Papers

AVS2001 Session MI+NS-MoA: Nano Magnetics

Monday, October 29, 2001 2:00 PM in Room 110

Monday Afternoon

Start	Invited?	Item
2:00 PM		MI+NS-MoA-1 Preparation and Magnetic Studies of Mass-Selcted Iron Clusters V. Senz, R.-P. Methling, A. Kleibert, J. Bansmann, K.E.H. Meiwes-Broer (Universitaet Rostock, Germany) We have investigated the magnetic properties of mass-selected iron clusters using the magneto-optical Kerr effect (MOKE) in the visible and soft x-ray energy range. Using a continuously working cluster source (Arc Cluster Ion source - ACIS) we codeposited mass-selected iron clusters into a matrix of evaporated silver atoms on a silicon substrate. The source is based on cathodic arc erosion in inert gas environment and supersonic expansion. Its intensity and stability allows an enhanced mass-separation which is achieved by means of an electrostatic quadrupole deflector. Magnetization curves were measured for cluster sizes of 8.1nm and 11.7nm. The hysteresis curves reveal the transition from the ferromagnetic to the superparamagnetic state in dependence on the cluster size and temperature. Recently, element-specific reflectivity measurements have been carried out in the energy range of the Fe 2p core levels using linearly polarized light (X-MOKE). The observed MOKE effect at the 2p levels is much larger than the respective value at the Fe 3d valence band due to the enhanced spin-orbit interaction.
2:20 PM		MI+NS-MoA-2 Fabrication of Nanomagnet Arrays using Anodic Porous Alumina J.H. Choi, H.-Y. Kang, W.-G. Park, Y. Kuk (Seoul National University, Korea) Anodic porous alumina has attracted increasing attention because of its naturally self-ordered porous structure and the capability for the fabrication of dots in nanometer scale. We present two fabrication processes of nanomagent arrays using anodic porous alumina. Electrochemically polished aluminum sheet is anodized in oxalic and sulfuric acid under constant voltage condition and porous alumina is used as a template. Co is electrodeposited in the pore of alumina and the deposition is stopped before Co fills the pore completely. Finally, ion milling is used to remove the upper side of alumina and get smooth surface. In the second process, we use anodic porous alumina as a mold for imprint lithography. Anodic porous alumina is placed on the PMMA/SiO₂/Co multilayer for the imprint. Hexagonal dot arrays are generated on PMMA and pattern-transferred to lower Co layer using reactive ion etching and ion milling. In both processes, nanomagnet arrays with the size of 40 ~ 100nm are successfully fabricated. The magnetic properties of the particles and their interactions have been investigated by spin polarized scanning tunneling microscopy, magnetic force microscopy and spin polarized scanning electron microscopy.
2:40 PM		MI+NS-MoA-3 Torques and Tunneling in Nanomagnets D.C. Ralph, E.B. Myers, M.M. Deshmukh, E. Bonet, F.J. Albert, R.A. Buhrman (Cornell University) When the size scale of magnetic devices is shrunk to nanometer dimensions, qualitatively new properties can emerge. I will discuss two recent examples. First, I will review investigations of a new mechanism -- spin-transfer -- by which applied currents can be used to manipulate the orientation of ferromagnetic moments. Unlike traditional schemes which utilize a magnetic field to control magnetic reorientation, spin-transfer is based on the exchange interaction. It is a torque that results when a spin-polarized current scatters from a magnetic element, and in the process transfers spin-angular momentum to the magnet. Depending on device geometry and the magnitude of the applied magnetic field, this torque can cause either controlled magnetic reversal or the excitation of high-frequency precession driven by a dc current. Another property that emerges only in devices containing metal grains smaller than about 10 nm in diameter is that the electronic states involved in electron transport can be resolved individually. I will discuss spectroscopic measurements of the electronic states which contribute to electron tunneling in cobalt nanomagnets containing about 1000 atoms, and how these states are influenced by exchange interactions, anisotropy forces, and applied magnetic fields. We find that each electronic state in given magnetic nanoparticle is described by a slightly different anisotropy energy, with fluctuations of order 1 to 3 percent. Individual states are not purely spin-up or spin-down, but have a mixed character. Spin-waves and non-equilibrium excitations play a central role in shaping the tunneling spectrum, even at low energies.
