Magnetic study of the UPt3 superconducting phases

In order to distinguish the UPt3 superconducting (s.c,) phases we have stud ied their magnetic properties at low fields in a SQUID magnetometer and up to fields > H-c2(0) with a capacitive torque-meter, With the SQUID we measu re the magnetic penetration depth and find the second s.c. transition at T- c(-) when the held is applied along the c-axis, but not with H perpendicula r to (c)over-cap. This result, combined with power-law behavior at low temp erature T. is most consistent with the two-dimensional E-2u s.c. order para meter. Below 20 mK we End an additional diamagnetic signal that we ascribe to the normal state magnetism. In high fields our torque measurements show a kink of the perpendicular magnetization component at the B-C phase line, pointing to an enhanced Ginzburg-Landau parameter in the C phase. (C) 2000 Elsevier Science B.V. All rights reserved.