  We measured the magnetic field dependence of the critical current of high
quality Nb-based planar Josephson tunnel junctions in the presence of a
controllable non-uniform field distribution. We found skewed and slowly
changing magnetic diffraction patterns quite dissimilar from the
Fraunhofer-like ones typical of a homogeneous field. Our findings can be well
interpreted in terms of recent theoretical predictions [R. Monaco, J. Appl.
Phys. vol.108, 033906 (2010)] for a uniform magnetic field gradient leading to
Fresnel-like magnetic diffraction patterns. We also show that Fiske resonances
can be suppressed by an asymmetric magnetic field profile.
