Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session S27: Focus Session: Magnetic Nanowires and Nanodots II |
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Sponsoring Units: GMAG DMP Chair: Daniel Ralph, Cornell University Room: Morial Convention Center 219 |
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S27.00001: Probing Magnetic Nanostructures on the Atomic Scale Invited Speaker: Magnetic nanostructures are increasing data storage capacities and are promising candidates for implementations of novel spin-based computation techniques. The relative simplicity and reduced dimensionality of nanoscale magnetic structures also make them attractive model systems for studying fundamental interactions between quantum spins. We used a scanning tunneling microscope to build individual magnetic nanostructures one atom at a time. By measuring their spin-excitation spectra with inelastic electron tunneling spectroscopy, we determined the orientation and strength of the anisotropies of individual Fe and Mn atoms on copper nitride. First-principles calculations indicate that the magnetic atoms become incorporated into a polar covalent surface molecular network, making them similar to the building blocks of molecular magnets. In linear chains of up to 10 Mn atoms, we observed excitations of the coupled atomic spins that can change both the total spin and its orientation. The large magnetic anisotropy and strong spin-coupling manifested in these structures, which provide atom-by-atom accessibility via local probes, have the potential to produce atomic-scale magnetic structures that have a stable magnetization at low temperatures. \par * This work was done in collaboration with C.-Y. Lin, A.F. Otte, M. Ternes, C.P. Lutz, B.A. Jones, and A.J. Heinrich at the IBM Almaden Research Center, San Jose, CA 95120 USA. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S27.00002: Initial Metallization and Transition Metal Diffusion in ZnO Single Crystals, CVD-Grown Films, and Nanostructures Senia Katalinic, Sylvie Rangan, Rodney Gateau, Pan Wu, Yicheng Lu, Robert Bartynski Transition metal doped ZnO is a promising candidate room temperature dilute magnetic semiconductor for spintronic applications. In previous studies indicate Fe or Mn dopants exhibit significantly different diffusion properties in ZnO. To explore whether this is an inherent property of ZnO or if it is related to non-ideal aspects of the films or nanostructures, we have studied the initial stages of Mn, Fe, and Cu metallization of the single crystal ZnO(0001)[Zn-terminated] and (11-20) surfaces, as well as MOCVD-grown epitaxial a-plane films using scanning tunneling microscopy and spectroscopy (STM and STS). While deposited Cu forms well defined islands, all three surfaces exhibit substantial disruption upon Fe deposition, including significant change in terrace widths and a markedly smaller fraction of atomic height steps. Upon annealing, Cu islands become mobile and coarsen, but the underlying ZnO structure is not strongly affected. Annealing with Fe on the surface, significant coarsening and roughening of the substrate occurs even at the modest annealing temperature of 200C, and this effect is enhanced upon annealing to 400 C. Initial results suggest that uptake of metals into the epi-film is predominantly determined by the properties of the (11-20) surface that terminates the film. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S27.00003: Unusual size-dependent magnetic anisotropy in Co nanomagnets made from self-organized fast laser processing H. Krishna, C. Miller, Z. Nussinov, A.K. Gangopadhyay, R. Kalyanaraman Unusual size-dependent magnetic anisotropy has been observed in hemispherical polycrystalline Co nanomagnet on SiO$_{2}$ substrates produced by fast pulsed-laser-induced self-organization. The magnetic states of these particles have been characterized by using magnetic force microscopy (MFM) and hysteresis measurements. The results for single domain particles up to a diameter of 180 nm, the magnetization direction of smaller sized particles tends to be in-plane, while the larger particles tend ot have out-of-plane orientation. This finding is not consistent with shape anisotropy which predicts a size-independent in-plane alignment. Microstructural analysis revealed that particles had a granular microstructure with the grain size increasing with particle size. This unusual behavior has been attributed to large residual tensile strain in the hemispherical nanoparticles due to the large heating/cooling rates ($\sim$10$^{10}$ K/s) under ns laser self-organization, the large thermal expansion mismatch and the negative magnetostrictive constant for polycrystalline Co. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S27.00004: Magnetocaloric effect (MCE) in ferrite nanoparticles James Gass, Hariharan Srikanth Enhancement of the magnetocaloric effect (MCE) in nanostructured materials is important for refrigeration applications particularly in potential spot cooling of MEMS and NEMS devices. We have investigated MCE in various classes of polydisperse and monodisperse soft ferrite nanoparticles with different blocking characteristics. Our observations indicate that in some systems, surface properties such as spin disorder and anisotropy lead to considerable enhancement of MCE. This is promising for potentially increasing MCE in nanoparticle systems through systematic engineering of the surfaces via core-shell or other approaches. We report on the magneto caloric effect (MCE) in several ferrite nanoparticle systems and compare them. Characterization of structural and magnetic properties was