Magnetic properties of the heavy-fermion superconductors UPt 3 and URu 2Si 2

Abstract

Magnetization measurements have been performed in order to investigate further the temperature dependence of the lower critical field H c 1, the Meissner effect and the critical currents on single crystals of the anisotropic Heavy-Fermion Superconductors UPt 3 and URu 2Si 2. For the UPt 3 single crystals, no marked anisotropy in the lower critical field has been observed, but a change in slope could be identified at Δ T≈;65 mK below T c . Extrapolation of the lower critical field to zero temperature gives a value of 6.5 mT in both directions ( H∥ c, H⊥ c). The lower critical field of URu 2Si 2 roughly shows a linear temperature dependence in the range from T c to ∼ 150 mK. At T ≈; 150 mK H c 1 seems to pass through a maximum before eventually dropping down. The magnetization measurements have shown that H c 1 of URu 2Si 2 is isotropic for H∥ c and H ⊥ c with a value H c 1 (0) of 3.3 mT. This isotropy of the H c 1 contrasts strongly with the anisotropy of upper critical field H c 2( T). The measured hysteresis of the magnetization of URu 2Si 2 is more important than in the case of UPt 3 and exhibits magnetization jumps at low temperatures. Applying the Bean-model, critical current densities of 3.8 × 10 3 A cm −2 and 2.4 × 10 4 A cm −2 are deduced for UPt 3 and URu 2Si 2 respectively at T/ T c ≈;0.2 and μ 0 H i ≈;80 mT.