Upper critical magnetic fields in single crystal MgB₂

The upper critical field H_c2 for the fields perpendicular and parallel to Mg and B planes of the magnesium diboride is presented in the temperature range from 5.4 K up to T_c as obtained by comparison of high-field magnetotransport, ac-susceptibility and specific heat measurements. The onset of the finite resistivity yields H_c2, while the end of the resistive transition to the normal state is due to surface effects. H_c2 perpendicular to the planes reveals a conventional temperature dependence with μ₀H_c2∥ab(0) = 3.5 T but the parallel critical field with μ₀H_c2∥ab (0) = 17 T has a positive curvature at temperatures above 20 K. Consequently, the anisotropy factor Γ = H_c2∥ab/H_c2⊥ab is temperature dependent with F equal to 4.8 at low temperatures and about 2 near T_c. The angular dependence of the upper critical field of an elliptic form is obtained at several temperatures.