Direct TEM observation of nanometric-sized defects in neutron-irradiated MgB2 bulk and their effect on pinning mechanisms

In this communication we present a transmission electron microscopy study on neutron-irradiated bulk samples. We give clear evidence that neutron irradiation creates nanometric amorphous regions within the crystal lattice. The density of these defects correctly scales with the neutron dose. These defects, whose size matches with the coherence length, act as pinning centers and are responsible for the strong increase of the critical current density at high field and for shifting the maxima of the pinning force curves toward higher fields.