  Permalloy magnetic tunnel junctions. Complementary low frequency noise
measurements are used to understand the conductance results. The obtained data
indicate the breakdown of the Coulomb blockade for thickness of the asymmetric
silicon layer exceeding 1.2\AA . The crossover in the conductance, the
dependence of the tunnelling magnetoresistance with the bias voltage and the
noise below 80K correspond to 1 monolayer coverage. Interestingly, the zero
bias magnetoresistance remains nearly unaffected by the presence of the silicon
layer. The proposed model uses Larkin-Matveev approximation of tunnelling
through a single impurity layer generalized to 3D and takes into account the
variation of the barrier shape with the bias voltage. The main difference is
the localization of all the impurity levels within a single atomic layer. In
the high thickness case, up to 1.8\AA, we have introduced a phenomenological
parameter, which reflects the number of single levels on the total density of
silicon atoms.
