Effects of Al doping on the superconducting and structural properties of MgB2

We have investigated the structural and superconducting properties of a series of Al doped MgB2 samples. The results show that the superconductivity of MgB2 is destroyed when Al is alloyed into MgB2 as reported previously. A structural instability was observed from the data of X-ray diffraction. Further study by TEM shows that the Al substitution leads to an ordering occupation on Mg sites. This corresponds to a superstructure with a repeat period of 2c along the c-axis. The superstructure was identified to be a form of MgAlB4 structure which intergrows with MgB2. Measurements on the low doping level samples showed that the upper critical field Bc2, irreversible field Birr and the critical current density Jc of these samples were reduced as the Al content was increased. Analysis of the upper critical field data suggests that the density of states near the Fermi surface N(EF) is reduced by Al doping. This is consistent with the decrease in the superconducting transition temperature Tc. The field dependence of pinning force is also discussed for these samples