the transport properties of the latter sensitive to the magnetization state of the particles grown on top. We will present magnetization measurements of magnetic nanoparticles/thin-film heterostructures with permalloy and the concentrated magnetic semiconductor EuS as film material. These results will be compared with measurements of non-interacting particles grown onto a non-magnetic gold film. We also studied the effect of the particle's magnetization state on the transport properties of a magnetic permalloy film. We find a distinct (negative) switching effect in the magnetoresistance that persists up to room temperature. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L42.00007: Magnetic multilayers on nanospheres Frank Treubel, Ildico Guhr, Till Ulbrich, Johannes Boneberg, G\"unter Schatz, Manfred Albrecht, Guohan Hu Nanoparticle media using arrays of monodisperse nanoparticles with high magnetic anisotropy are assumed to be the ideal future magnetic recording media [1]. However, key requirements like control of the magnetic anisotropy orientation along with magnetic domain isolation have not been achieved so far. Here, we report on a combination of a two-dimensional topographic pattern formed of self-assembled polystyrene particles [2] with sizes as small as 20 nm and magnetic film deposition. The so formed nanostructures on top of a sphere are monodisperse and reveal a uniform magnetic anisotropy which can be tailored by changing the stack of a Co/Pd multilayer film and the deposition angle. Magnetic exchange isolation depends strongly on the total film thickness and the particle size as observed by MFM imaging and MOKE studies. Moreover, results on the switching mechanism as a function of nanostructure size will be presented. [1] M. Albrecht et al., Physik Journal, 10 (2003) [2] F. Burmeister et al., Appl. Surf. Sci. 144-145, 461 (1999) [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L42.00008: Observation of an anisotropy-induced antiparallel-parallel switching at the interface of Fe3O4/Mn3O4 superlattice on MgO(011) Gung Chern, Y.C. Wang An anisotropy-induced magnetic phase transition is first time observed from magnetization vs. field measurement in an antiferromagnetic coupled Fe3O4/Mn3O4 superlattice on MgO(011). Relative to a twisted phase transition previously found in isotropic layer systems, the present transition only occurs along the easy axis in the plane. An abrupt increased magnetization associated with the on-set of the transition corresponds to a direct switching of spin from an antiparallel state to a parallel state at the interface. Large magnetic hysteresis associated with the spin switching are observed on H greater than 0 and H smaller than 0 and thus 4 stable magnetization stages exist in the present system. The critical external field provides a direct estimate of the anisotropy energy of the superlattice. Magnetic hysteresis curves measured at various temperatures further provide a quantitative understanding of the interface coupling of Fe3O4/Mn3O4 superlattices. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L42.00009: Kondo physics with organic molecules Violeta Iancu, Aparna Deshpande, Alexander Govorov, Saw-Wai Hla The presence of a magnetic impurity on a nonmagnetic metal substrate is known to give rise to a Kondo resonance in the local density of states (LDOS) of the sample. We have carried out a low temperature STM study on a Co-porphyrin molecule, 5, 10, 15, 20-Tetrakis-(4-bromophenyl)-porphyrin-Co (II) 98{\%}, adsorbed on a Cu(111) substrate. Single molecules as well as self-assembled molecular layers (SAM) were studied by scanning tunneling spectroscopy at 12 K. A suppression of the LDOS at the Fermi energy was observed and is explained in terms of the Kondo resonance. Electronic properties of molecules are crucial for the design of new molecular electronic devices. This work is financially supported by the US-DOE grant: DE-FG02-02ER46012. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L42.00010: Low-Temperature Expansion of a Model Describing Helical Magnetic Phases in Rare-Earth Heterostructures Douglas Lovelady, Isaac Brodsky, David Rabson The variety of magnetic phases observed in rare-earth heterostructures at low temperatures, such as Ho/Y, may be elucidated by an ANNNI-like model Hamiltonian. In previous work modeling bulk Ho, such a Hamiltonian with a one-dimensional parameter space (possibly pressure) produced a single multicritical point, in consequence of which there was no long-range order at zero temperature. In contrast, the parameter space of the heterostructure model is three-dimensional, and instead of an isolated multicritical point, we find two-dimensional multicritical regions. In an example of Villain's ``order from disorder,'' an infinitesimal temperature breaks the ground-state degeneracy. In first order of a low-temperature expansion, we find that the degeneracy is broken everywhere in a multicritical region except on a line. In higher orders, the line may give way to a set of isolated multicritical points, or it may vanish entirely, or it may remain a multicritical line. We present exact computational results on the fate of a multicritical region in this model. