1. Departamento de Fisica Aplicada, Facultad de Ciencias, Universidad de Alicante, San Vicente del Raspeig, E-03790 Alicante, Spain
2. Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
3. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA

Correspondence to: Carlos Untiedt¹ Correspondence and requests for materials should be addressed to C.U. (Email: untiedt@ua.es).

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 nanostructures^{3, 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 systems^{4, 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.

1. Departamento de Fisica Aplicada, Facultad de Ciencias, Universidad de Alicante, San Vicente del Raspeig, E-03790 Alicante, Spain
2. Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
3. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA

Correspondence to: Carlos Untiedt¹ Correspondence and requests for materials should be addressed to C.U. (Email: untiedt@ua.es).

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