Influence of aluminum substitution on the vortex matter properties ofMgB2

Abstract

The vortex matter phase diagrams of aluminum doped ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2}$ crystals, deduced from local Hall ac-susceptibility (for $\mathbf{H}\ensuremath{\parallel}c$ axis) and bulk dc-magnetization measurements (for $\mathbf{H}\ensuremath{\parallel}c$ axis and $ab$ plane) are reported. As in pristine and carbon doped $\mathrm{Mg}{\mathrm{B}}_{2}$, 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 $0.05\ensuremath{\leqslant}x\ensuremath{\leqslant}0.5$. For low aluminum content the ${H}_{c2}^{c}(T)$ line lies slightly above the corresponding one of the pristine $\mathrm{Mg}{\mathrm{B}}_{2}$, but for higher aluminum content, ${T}_{c}$, ${H}_{c2}^{ab,c}(0)$, and anisotropy parameter $\ensuremath{\gamma}={H}_{c2}^{ab}(0)∕{H}_{c2}^{c}(0)$ take lower values when compared to pristine $\mathrm{Mg}{\mathrm{B}}_{2}$. Similarly with the pristine $\mathrm{Mg}{\mathrm{B}}_{2}$ 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.