A magnetic field gradient sensor includes a support and a structure having at least a first and a second mobile element, at least one magnetic sensor, each magnetic sensor being mechanically secured to one of the first and/or second mobile elements so as to be able to apply a mechanical force to the structure in the presence of a magnetic field gradient, a coupler for coupling between the first and second mobile elements so that the structure can be moved in at least one balanced mechanical mode in the presence of a magnetic field gradient, and a sensor for measuring the movement of the structure at least in balanced mode.