Anomalous temperature dependence of the mu(+) Knight shift and the phase diagram of UPd3

We report on first transverse field (0.6 T) measurements on a single crysta l of dhcp-UPd3 as a function of orientation and temperature (2-300 K). The TF-mu SR signal showed a splitting into two components with an amplitude ra tio 2:1. Rotating the applied field B-ext in the basal plane of the crystal no anisotropy in the precession frequencies was observed. implying axially symmetric mu(+)-positions. The three known phase transitions in UPd3 (two reflecting quadrupolar order below similar to 7.8 K and similar to 6.5 K an d antiferromagnetic order below similar to 4.5 K) were visible more or less pronounced in the Knight shift and/or the relaxation rates. Generally, the majority component was more strongly affected. The temperature dependencie s of the Knight shifts are complex, showing sign changes and changes in slo pe. We interpret this to reflect the different magnetic responses of the U- ions at the quasi-cubic and the hexagonal sites (a-site, c-site). Below T-N similar or equal to 4.5 K we do not observe any substantial additional lin e broadening in contrast to expectations. (C) 2000 Elsevier Science B.V. Al l rights reserved.