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Journal of Applied Physics / Volume 101 / Issue 11 / MAGNETISM AND SUPERCONDUCTIVITY

J. Appl. Phys. 101, 113913 (2007); http://dx.doi.org/10.1063/1.2736303 (7 pages)

Observation of Co/CoO nanoparticles below the critical size for exchange bias

A. N. Dobrynin1, K. Temst1, P. Lievens1, J. Margueritat2, J. Gonzalo2, C. N. Afonso2, E. Piscopiello3, and G. Van Tendeloo3

1Laboratorium voor Vaste-Stoffysica en Magnetism & INPAC-Institute for Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
2Laser Processing Group, Instituto de Optica, CSIC, Serrano 121, 28006 Madrid, Spain
3Electronenmicroscopie voor Materiaalonderzoek, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

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(Received 14 November 2006; accepted 28 March 2007; published online 11 June 2007)

We compare the magnetic properties of pure and oxidized Co nanoparticles embedded in an amorphous Al2O3 matrix. Nanoparticles with diameters of 2 or 3 nm were prepared by alternate pulsed laser deposition in high vacuum conditions, and some of them were exposed to O2 after production and before being embedded. The nanoparticles are organized in layers, the effective edge-to-edge in-depth separation being 5 or 10 nm. The lower saturation magnetizations per Co atom for the samples containing oxidized nanoparticles provide evidence for the formation of antiferromagnetic CoO shells in the nanoparticles. None of the samples with Co/CoO nanoparticles show exchange bias, while vertical hysteresis loop shifts and enhanced coercivities (as compared to samples with pure Co nanoparticles) are observed. This constitutes evidence for the nanoparticles size being in all cases smaller than the critical size for exchange bias. The difference in coercivity versus temperature dependences for the samples with pure and oxidized Co nanoparticles shows that the exchange anisotropy in Co/CoO nanoparticles appears at temperatures lower than 50 K.

© 2007 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. SAMPLES PREPARATION AND CHARACTERIZATION
    1. Pulsed laser deposition
    2. Transmission electron microscopy
    3. Rutherford backscattering spectrometry
  3. MAGNETIZATION MEASUREMENTS
  4. DISCUSSION
  5. CONCLUSIONS

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KEYWORDS and PACS

Keywords

cobalt, cobalt compounds, ferromagnetic materials, antiferromagnetic materials, nanoparticles, exchange interactions (electron), nanotechnology, pulsed laser deposition, magnetic hysteresis, coercive force

PACS

  • 75.50.Cc

    Other ferromagnetic metals and alloys

  • 75.50.Ee

    Antiferromagnetics

  • 75.50.Tt

    Fine-particle systems; nanocrystalline materials

  • 75.30.Et

    Exchange and superexchange interactions

  • 75.60.Ej

    Magnetization curves, hysteresis, Barkhausen and related effects

  • 61.46.Df

    Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.2736303

PUBLICATION DATA

ISSN

0021-8979 (print)  
1089-7550 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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