The Kondo effect in ferromagnetic atomic contacts

TL;DR

This study provides direct evidence of Kondo physics in atomic-scale ferromagnetic contacts, revealing how nanoscale geometry and electronic correlations influence magnetism in materials like iron, cobalt, and nickel.

Contribution

It demonstrates that Kondo effects occur in ferromagnetic atomic contacts, highlighting the importance of electronic correlations and geometry at the nanoscale, which differ from bulk properties.

Findings

Observation of Fano-Kondo resonances in conductance measurements.

Material-dependent lognormal distribution of Kondo resonance widths.

Resonances broaden and vanish with increasing temperature, consistent with Kondo physics.

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 temperature. The Kondo effect creates a sharp resonance at the Fermi energy, affecting the electrical properties of the system;this appears as a Fano-Kondo resonance in the conductance characteristics as observed in other artificial nanostructures. The study of hundreds of contacts shows material-dependent lognormal distributions of the resonance width that arise naturally from Kondo theory. These resonances broaden and disappear with increasing temperature, also as in standard Kondo systems. 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.