Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures III |
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Sponsoring Units: DMP GMAG Chair: John Wendelken, Oak Ridge National Lab Room: LACC 150B |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H42.00001: Magnetic Anisotropy in Surface Supported Nanostructures Invited Speaker: From a fundamental point of view, only three issues define the ultimate density limit of magnetic information storage. First, the magnetic anisotropy energy $K$ has to be beyond 1.2~eV for the memory to nonvolatile at 300~K. Second, the write and read signals have be to uniform in order to enable the ultimate limit of single particle bits. Third, mutual interactions between adjacent bits have to be minimized, which is achieved for an out-of-plane easy axes. \\ We present measurements on surface supported nanostructures which are considered as model systems for the study of magnetism in nanostructures and for the exploration of the ultimate density limit of magnetic information storage. We report on the evolution of $K$ as a function of size, starting from single atoms, and going up in size almost atom by atom for Co on Pt(111) using XMCD measurements. Monomers have 200--times the Co$_{\rm hcp}$ bulk anisotropy. Larger 2D nanostructures are investigated by MOKE confirming a strong coordination effect on the anisotropy energy. For Co/Pt(111) the low-coordinated edge atoms have 20--times the bulk anisotropy and favor out-of-plane magnetization. Currently, the best homogeneity of magnetic properties is obtained for Co islands self-assembled on Au(788). The temperature dependence of the zero-field susceptibility $\chi$ shows that mutual interactions between the monodomain particles are absent at a density of 26 Tera particles/in$^{2}$. We finally present data on bimetallic (FeCo, FePt, CoPt) nanostructures. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H42.00002: STM and SQUID studies of quantum engineered Co magnetic nanoplateles on Si. U. M. Mirsaidov, J.-L. Li, S.-Y. Qin, C.K. Shih, J. T. Markert, Z. Zhang, J.-F. Jia, Q.-K. Xue Self-organized Co platelets with uniform size and shape were grown on a template of ordered Al cluster arrays on Si(111)-7$\times $7 surfaces. The Al nanocluster array not only suppresses reaction between Si and Co, but also enables formation of well-defined Co nanoplatelets. These platelets are equilateral triangles with fixed orientation and two-monolayer thickness, and the area of individual platelets is half of N$^{2}$ of the 7$\times $7 unit cells. Despite their small volume (a few nm$^{3})$, these magnetic nanoplatelets exhibit unusually high blocking temperature ($>$100 K). The hysterisis curves obtained for these platelets suggest that easy axis of magnetization is perpendicular to the substrate plane. The perpendicular magnetization, high blocking temperature and growth on a silicon surface may make these platelets appealing for technological applications. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H42.00003: Kinetic pathway for the formation of Fe nanowires on stepped Cu(111) surfaces Yina Mo, Kalman Varga, Efthimios Kaxiras, Zhenyu Zhang We report the discovery of a novel kinetic pathway for the formation of one-dimensional Fe nanowires of single atom width on stepped Cu(111) surfaces. This pathway, identified through extensive total-energy calculations within density functional theory, establishes that the stable structure involves a row of Fe atoms in the upper edge of a step. The formation of the surface wire is preceded by facile incorporation of an initial row of Fe atoms into the surface layer at one lattice constant away from the step edge, which then acts as an attractor for the second, exposed row of atoms. Under this new frame of wire structure, magnetic properties of these wires are studied. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H42.00004: Carbon coated iron nanowires: a theoretical study Mariana Weissmann, Griselda Garcia, Miguel Kiwi, Ricardo Ramirez Several properties of hybrid systems made of iron nanowires coated with carbon are computed from first principles. In particular, we focus on how the presence of carbon determines the magnetic ordering. A quasi one-dimensional fcc (or hcp) Fe structure favors ferromagnetic ordering, but when encapsulated into a C tube antiferromagnetic ordering can become favorable. The spin polarization at the Fermi level is large for the bare nanowires, but it decreases due to hybridization with the carbon coating. Implications of these results for the fabrication of nanodevices, as well as for the appearance of exchange bias, are discussed. