We present experimental results showing an increase of stress of about 150kPa for a weak applied magnetic field (H<10kA/m) in an aqueous suspension of carbony iron particles coated with a superplasticizer molecule used in cement industry. These values, which are several orders of magnitude larger than those classically obtained with magnetorheological suspensions at such low field, can open the way to new applications. These high values result from the triggering of a discontinuous shear thickening induced by the magnetic field. A phase diagram is presented for a volume fraction of carbonyl iron particles of 62%, showing two domains in the plane, magnetic field versus shear rate. The lower one is liquid of quite low viscosity and the upper one corresponds to a jammed phase where the particles are in frictionnal contacts and can only move under very high stresses. The transition between the two states is monitored by the ability of the superplasticizer molecule to resist to the compression forces both hydrodynamic and magnetic.