  In this work we explore the rheology of very concentrated (0.55<$\Phi$<0.67)
suspensions of carbonyl iron (CI) particles coated by a small polymer. A strong
DST is observed in a large range of volume fraction presenting some
specificities relatively to other systems. In particular, in a given range of
volume fraction, the DST transition appears suddenly without being preceded by
shear thickening. The presence of a frictional network of particles is
confirmed by a simultaneous measurement of the electric resistance of the
suspension and of the rheological curve. Using the Wyart-Cates model we show
that, increasing the volume fraction, the fraction of frictional contacts grows
more and more quickly with the stress that disagrees with the prediction of
computer simulations. The same kind of behavior is observed in the presence of
a magnetic field with, in addition, a very strong increase of the viscosity
with the magnetic field before the transition. We interpret this behavior by
the interpenetration of the polymer layer under the effect of the shear
stress-and of the magnetic stress-followed by the expulsion of the polymer out
of the surfaces. Besides we point that, above the DST transition, we do not
observe a jamming in the range of volume fraction whereas it is predicted by
the W-C model. Based on the fact that in the absence of shear flow, the polymer
should come back to the surface and destroy the frictional contacts we can
predict an asymptotic non-zero shear rate and reproduce the experimental
behavior.
