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

Paper MI+NS-MoA2
Magnetic Resonance and X-ray Magnetic Circular Dichroism of Monodisperse Co Nanoparticles

Monday, November 4, 2002, 2:20 pm, Room C-205

Session: Self-Assembly and Nanomagnetism
Presenter: U. Wiedwald, Technical University Braunschweig, Germany
Authors: U. Wiedwald, Technical University Braunschweig, Germany
M. Spasova, Technical University Braunschweig, Germany
M. Hilgendorff, Hahn-Meitner-Institut, Germany
M. Ulmeanu, Technical University Braunschweig, Germany
E.L. Bizdoaca, Technical University Braunschweig, Germany
M. Giersig, Hahn-Meitner-Institut, Germany
M. Farle, Technical University Braunschweig, Germany
Correspondent: Click to Email
Monodisperse Co nanoparticles with an medium diameter of 12 nm (@sigma@ < 5%) are prepared by the thermolysis of Co@sub 2@(CO)@sub 8@.@footnote 1@ After using a size-selective separation technique they are redispersed in toluene. For structural and magnetic investigations the samples are prepared by controlled evaporation of the solvent on various substrates. Detailed TEM analysis of the structure of Co nanoparticles shows that they consist of a 8-9 nm metallic Co core and a 1-2 nm thick CoO shell, both crystallized in fcc structure.@footnote 2@ Angular dependent ferromagnetic resonance (FMR) of these arrays of Co nanoparticles reveals a remanent magnetization at 300 K that is due to long-range dipolar coupling between the nanoparticles.@footnote 3,4@ A preliminary g-factor analysis yields a value of g = 2.15±0.02 that is close to bulk fcc value and in good agreement with the structural analysis. X-ray absorption fine structure (XAFS) shows a multiplet structure in the Co L@sub 3@ edge that is due to a superposition of metallic Co and Co in an oxidic environment. No evidence for x-ray magnetic circular dichroism (XMCD) at the L@sub 2,3@ edges is found for as prepared ex-situ samples for temperatures between 20 K and 300 K. After slight Ar@super +@ ion etching a well resolved XMCD signal with a strongly enhanced orbital-to-spin-moment ratio is observed. The results of the FMR and XMCD analysis will be compared and discussed in terms of the contributions from the antiferromagnetic CoO shell. This project is supported by the European Community, contract no. HPRN-CT-1999-00150 and the Access to Research Infrastructure action of the Improving Human Potential Programme. M. H. thanks the Deutsche Forschungsgemeinschaft, Az.: II C 10 - SPP 1072 for support. @FootnoteText@ @footnote 1@ M. Hilgendorff et al., Aust. J. Chem. 54 (2001)@footnote 2@ M. Spasova et al., Proc. Spring MRS Meeting, San Francisco (2002), submitted@footnote 3@ U. Wiedwald et al., J. Vac. Sci. Technol. A 19 (2001)@footnote 4@ M. Spasova et al., J. Magn. Magn. Mat. 240 (2002)