Reversible magnetization of MgB 2 single crystals with a two-gap nature

We present reversible magnetization measurements on MgB 2 single crystals in magnetic fields up to 2.5 T applied parallel to the crystal's c axis. This magnetization is analyzed in terms of the Hao-Clem model, and varioussuperconducting parameters, such as the critical fields [H c (0) and H c 2 (0)], the characteristic lengths [ξ(0) and λ(0)], and the Ginzburg-Landau parameter, K are derived. The temperature dependence of the magnetic penetration depth λ(T), obtained from the Hao-Clem analysis could not be explained by theories assuming a single gap. Our data are well described by using a two-gap model.