Determination of superconducting anisotropy from magnetization data on random powders as applied to LuNi2B2C, YNi2B2C and MgB2

The recently discovered intermetallic superconductor MgB2 appears to have a highly anisotopic upper critical field with H c2 /H min c2 ≡ γ > 5. In order to determine the temperature dependence of both H c2 and H min c2 we propose a method of extracting the superconducting anisotropy from the magnetization M(H,T ) of randomly oriented powder samples. The method is based on two features in (∂M/∂T )H : the onset of diamagnetism at T max c , that is commonly associated with Hc2, and a kink in ∂M/∂T at a lower temperature T c . Results for LuNi2B2C and YNi2B2C powders are in agreement with anisotropic Hc2 obtained from magneto-transport measurements on single crystals. Using this method on four different types of MgB2 powder samples we are able to determine H c2 (T ) and H min c2 (T ) with γ ≈ 6. 74.25.Dw, 74.60.Ec, 74.25.Ha Typeset using REVTEX