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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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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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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