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Collective behaviour in two-dimensional cobalt nanoparticle assemblies observed by magnetic force microscopy

Nature Materials volume 3, pages 263268 (2004) | Download Citation

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Abstract

The use of magnetic nanoparticles in the development of ultra-high-density recording media is the subject of intense research. Much of the attention of this research is devoted to the stability of magnetic moments, often neglecting the influence of dipolar interactions. Here, we explore the magnetic microstructure of different assemblies of monodisperse cobalt single-domain nanoparticles by magnetic force microscopy and magnetometric measurements. We observe that when the density of particles per unit area is higher than a determined threshold, the two-dimensional self-assemblies behave as a continuous ferromagnetic thin film. Correlated areas (similar to domains) of parallel magnetization roughly ten particles in diameter appear. As this magnetic percolation is mediated by dipolar interactions, the magnetic microstructure, its distribution and stability, is strongly dependent on the topological distribution of the dipoles. Thus, the magnetic structures of three-dimensional assemblies are magnetically soft, and an evolution of the magnetic microstructure is observed with consecutive scans of the microscope tip.

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Acknowledgements

We thank the technical help and scientific discussion with Peter Nilson from Digital Instruments and Antonio Turiel from the University of Barcelona. Funding came from SEUID MAT2003-01124, DURSI 2001SGR00066, NIH 1 R01 RR-14891-01 and DE-AC03-76SF00098.

Author information

Affiliations

  1. Physics Department, University of Barcelona, 08028 Barcelona, Spain

    • Victor F. Puntes
    •  & Neus G. Bastus
  2. Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley 94720, USA

    • Pau Gorostiza
  3. College of Chemistry, University of California, Berkeley, Berkeley 94720, USA

    • Deborah M. Aruguete
    •  & A. Paul Alivisatos

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The authors declare no competing financial interests.

Corresponding author

Correspondence to Victor F. Puntes.

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https://doi.org/10.1038/nmat1094

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