The Kondo effect in the presence of magnetic impurities

We measure transport through gold grain quantum dots fabricated using electromigration, with magnetic impurities in the leads. A Kondo interaction is observed between dot and leads, but the presence of magnetic impurities results in a gate-dependent zero-bias conductance peak that is split due to an RKKY interaction (I) between the spin of the dot and the static spins of the impurities. Both ferromagnetic and anti- ferromagnetic interactions have been observed in different samples. A magnetic field restores the single Kondo peak in the case of an anti-ferromagnetic RKKY interaction, whereas the splitting is enhanced in the case of ferromagnetic interaction. A gate electrode can change the relative interaction strength $T_K/I$. This system provides a new platform to study Kondo and RKKY interactions in metals at the level of a single spin.