Anisotropic dc magnetization of superconducting UPt3 and antiferromagneticordering below 20 mK

We have measured the de magnetization of three high-quality single crystals of UPt3 with a SQUID magnetometer down to approximate to 5 mK in magnetic fields below 2 mT. With (B) over right arrow in the (c) over cap direction we find a double superconducting transition, while for (B) over right arrow parallel to (a) over cap we observe only one. At lower temperatures the te mperature dependence of the magnetization follows power laws indicating unc onventional behavior for the penetration depth. Below 20 mK a steep diamagn etic drop occurs, coinciding with the specific heat anomaly which we found earlier at 18 mK, pointing to static antiferromagnetic order.