Influence of Al Doping on the Critical Fields in Magnesium Didoride Single Crystals

The lower (Hc1) and upper (Hc2) critical fields of AlxMg1−xB2 single crystals have been deduced from specific heat and magnetization measurements (with x = 0, 0.1 and 0.2). We show that Hc1 and Hc2 are both decreasing with increasing doping content. The corresponding anisotropy parameter ΓHc2 (0) value then also decreases from ∼ 5 in pure MgB2 samples down to ∼ 1.6 for x = 0.2 whereas ΓHc1 (0) remains on the order of 1 in all measured samples.