  Huge values of high field magnetoresistance have been recently reported in
large arrays of CoFe nanoparticles embedded in an organic insulating lattice in
the Coulomb blockade regime. An unusual exponential decrease of
magnetoresistance with increasing voltage was observed, as well as a
characteristic scaling of the magnetoresistance amplitude versus the
field-temperature ratio. We propose a model which takes into account the
influence of paramagnetic impurities on the transport properties of the system
to describe these features. It is assumed that the non-colinearity between the
core spins inside the nanoparticles and the paramagnetic impurities can be
modelled by an effective tunnel barrier, the height of which depends on the
relative angle between the magnetization of both kind of spins. The influence
on the magnetotransport properties of the height and the thickness of the
effective tunnel barrier of the magnetic moment of the impurity, as well as the
bias voltage are studied. This model allows us to reproduce the large
magnetoresistance magnitude observed and its strong voltage dependence, with
realistic parameters.
