Abstract:

The vortex matter phase diagrams of aluminum doped Mg1-xAlxB2 crystals, deduced from local Hall ac-susceptibility (for H parallel to c axis) and bulk dc-magnetization measurements (for H parallel to c axis and ab plane) are reported. As in pristine and carbon doped MgB2, aluminum substituted crystals display the peak effect in the critical current. The peak effect is located very close to the H-c2(c)(T) line, while it disappears below a characteristic magnetic field H-* that depends on Al content. The absence of significant bulk pinning below the onset of the peak effect implies that the Bragg glass phase is present there. In some of the crystals the peak effect is not present as a sharp negative peak of the real part of the local ac susceptibility, but it appears as a negative double-peak feature. This observation may be related with the miscibility gap that occurs for 0.05 <= x <= 0.5. For low aluminum content the H-c2(c)(T) line lies slightly above the corresponding one of the pristine MgB2, but for higher aluminum content, T-c, H-c2(ab,c)(0), and anisotropy parameter gamma=H-c2(ab)(0)/H-c2(c)(0) take lower values when compared to pristine MgB2. Similarly with the pristine MgB2 crystals for the superconducting aluminum substituted crystals, the anisotropy parameter decreases monotonously as temperature increases as well. All the experimental observations could be qualitatively explained within the clean two-band approximation.