  Transverse-field muon spin relaxation measurements have been carried out on
the heavy-fermion superconductor UPt3 doped with small amounts of Pd. We find
that the critical Pd concentration for the emergence of the large-moment
antiferromagnetic phase is ~0.6 at.%Pd. At the same Pd content,
superconductivity is completely suppressed. The existence of a magnetic quantum
critical point in the phase diagram, which coincides with the critical point
for superconductivity, provides evidence for ferromagnetic spin-fluctuation
mediated odd-parity superconductivity, which competes with antiferromagnetic
order.