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H42.00005: Thermal Processing and Overlayer Effects of Ultra-Thin Fe Layers on GaAs(001) Justin Shaw, Charles Falco We used Brillouin Light Scattering (BLS) and Magneto-Optical Kerr Effect (MOKE) measurements to study the magnetic properties of ultra-thin Fe layers undergoing various growth and post-deposition thermal processes. We found that the critical Fe thickness required for the onset of ferromagnetism can be significantly reduced through annealing processes. Iron islands coalesce and become more ordered following these annealing processes, which also results in the formation of a stable ferromagnetic phase in the Fe layer. The thermal processing also has a profound impact on the magnetic anisotropy and surface structure. In situ Scanning Tunneling Microscopy (STM) revealed that high temperature post-deposition annealing results in the formation of rectangular pits while high temperature growth results in a rough faceted microstructure. In addition, we found that Al and Au overlayers suppress the uniaxial anisotropy at the interface. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H42.00006: Enhanced Magnetism in Cobalt Nanoparticles Gene Venturini, Jess Wilcoxon, Paula Provencio We report an enhanced magnetic response for dilute Co nanoparticles with diameters d $<$ 2 nm in frozen organic matrices. Slight variations of an air- and water-free inverse micellar synthesis produced isolated Co particles with two sizes, d = 1.8 nm and d $<$ 1 nm. The 1.8 nm particles exhibit a Langevin response from 5 to 200 K with a saturation magnetization 10-20\% above bulk hcp Co; size estimates from Langevin fits and HRTEM images were consistent. Interface effects were studied by changing the surfactant, resulting in increased (amines) and decreased (thiols) magnetism at high fields. The smaller Co particles have a magnetic response at 5 K that fits a Brillouin function with a spin of 8-10 Bohr magnetons and an estimated saturation magnetization well above both bulk Co and Co(II) ions. However, the magnetic moment of these small clusters decreases rapidly with increasing temperature. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H42.00007: Are catalytic properties preserved at the nanoscale?: The case of cobalt. Fernando Reboredo, Giulia Galli It is generally believed that the chemical activity of metal catalysts would be increased at the nanoscale because of the increment of surface to volume ratio. However, reducing the size of a metal particle to few hundred of atoms changes its electronic and chemical properties, with respect to the bulk. Therefore, whether small metal nanoparticles remain good catalyst or are better catalysts than bulk materials remains an open question. Bulk cobalt surfaces are known to catalyze important chemical reactions. In this talk we report ab-initio calculations of the chemisorption of CO on small Co clusters, in particular chemisorption energies a function of the size of the Co cluster and the surface coverage. Our results show a significant change in the total magnetic moment of the clusters as a function of CO coverage, suggesting that Co clusters may be used as gas sensors. We also discuss the changes in catalytic activity of the cluster as a function of size. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H42.00008: Vacancy-Induced Magnetic Structures in Graphene Sheet Hosik Lee, Myungjun Han, Gundoo Lee, Jaejun Yu The observations of magnetic signals in doped fullerenes and other carbon-based systems have initiated numerous studies on the origin of magnetism in these materials. Recently carbon-adatoms or vacancies in graphene and carbon nanotubes are suggested to be an origin of the defect-related magnetism in carbon-based systems.\footnote{P.O. Lehtinen, A. S. Foster, Y. Ma, A.V. Krasheninnikov, and R.M. Nieminen1, Phys. Rev. Lett. {\bf 93}, 187202 (2004)} To investigate the vacnacy-induced structures and magnetic properties of graphene sheet, we carried out first-principles density-functional calculations on the graphene sheet with various vacancy configurations. From the results, it is found that there exist stable ferromagnetic ground states at several different vacancy configurations, and the minimal size and shape of the ``ferromagnetic hole'' is determined. Among the configurations considered in this work, a triangular shaped structure is found to be energetically favored. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H42.00009: Local Spin Anisotropy Effects upon the Magnetization of Single Molecule Dimers Richard Klemm, Dmitri Efremov We present an exactly solvable model of equal spin $s_1$ single molecule magnetic dimers. The spins within each dimer interact via the Heisenberg and the most general quadratic global and local anisotropic spin exchange interactions, and with the magnetic induction ${\bf B}$. For antiferromagnetic couplings and $s_1>1/2$, the low temperature magnetization ${\bf M}({\bf B})$ exhibits a rich variety of steps of unequal width, the structure and anisotropy of which depend upon the various local anisotropic spin exchange energies. Specific numerical results for Fe2 $s_1=5/2$ dimers will be presented. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H42.00010: Theoretical investigation of noncollinear magnetism for Mn$_{5}$ and Mn$_{6}$ Tsuguo Morisato, Shiv Khanna, Yoshiyuki Kawazoe The presence of odd-number rings in systems marked by nearest -neighbor antiferromagnetic (AFM) coupling can lead to situations where it is not possible to satisfy all interactions to find the ground state. This leads to so called ``frustration.'' Since atomic clusters tend to take structures containing triangles or tetrahedra, this can cause magnetic frustration. It is therefore possible that some transition metals like chromium or manganese, whose crystals tend to be AFM, show noncollinear (NCL) magnetic configuration in small clusters. In the present study, we carry out first-principles calculations with NCL magnetism for Mn$_{5}$ and Mn$_{6}$ clusters. We adopt the projected augmented-wave method of the program called VASP (Vienna \textit{ab initio} simulation package) for this calculation. From the results, we can see Mn$_{5}$ may take NCL magnetic configuration as one of the ground states isomers, while other isomers have collinear configurations. For the Mn$_{6}$, we found the NCL magnetic configuration is highly expected as the ground state. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H42.00011: Isomers and Isomags in MnnX (n = 5-6, X=N, O) Clusters W.-J. Zheng, J.M. Nilles, K.H. Bowen, N.O. Jones, S.N. Khanna, P. Jena, T. Baruah, M.R. Pederson It is shown that the addition of N or O to pure Mn$_{n}$ clusters can lead to situations where the ground state is marked by configurations having comparable binding energies, same total spins but differing distribution of local moments. We call these ``isomags'' and demonstrate their existence through a synergistic approach combining negative ion photo-detachment spectra and theoretical investigations. In the case of Mn$_{5}$O and Mn$_{6}$O clusters show that the O atom occupies either bridge or hollow sites. While it occupies a bridge site with a binding energy of 6.88 eV in Mn$_{5}$, it prefers a hollow site with a binding energy of 7.18 eV in Mn$_{6}$ clusters. Further, Mn$_{5}$O is shown to possess two magnetic isomers with total spins of 13 and 5 $\mu _{B}$ that contain three and five isomags respectively. Mn$_{6}$O possesses three isomers, two with moments of 8 and 2 $\mu _{B}$ in addition to a non-magnetic state. It is shown that the isomags, although undetected in magnetic deflection experiments, can be traced via their electronic structure in experimental photoelectron spectra. Corresponding results on Mn$_{n}$N will also be presented. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H42.00012: Effect of Hydrogen Absorption on the Magnetic Moment and Anisotropy of Fen and Con Clusters M.R. Beltran, N.O. Jones, S.N. Khanna, T. Baruah, M.R. Pederson Theoretical investigations have been carried out to examine the effect of H absorption on the magnetic moment and anisotropy of small Fe$_{n}$ and Co$_{n}$ clusters. Our studies use a gradient corrected density functional approach and cover clusters containing up to 4 transition metal and 2 H atoms. We show that the successive addition of H atoms can lead to monotonic or oscillatory change from the free cluster magnetic moment. An analysis of the density of electronic states shows that the variations in the magnetic moment can be related to the location of the lowest unoccupied molecular orbital in the parent cluster. It is shown that the addition of hydrogen can substantially change the magnetic anisotropy, yielding a particularly high magnetic anisotropy for Co$_{3}$H$_{2}$ that is higher than the known molecular nano-magnets. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H42.00013: Spin-dependent Fabry-P\'erot interferometer of a Cu thin film grown on fcc Co(001) Y.Z. Wu, A.K. Schmid, M.S. Altman, X.F. Jin, Z.Q. Qiu Spin-dependent electron reflection from a Cu thin film grown on Co/Cu(001) was studied using Spin Polarized Low Energy Electron Microscopy (SPLEEM). We find that the electron reflectivity from the Cu thin film exhibits a Fabry-P\'{e}rot type interference. As the Cu thickness increases, the energies of the interference peaks decrease (or increase) for k-vector greater (or less) than half of the Brillouin zone (BZ) vector. This interference effect can be well explained using the phase accumulation model. Moreover, we observed spin-dependent reflectivity from the Cu thin film. SPLEEM images of the Cu film reveal the magnetic domain structure of the underlying Co film, with the domain contrast oscillating with the electron energy and the Cu film thickness. This observation is attributed to the spin-dependent reflectivity at the Cu/Co interface. [Preview Abstract] |
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