done using XRD, TEM, DC and AC magnetization, and transverse susceptibility. The change in entropy was calculated using the thermodynamic Maxwell relation from the family of M-H curves taken at different temperatures. The specific role of surface anisotropy and surface structure in ferrite nanoparticles and correlation to the MCE will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S27.00005: A magnetic nanoparticle as an ultimate voltage-controlled nanomagnet Igor Ovchinnikov, Kang Wang We argue that when the conduction band edge of a magnetic nanoparticle is pinched between the majority and minority spins' Fermi energies, the capacitively injected carriers go almost entirely into one of the spin groups, thus varying the magnetization value at nearly 100\% efficiency. Among the IV-group transition metals, Cobalt and Nickel satisfy this requirement. Our proposition designs a source of strong local voltage-controlled nano-scale magnetic fields, which can be an ultimate alternative to the recent quest for DMS devices, in which the efficiency is limited to the magnetic doping concentration $\alt 5$\%. We theoretically support the idea for several transition metals' within the LSDA approach augmented by the kinetic energy functionals, built from the previous ab-initio density of states profiles. Our simulations show that, \emph{e.g.}, for a $N_a=50$ Nickel atom nanoparticle the original spin polarization $2S \sim 0.66 N_a$ gets completely suppressed at the gate voltage of $\approx 15$ Volts. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S27.00006: Anisotropy-Compensated Magnetic Nanostructures Ralph Skomski, Tom A. George, D.J. Sellmyer Nanostructuring can be used to tailor the magnetic anisotropy K$_ {1}$ as function of temperature, which is important in permanent magnetism and magnetic recording. Anisotropy is an atomic quantity, but the ferromagnetic exchange ensures an anisotropy averaging over a few nanometers, in contrast to the absence of nanoscale Curie-temperature averaging [1]. An intriguing and largely overlooked feature is the possibility of temperature- dependent anisotropy zeros, which yields a potential write-field reduction in magnetic recording. On an atomic scale, this effect is well-known but limited to a relatively narrow range of rare- earth transition-metal intermetallics. Nanostructuring greatly extends the range of materials. Explicitly considered structures are thin films, where the determination of the anisotropy zero (s) yields algebraic equations whose roots depend on the thicknesses, Curie temperatures, and zero-temperature anisotropies of the involved phases. On a somewhat larger length scale (> 5 to 10 nm), there is no longer a well-defined anisotropy, but the corresponding micromagnetic corrections are easily incorporated into the theory. - [1] R. Skomski, ``Simple Models of Magnetism,'' University Press, Oxford 2008. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S27.00007: Lattice Induced Configurational Anisotropy in Nanomagnets Wen Zhang, Noah Bray-Ali , Stephan Haas The study of magnetic nanoparticles is evolving into a rich and rapidly growing area, featuring many novel phenomena and potential applications. One of the most important properties of these systems is the magnetic anisotropy, which determines the blocking temperature. Besides the well-known crystalline and shape anisotropies, the competition of exchange and magnetostatic interactions in nanomagnets leads to the formation of a configurational anisotropy, resulting from small deviations of the magnetization from uniformity within the nanostructures. In this talk, I discuss a new type of anisotropy, i.e. the lattice induced configuration anisotropy, which we have studies using Monte Carlo simulations. In particular, a scaling approach has been shown to be effective in obtaining the magnetic properties of nanoparticles. The relationship between anisotropy and blocking temperature will also be discussed. Understanding the influence of anisotropy opens up a new path to designing nanostructured magnetic materials with novel functionalities. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S27.00008: Between ferro- and para- magnetism. Electron magnetic resonance and quantal effects in superparamagnetic nanoparticles Natalia Noginova, Maxim Noginov, Vadim A. Atsarkin Nanometer-scale magnetic objects are at the interface between quantum dynamics of several interacting spins and classical thermodynamics of multi-particle systems. We present results of electron magnetic resonance (EMR) studies in suspensions of two different systems with superparamagnetic iron oxide nanoparticles with average size of 5 nm and 9 nm correspondingly. It is shown that both types of particles demonstrate common EMR behavior, including the broad spectral component, temperature-dependent narrow component with g-factor of $\sim $ 2, and additional low-field signals observed at the fields B$_{0k}$ = B$_{0}$/k, where B$_{0}$ is the resonance field of the main resonance, and k = 2, 3, and 4. These lines correspond to the transitions at the double, triple, etc., resonance frequencies and can be described in terms of the non-secular spin operators arising from the single-particle magnetic anisotropy or/and inter-particle dipole-dipole interactions. These features are common for small quantum systems and not expected in classical case. The relative intensity of the narrow component and low-field signals rapidly decreases with cooling or increase of particle size, marking gradual transition to classical FMR behavior. