Coexistence of the Kondo effect and a ferromagnetic phase in magnetic tunnel junctions

Planar macroscopic magnetic tunnel junctions exhibit well defined zero-bias anomalies when a thin layer of ferromagnetic CoFe(B) nanodots is inserted within a MgO based tunnel barrier. The conductance curves exhibit a single and a double peak, respectively, for antiparallel and parallel alignment of the magnetizations of the electrodes that sandwich the tunnel barrier. This leads to a suppression of the tunneling magnetoresistance near zero bias. We show that the double-peak structure indicates that the zero-bias anomaly is spin split due to a magnetic exchange interaction between the magnetic nanodots and the ferromagnetic electrodes. Using a model based on an Anderson quantum dot coupled to ferromagnetic leads, we show that these results imply the coexistence of a Kondo effect and ferromagnetism.