Study of the positive muon Knight shift in UNi2Al3: Evidence for a tetravalent U4+-state and crystalline electric field splitting

We report on transverse field (TF) Muon Spin Rotation (mu SR) measurements on a single crystal of the hexagonal heavy fermion superconductor UNi2Al3 b etween 5 K and 300 K. From the measured muon (mu(+)) Knight shift (KS) in t he easy (a, b)-plane and along the c-axis we extracted the local magnetic s usceptibility tensor <(chi)over left right arrow>(local), while arises from the nearest U-neighbors. By comparison with the bulk susceptibility <(chi) over left right arrow>(tot) it is found that <(chi)over left right arrow>(l oc) and <(chi)over left right arrow>(tot) agree well above 150 K but deviat e considerably in the basal plane below 150 K, due to the disturbance intro duced by the mu(+). We succeed in reproducing both <(chi)over left right ar row>(tot) (T) and <(chi)over left right arrow>(loc) (T) on the basis of a c rystalline electric field (CEF)-approach assuming U to be in the tetravalen t (U4+) state. The disturbance introduced by the mu(+) affects the CEF-Hami ltonian in an expected manner, suggesting strongly that a CEF-picture imply ing a rather local 5f-electron wave function is indeed valid. Reanalyzing o lder data on UPd2Al3 we arrive at the same conclusion. A necessary conditio n for extracting the local susceptibility was the knowledge of the mu(+)-si te, this information was derived from the analysis of the TF-relaxation rat es. At low temperatures we found about 30% of the implanted mu(+) at the d- site and none at this site above 200 K. The majority fraction was found to be in a tunneling state over six m (or k)-sites around the b-site. No long range diffusion was seen up to room temperature.