Shot noise in Cr-doped and undoped Co/Al$_{2}$O$_{3}$/Py magnetic tunnel junctions

We have found that shot noise in Co(80{\AA}) /Al$_{2}$O$_{3}$(14{\AA})/Py(100{\AA}) magnetic tunnel junctions (MTJs) is reduced with respect to Poisonian value. The Fano factor, obtained at frequencies (100$<$f$<$1000Hz), temperatures (T$<$10K) and biases (below 150 meV) where the shot noise dominates, varies between F $\approx $0.8 and 0.65, indicating correlated electron tunnelling. Doping of the insulating barrier with Cr inclusions suppresses the conductivity and tunnelling magnetoresistance, and restores the Fano factor to a value corresponding to uncorrelated transport (F$\approx $1). These results indicate an enhanced cross-correlation between electrons due to trapping or spin- flip assisted tunnelling in the undopped MTJs, and a possible Coulomb blockade in the Cr doped MTJs.