Grain Boundary Effects and AC-Susceptibility Measurements in Aluminum-Doped MgB₂Superconductor

We have used the AC susceptibility for MgAlB₂ samples to investigate the grain boundary effects and the Al-doping effects on the critical current density. The Cole-Cole plots were used to obtain the grain boundary effects on the critical current density. We found that the grain boundaries are highly transparent to current flow and have virtually no effects on the critical current density in MgB₂pure and Al-doped samples. Moreover, the lower critical field H _c1 is found to be enhanced with Al doping. The highest H _c1 value is achieved with the 4.5 % concentration of Al.