Anisotropy in superconducting MgB 2: a comparison of SQUID and torque measurements

Abstract

The reversible parameters of superconducting MgB 2 single crystals obtained by SQUID and by torque magnetometry are compared. Both methods lead to almost identical values for H c2 c and λ ab . This justifies the usual evaluation of torque data, which directly reveals the anisotropy of the upper critical field ( γ H ) and of the magnetic penetration depth ( γ λ ). We show that this allows a rather accurate determination of γ λ , whereas some uncertainties remain for γ H . Still, we find γ H = γ λ for 21 K<T<30 K and γ H ≅ γ λ for lower temperatures ( H≳ H c2 c /3).