Using a double-axis vibrating sample magnetometer, detailed magnetic measurements were made of the lower critical field HC1 for fields parallel to the 2 crystallographic directions of MgB2 single crystals. In addition, by using a novel Hall probe magnetometer, measurements were made of high precision magnetization loops. From these, the upper critical field HC2 was directly determined for both field orientations. The results suggested that HC1 was much larger than suggested by most previous estimates and that, consequently, the Ginzburg–Landau parameter was less than 5. It was found that the anisotropy parameter was about 2, regardless of temperature, over the measured range.

Superconducting Critical Fields and Anisotropy of a MgB2 Single Crystal. G.K.Perkins, J.Moore, Y.Bugoslavsky, L.F.Cohen, J.Jun, S.M.Kazakov, J.Karpinski, A.D.Caplin: Superconductor Science and Technology, 2002, 15, 1156-9