Penetration depth and torque of superconducting UPt₃

In order to identify the properties of the internal superconducting phases of UPt₃ we studied the DC penetration depth in a SQUID system and the anisotropic s.c. response in a magnetic torque meter on three high-quality single crystals down to 5 mK. Near T_C and with B in the c-direction we find a distinct s.c. double transition. With B along the a-axis there is only a single transition. These results seem to be in favor of a two-dimensional s.c. order parameter. At lower temperatures a power law λ ∼ T² is found for B∥c, whereas in the a-direction a T¹ dependence occurs. However, both power laws do not continue to very low temperatures, but are followed by a slight 'paramagnetic' increase below 30 mK and a sharp 'diamagnetic' drop below 20 mK which points to long-range antiferromagnetic order at low temperatures. The s.c. magnetization curves studied in the capacitive torque meter do not show any sign of the internal phase lines but a crossing line parallel to B_c2(T), similar to the peak effect. This behavior is interpreted in terms of anisotropic flux pinning.