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The transition between Kondo and Coulomb blockade effects in discontinuous double magnetic tunnel junctions is explored as a function of the size of the CoPt magnetic clusters embedded between AlOx mnnel barriers. A gradual competition between cotunneling enhancement of the tunneling magnetoresistance (TMR) and the TMR suppression due to the Kondo effect has been found in these junctions, with both effects having been found to coexist even in the same sample. It is possible to tune between these two states with temperature (at a temperature far below the cluster blocking temperature). In addition, when further decreasing the size of the CoPt clusters, another gradual transition between the Kondo effect and direct tunneling between the electrodes takes place. This second transition shows that the spin-flip processes found in junctions with impurities in the barrier are in fact due to the Kondo effect. A simple theoretical model able to account for these experimental results is proposed. [ABSTRACT FROM AUTHOR]

In a previous work [Phys. Rev. B 85, 214408 (2012)], the conductance and the tunneling magnetoresistance in discontinuous high anisotropy magnetic tunnel junctions was experimentally studied. Different conduction regimes (sequential tunneling, co-tunneling, Kondo effect, and direct tunneling) and gradual transitions between them were found as a function of the temperature and the size of the clusters within the barrier. A simple theoretical model was suggested able to account for the experimental results even assuming no dispersion of the distribution of the size of the clusters. Here, we study, within this theoretical framework, the effect of the volume of the clusters within the barrier, and the effect of the thickness of the insulating barrier (or the tunnel resistance), on the transition between Kondo effect and co-tunneling regime. Clarifying the role of both parameters is of importance to understand and experimentally control the transition between the different conduction regimes. [ABSTRACT FROM AUTHOR]