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 aniso tropic s.c. response in a magnetic torque meter on three high-quality singl e crystals down to 5 mK. Near T-c and with B in the c-direction we find a d istinct s.c, double transition. With B along the a-axis there is only a sin gle transition These results seem to be in favor of a two-dimensional s.c, order parameter. At lower temperatures a power law lambda similar to T-2 is found for B\\c, whereas in the a-direction a T-1 dependence occurs. Howeve r, both power laws do not continue to very low temperatures? but are follow ed by a slight 'paramagnetic' increase below 30 mK and a sharp 'diamagnetic ' drop below 20 mK which points to long-range antiferromagnetic order at lo w temperatures. The s.c.magnetization curves studied in the capacitive torq ue meter do not show any sign of the internal phase lines but a crossing li ne parallel to B-c2(T), similar to the peak effect. This behavior is interp reted in terms of anisotropic flux pinning. (C) 1999 Elsevier Science B.V. All rights reserved.