The flow of concentrated suspensions of solid particles can be suddendly blocked by the formation of a percolated network of frictionnal contacts above a critical value of the applied stress. Suspensions of magnetic particles coated with a superplastifier molecule were shown to produce a strong jamming transition. We find that, for these supensions with an abrupt discontinuous shear thickening , a model using a divergence of the viscosity at a volume fraction which depends on the applied stress does not well describe the observed behavior both below and above the critical stress. At a constant applied stress above the critical one, we have a stick slip behavior of the shear rate whose period can be predicted and scales as the square root of the relaxation time of the frictional contacts. The application of a small magnetic field allows to continuously decrease the critical shear rate and it appears that the yield stress induced by the magnetic fiekd does not contribute to the jamming transition. At last it is shown that this jamming transition also appears in the extrusion of the suspension through a die, but with a much slower dynamics than in the case of stress imposed on a rotational geometry.