AVS 49th International Symposium
    Magnetic Interfaces and Nanostructures Monday Sessions
       Session MI+NS-MoA

Paper MI+NS-MoA5
Structure and Magnetism of Colloidal Composite AgCo Nanoparticles

Monday, November 4, 2002, 3:20 pm, Room C-205

Session: Self-Assembly and Nanomagnetism
Presenter: M. Spasova, Technishe Universität Braunschweig, Germany
Authors: M. Spasova, Technishe Universität Braunschweig, Germany
T. Radetic, Lawrence Berkeley Laboratory
N.S. Sobal, Hahn-Meitner-Institut Berlin, Germany
C. Raeder, Technishe Universität Braunschweig, Germany
M. Hilgendorff, Hahn-Meitner-Institut Berlin, Germany
U. Dahmen, Lawrence Berkeley Laboratory
M. Giersig, Hahn-Meitner-Institut Berlin, Germany
M. Farle, Technishe Universität Braunschweig, Germany
Correspondent: Click to Email
Monodisperse, air-stable Ag@sub 100-x@Co@sub x@ composite nanoparticles with a mean diameter of 12 nm have been synthesized by methods of colloidal chemistry.@footnote 1@ The composition x was varied between 20 and 73 at.% Co. High resolution Transmission Electron Microscopy (TEM) and selected area electron diffraction have showed that the nanoparticles consist of precipitates of fcc Co and fcc Ag grains. No evidence for alloy formation was observed. Element-specific TEM images obtained by electron energy-loss spectroscopy and X-ray microanalysis indicate that Co is predominantly found in the surface region of the particles and the particles have an Ag@sub core@Co@sub shell@ structure. No Co oxide formation was observed in spite of Co being located at the particle surface. Magnetic properties of arrays of the nanoparticles on Si substrates were investigated by angular dependent ferromagnetic resonance and SQUID magnetometry between 5 and 300 K. The blocking temperature is found to depend on the particle composition. It increases with increasing Co content. At room temperature the nanoparticles containing 73 at.% Co are below and the Ag@sub 55@Co@sub 45@ nanoparticles are above their respective blocking temperatures. At lower temperatures contributions from additional magnetic phases are observed. The magnetization curves were analyzed as a function of temperature taking into account both paramagnetic and ferromagnetic contributions. The results are discussed in context to contributions from the Co/Ag interfacial and surface magnetism. The work has been supported through EC contract no. HPRN-CT-1999-00150. @FootnoteText@ @footnote 1@ N.S. Sobal at el., Nano Letters, in press.