  We report on muon spin rotation/relaxation and $^{119}$Sn nuclear magnetic
resonance (NMR) measurements to study the microscopic superconducting and
magnetic properties of the Heusler compound with the highest superconducting
transition temperature, \ypd\ ($T_c=5.4$ K). Measurements in the vortex state
provide the temperature dependence of the effective magnetic penetration depth
$\lambda(T)$ and the field dependence of the superconducting gap $\Delta(0)$.
The results are consistent with a very dirty s-wave BCS superconductor with a
gap $\Delta(0)=0.85(3)$ meV, $\lambda(0)= 212(1)$ nm, and a Ginzburg-Landau
coherence length $\xi_{\mathrm{GL}}(0)\cong 23$ nm. In spite of its very dirty
character, the effective density of condensed charge carriers is high compared
to the normal state. The \mSR data in a broad range of applied fields are well
reproduced by taking into account a field-related reduction of the effective
superconducting gap. Zero-field \mSR measurements, sensitive to the possible
presence of very small magnetic moments, do not show any indications of
magnetism in this compound.
