Anisotropic dc magnetization of superconducting upt₃and antiferromagnetic ordering below 20 mk

We have measured the dc magnetization of three high-quality single crystals of UPt₃with a SQUID magnetometer down to ≈5 mK in magnetic fields below 2 mT. With B虀 in the ĉ direction we find a double superconducting transition, while for B虀||â we observe only one. At lower temperatures the temperature dependence of the magnetization follows power laws indicating unconventional behavior for the penetration depth. Below 20 mK a steep diamagnetic drop occurs, coinciding with the specific heat anomaly which we found earlier at 18 mK, pointing to static antiferromagnetic order.