Elementary pinning forces measured using low temperature scanning electron microscopy

Magnetic flux penetrating Josephson tunnel junctions parallel and transverse to the tunnel barrier has been imaged using low temperature scanning electron microscopy. Employing this technique we have studied the behavior of magnetic flux quanta under the influence of external forces. The strength of pinning centers acting on vortices existing within the barrier has been determined by measuring the turning angle of these flux quanta in rotating magnetic fields. Trapped transverse flux quanta could be depinned by applying a sufficient Lorentz force or electron beam irradiation. The strength of the pinning centers has been determined.