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L42.00011: Effect of Dipolar Interactions on the Magnetization of Single-Molecule Magnets in a cubic lattice Marisol Alcantara Ortigoza, Richard A. Klemm, Sergey Stolbov, Talat S. Rahman Since the one-body tunnel picture of single-molecule magnets (SMM) is not always sufficient to explain the fine structure of experimental hysteresis loops, the effect of intermolecular dipolar interactions has been investigated on an ensemble of 100 3D-systems of 5X5X4 particles, each with spin S = 5, arranged in a cubic lattice. We have solved the Landau-Lifshitz-Gilbert equation for several values of the damping constant, the field sweep rate and the lattice constant. We find that the smaller the damping constant is, the stronger the maximum field needs to be to produce hysteresis. Furthermore, the shape of the hysteresis loops also depends on the damping constant. We also find that the system magnetizes and demagnetizes faster with decreasing sweep rates, resulting in smaller hysteresis loops. Variations of the lattice constant within realistic values (1.5nm and 2.5nm) show that the dipolar interaction plays an important role in magnetic hysteresis by controlling the relaxation process. Examination of temperature dependencies (0.1K and 0.7K) of the above will be presented and compared with recent experimental data on SMM. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L42.00012: Dispersion of magnetic nanoparticles in polymer films J. Gass, J. Almand, P. Poddar, H. Srikanth Magnetic nanoparticles embedded in polymer matrices have excellent potential for EMI shielding and biomedical applications. However, uniform dispersion of particles in polymers without agglomeration is quite challenging. We have fabricated PMMA/polypyrrole bilayer structures embedded with Fe$_{3}$O$_{4}$ magnetic nanoparticles (mean size $\sim $ 12 nm) synthesized using wet chemical methods. The magnetic polymer nanocomposites were spin-coated on various substrates. Agglomeration-free dispersion of nanoparticles was achieved by coating the particles with surfactants and by dissolving both the particles and PMMA in cholorobenzene. Structural characterization was done using XRD and TEM. Magnetic properties of the bilayer structures indicated that the superparamagnetic and ferromagnetic response of the polymer nanocomposites including parameters such as the coercivity, remanence and saturation magnetization could be systematically varied with controlled amounts of nanoparticle dispersions in the polymer media. The RF impedance up to frequencies of 3 \textit{GHz }measured using a vector network analyzer will also be presented. Overall, we demonstrate that magnetic polymer nanocomposite films are excellent candidates for EMI suppression applications. Work supported by NSF through Grant No. ECS 0140047 [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L42.00013: Chromium Oxide Clusters as Building Blocks to Nanoscale Materials S.N. Khanna, N.O. Jones, D.E. Bergeron, A.W. Castleman It is shown that by varying the formation conditions, two distinct families of stable chromium oxide nanoparticles can be generated, each with unique electronic and magnetic properties. The clusters are found to demonstrate remarkable stability, acting as building blocks and providing the framework to form extended structures. By use of gradient corrected density-functional theory and mass spectra data, we demonstrate that different classes of stable oxygen-passivated chromium oxide clusters have class-specific electric and magnetic properties. More specifically, irrespective of cluster size, the Cr$_{n}$O$_{2n+2}$ cages are ferromagnetic, and the saturated rings of Cr$_{n}$O$_{3n}$ are nonmagnetic. The rings are characterized by high electron affinities in addition to their stability and an investigation into the formation of ionic molecules upon combination with alkali atoms is addressed. [Preview Abstract] |
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