Anisotropy of the lower and of the upper critical fields in Mg1-xAlxB2 single crystals

Temperature dependence of the lower (Hc1) and of the upper critical field (Hc2) was derived from magnetization and torque measurements of the high quality Mg1-xAlxB2 single crystals with Al content x = 0.0, 0.012, 0.077, and 0.095. The anisotropy of Hc1 is close to 1.5. The lower critical field, as well as its anisotropy, γHc1, at low temperatures slightly decrease as the result of Al substitution. It was found that the upper critical field is reduced for H parallel to the ab-plane, whereas it is slightly increased for H parallel to the c-axis, in Al substituted crystals with small Al content, and then decreased for higher Al content. A significant reduction of Hc2 anisotropy, γHc2, is observed at lower temperatures. For the crystal with Al content x = 0.095, it was found that γHc2 is equal to about 2.5, whereas it is equal to about 6 for unsubstituted crystals