Lack of Kondo screening at nanocontacts of nearly magnetic metals

¹  International School for Advanced Studies (SISSA), and CRS Democritos, CNR-INFM - Via Beirut 2-4, I-34014 Trieste, Italy, EU
²  Laboratorio Regionale SuperMat, CNR-INFM, and Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno I-84081 Baronissi, Salerno, Italy, EU
³  Centre de Physique Théorique, Ecole Polytechnique, CNRS - 91128 Palaiseau, France, EU
⁴  The Abdus Salam International Centre for Theoretical Physics (ICTP) - P.O. Box 586, I-34014 Trieste, Italy, EU

Corresponding author: fabrizio@sissa.it

Received: 14 May 2009
Accepted: 20 July 2009

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. 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.