Ferromagnetic Kondo Effect at Nanocontacts

TL;DR

This paper investigates the ferromagnetic Kondo effect at nanocontacts, showing that a permanent magnetic moment can be stabilized through ferromagnetic exchange coupling, leading to distinct spectroscopic signatures.

Contribution

It introduces a model including ferromagnetic exchange coupling that stabilizes a permanent magnetic moment via the ferromagnetic Kondo effect, contrasting with previous mean field expectations.

Findings

Permanent moments do not form in the standard Anderson model due to Kondo screening.

Adding ferromagnetic exchange stabilizes a permanent magnetic moment.

Spectroscopic signatures include a sign inversion of the zero-bias anomaly.

Abstract

Magnetic impurities bridging nanocontacts and break junctions of nearly magnetic metals may lead to permanent moments, analogous to the giant moments well known in the bulk case. A numerical renormalization group (NRG) study shows that, contrary to mean field based expectations, a permanent moment never arises within an Anderson model, which invariably leads to strong Kondo screening. By including in the model an additional ferromagnetic exchange coupling between leads and impurity, the NRG may instead stabilize a permanent moment through a ferromagnetic Kondo effect. The resulting state is a rotationally invariant spin, which differs profoundly from mean field. A sign inversion of the zero-bias anomaly and other spectroscopic signatures of the switch from regular to ferromagnetic Kondo are outlined.