3:20 PM		MI+NS-MoA-5 Self-assembled Magnetic Dots / Antidots and Dot Chains: Epitaxial Co/Ru(0001)^* D. Li, C. Yu, J. Pearson, S.D. Bader (Argonne National Laboratory) We have grown ~ 0-420 nm thick epitaxial Co wedges on flat and grooved Ru(0001) with molecular beam epitaxy at 350 °C to investigate self-assembly in metals and their magnetic properties utilizing ex-situ atomic force microscopy and magnetic force microscope. Three-dimensional islands (dots) or a flat film network with deep holes (antidots) in well-defined truncated pyramidal shapes appear below or above ~ 20 nm, respectively.¹ The lateral sizes of these dots/antidots, as well as their spatial distribution on the flat substrates, tend to be uniform at a lengthscale of ~ 10² nm in diameter and ~ 10⁰ nm in height. This growth mode is mainly driven by strain as a result of an 8% lateral mismatch between the basil plane lattice constants of bulk Co and Ru. On grooved Ru substrates, these self-assembled Co dots align into linear chains along the top and bottom of the grooves. The average dot-to-dot distance within a chain changes from ~ 500 nm to connecting into uniform stripes as a function of coverage. Magnetically, the dots are single domain with in-plane anisotropy. The dot chains have uniaxial anisotropy along the grooves and exhibit dipolar ferromagnetic inter-dot interaction. ^* Supported by DOE BES-MS #W-31-109-ENG-38. ¹ Chentao Yu, Dongqi Li, J. Pearson, and S.D. Bader, Appl. Phys. Lett. 78, 1228 (2001).
3:40 PM		MI+NS-MoA-6 Greatly Enhanced Magnetic Anisotropies in Pure and Bimetallic Co Nanostructures on Pt(111) T. Cren, S. Rusponi, N. Weiss, M. Epple, H. Brune (Ecole Polytechnique Federale de Lausanne, Switzerland) We report on the enhancement of the magneto-crystalline anisotropy energy K in low dimensional Co islands induced by firm electronic coupling with the underlying Pt(111) substrate. The Co islands were created on Pt(111) using kinetically controlled MBE growth. The correlation between structure and magnetism has been studied by STM and in-situ Surface Magnetic Optical Kerr Effect (SMOKE). We generally observe that the magnetism of the islands is governed by the out-of-plane anisotropy and the M(H)-loops are well described by a two-state Ising model. For pure Co islands, of roughly 1000 atoms size, the shape has only little influence on magnetism (µ=2.2µ_B/atom for ramified and µ=1.9µ_B/atom for compact islands; bulk value 1,7µ_B). From the blocking temperature we deduce an anisotropy energy of K=0.37meV/atom which is greatly enhanced as compared to the Co bulk value of K=0.7µmeV/atom. For double layers islands K is reduced by a factor of two (K=0.16meV/atom) which clearly demonstrates the role of the Co/Pt interface. Finally, we show that the anisotropy can further be enhanced by decoration with Pd or Pt and by bimetallic alloy islands.
4:00 PM		MI+NS-MoA-7 Invited Paper A.D. Kent (New York University)
5:00 PM		MI+NS-MoA-10 Collective Behavior in Patterned Nanomagnetic Dot Array - A Promising Route to Magnetic Data Processing at Room Temperature A.J. Bennett, J.M. Xu (Brown University) It is well known that nanomagnetics could greatly improve data storage. In this work, through theory and experiment, we show that nanomagnetic patterned arrays are equally promising for data processing. Such arrays offer many potential advantages over CMOS circuits of the same scale: power dissipation drops through magnetostatic signal transport replacing resistive and radiative transmission lines; noise resistance is increased by low environmental coupling; interconnection problems are mitigated by signal transfer via "wireless" interactions. Magnetic interaction simulations using typical parameters suggest that room temperature operation is feasible. Experimental evidence and first-principles analysis will be presented to support this finding. We demonstrate specific nanomagnetic arrays which exhibit basic logic functions. We also show that the implementation of these arrays is within the reach of a hybrid strategy of e-beam lithography and a new non-lithographic nanofabrication technique our lab has developed (to be described in a separate report). Modeling collective behavior and designing nanomagnetic array logic represent new challenges which are met by a full-interaction-matrix Monte Carlo technique we developed. This approach enables simulation of nanodisk lattices as well as engineered branched arrays and gates for general logic. Unlike a nearest-neighbor model, our approach includes all interactions; thus, we may predict and compensate for problems arising from long-range interactions which arise in large circuits. In conclusion, magnetic nanostructures and nano-array gates show significant promise for nanoscale, room-temperature information processing.