Lack of Kondo screening at nanocontacts of nearly magnetic metals

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. In the presence of an additional ferromagnetic intersite exchange coupling between leads and impurity, realistic for nearly ferromagnetic leads, the NRG may instead stabilize a permanent moment. The resulting state is a rotationally invariant spin, which differs profoundly from mean field and whose physical properties are those of a ferromagnetic Kondo model as opposed to the more conventional antiferromagnetic one, where the localized moment gets eventually screened by the conduction electrons. A sign inversion of the zero-bias anomaly and other spectroscopic signatures of the switch from regular to ferromagnetic Kondo are outlined.