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The Kondo effect in ferromagnetic atomic contacts

Nature volume 458, pages 1150–1153 (2009)Cite this article

Abstract

Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk, electronic conduction in these materials takes place mainly through the s and p electrons, whereas the magnetic moments are mostly in the narrow d-electron bands, where they tend to align. This general picture may change at the nanoscale because electrons at the surfaces of materials experience interactions that differ from those in the bulk. Here we show direct evidence for such changes: electronic transport in atomic-scale contacts of pure ferromagnets (iron, cobalt and nickel), despite their strong bulk ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of local magnetic moments by the conduction electrons below a characteristic temperature1. The Kondo effect creates a sharp resonance at the Fermi energy, affecting the electrical properties of the system; this appears as a Fano–Kondo resonance2 in the conductance characteristics as observed in other artificial nanostructures3,4,5,6,7,8,9,10,11. The study of hundreds of contacts shows material-dependent log-normal distributions of the resonance width that arise naturally from Kondo theory12. These resonances broaden and disappear with increasing temperature, also as in standard Kondo systems4,5,6,7. Our observations, supported by calculations, imply that coordination changes can significantly modify magnetism at the nanoscale. Therefore, in addition to standard micromagnetic physics, strong electronic correlations along with atomic-scale geometry need to be considered when investigating the magnetic properties of magnetic nanostructures.

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Figure 1: Conductance of a monatomic contact.
Figure 2: Histograms of inferred Kondo temperatures for iron, cobalt and nickel.
Figure 3: Evolution of the Fano resonances with increasing temperature.
Figure 4: Electronic structure for a nickel chain and a nickel nanocontact.

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Acknowledgements

We thank E. Tosatti, R. Aguado and J. Ferrer for discussions, G. Scott and G. Saenz-Arce for experimental support and V. Esteve for technical support. This work was partly supported by the European Union through MolSpinQIP and Spanish MEC (grants MAT2007-65487, 31099-E and CONSOLIDER CSD2007-0010). D.J. acknowledges funding by the US National Science Foundation (NSF) under grant DMR-0528969. D.N. acknowledges the support of NSF grant DMR-0347253, the David and Lucille Packard Foundation and the W. M. Keck Program in Quantum Materials.

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Authors and Affiliations

  1. Departamento de Fisica Aplicada, Facultad de Ciencias, Universidad de Alicante, San Vicente del Raspeig, E-03790 Alicante, Spain,

    M. Reyes Calvo, Joaquín Fernández-Rossier, Juan José Palacios & Carlos Untiedt

  2. Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA,

    David Jacob

  3. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA,

    Douglas Natelson

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  1. M. Reyes Calvo
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  2. Joaquín Fernández-Rossier
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  3. Juan José Palacios
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  6. Carlos Untiedt
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Correspondence to Carlos Untiedt.

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Calvo, M., Fernández-Rossier, J., Palacios, J. et al. The Kondo effect in ferromagnetic atomic contacts. Nature 458, 1150–1153 (2009). https://doi.org/10.1038/nature07878

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