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S27.00009: Finite size effect in shell nanoparticles Joshua Koch, Renat Sabirianov The magnetic properties of single layer shell particles studied as function of the particle's size using Monte Carlo method with free boundary conditions. We formed truncated octahedron shell-nanoparticles of 4-30 lattice spacings across mimicking particles from 2-12 nm in size. The classical Heisenberg model with nearest neighbor ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions shows the existence of the well defined ground state. FM nanoparticles have susceptibility maximum decreasing with the increase of the nanoparticle size. Finite size scaling analysis predicts small Curie temperature for shells of the large size. The AFM particles built as truncated octahedron with (001) and (111) planes of the cubic lattice show freezing in the noncollinear structure with very low magnetization. The freezing temperature determined as maximum in specific heat in particles with AFM does not change strongly with the size of the shell in the studied particle sizes. We clearly observe the effects of edges and corners on the properties of shell particles resulting in deviation from simple scaling behavior. We fit the size dependence variation of thermodynamic properties maxima using the idea of the continuous dimensionality and the fact that infinite 2D systems do not exhibit long range order at finite temperature in the continuous symmetry. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S27.00010: Low T magnons condensation in FePd and FePt nano-particulate films R. A. Lukaszew, K. Yang, J. R. Skuza, C. Clavero Recently there have been interesting reports on experimental deviations from the Bloch T$^{3/2}$ law at low temperature in magnetic nanostructures. Specifically it has been reported an ``upturn'' of the T$^{3/2}$ law in the magnetization in some nanomagnetic materials. [1] This behavior has been attributed to a Bose-Einstein Condensation of magnons in the nanostructured materials due to more disorder in the spins leading to finite entropy as opposed to the case of crystalline bulk ferromagnets where it can be assumed that all the spins are in the direction of a saturating magnetic field and hence perfectly ordered. In order to further test these ideas, we have carried out systematic low temperature magnetization studies (SQUID) on FePd and FePt nano-particulate thin films. The microstructure and morphology of the thin films have also been extensively characterized with X-Rays, TEM and AFM/MFM. We will show our experimental data and point out significant differences and also similarities in the analyzed samples. [1]. Extension of the Bloch $T^{3/2}$ Law to Magnetic Nanostructures: Bose-Einstein Condensation, E. Della Torre, L. H. Bennett and R. E. Watson, \textit{Phys. Rev. Lett}. \textbf{94}, 147210 (2005) [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S27.00011: Strong-field interactions between a nanomagnet and a cavity mode O. Soykal, M. E. Flatt\'e We analyze the interaction of a nanomagnet with a single mode of a microcavity in a fully quantum-mechanical treatment. We consider a spherical cavity roughly 1 mm$^3$ in volume, and a nanomagnet consisting of $10^9$ spins treated as a macrospin, in the presence of a static magnetic field. For an initial configuration of no photons in the cavity and the macrospin oriented antiparallel to the field, the interaction Hamiltonian contains magnet-microwave mode coupling terms that exceed several GHz. Thus for quality factors in excess of 100, strong-field effects should be observable in the nanomagnet/cavity dynamics. Coherent states of the nanomagnet/photon system are characterized by large oscillations in the photon number (and nanomagnet spin), and are characterized by exceptionally long dephasing times. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S27.00012: Magnetism of Fe double wires deposited on Ir(100) Riccardo Mazzarello, Andrea Dal Corso, Erio Tosatti Bulk bcc Fe is a prototypical ferromagnet (FM), but a single monolayer of Fe on W(001) has been known to be antiferromagnetic (AFM). Very recent spin-polarized STM experiments on Fe double chains deposited on Ir(100) 5x1 [1] showed that these adsorbed nanowires are AFM too [2]. We study the magnetic properties of this system using ab-initio density functional theory and both scalar-relativistic and fully-relativistic ultrasoft pseudo-potentials. In particular, we address the energetics of FM and AFM configurations of several experimentally relevant structures. The AFM configuration is always energetically favored, which is in agreement with experiment but does not yet allow to distinguish between different structures. Investigation of the magnetic anisotropy induced by the spin-orbit interaction is in progress. [1] L. Hammer, W. Meier, A. Schmidt, and K. Heinz, Phys. Rev. 67, 125422 (2003). [2] R. Wiesendanger, private communication. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S27.00013: Tailoring magnetic order in nanowires by alloying 5d Transition Metal elements Javier Guevara, Tristana Sond\'on, Andres Saul The magnetism of pure-element nanowires have been theoretically studied and show, in the case of Au, Pt, and Ir, none or very low magnetic moment values but Os. In this work we study the magnetic properties of A$_{1/2}$B$_{1/2}$ nanowires, being A,B=Os, Ir, or Pt, by using the ab-initio Wien2k code. These alloyed nanowires have large magnetic moment values, and also giant MAE of different signs. We show the evolution of the spin and orbital magnetic moments as the magnetization axis is being varied. [Preview Abstract] |
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