misfit dislocations evidence in MgO barrier epitaxially grown on magnetic electrodes by electron diffraction (RHEED) in real time (video available). The RHEED patterns modifications induced by the dislocations are highlighted by using a subtraction images process (video available). Explanation of the RHEED patterns modification using kinematic theory Playing with the misfit between MgO and Fe1-xVx by varying x to accurately measure the critical thickness for plastic relaxation dependence with the misift.