Study of the positive muon Knight shift in \(\): Evidence for a tetravalent \(\)-state and crystalline electric field splitting
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We report on transverse field (TF) Muon Spin Rotation (μSR) measurements on a single crystal of the hexagonal heavy fermion superconductor UNi2Al3 between 5 K and 300 K. From the measured muon \(\) Knight shift (KS) in the easy (a,b)-plane and along the c-axis we extracted the local magnetic susceptibility tensor [0pt] \(\), which arises from the nearest U-neighbors. By comparison with the bulk susceptibility [0pt] \(\) it is found that [0pt] \(\) and [0pt] \(\) agree well above 150 K but deviate considerably in the basal plane below 150 K, due to the disturbance introduced by the \(\). We succeed in reproducing both [0pt] \(\) and [0pt] \(\) on the basis of a crystalline electric field (CEF)-approach assuming U to be in the tetravalent \(\) state. The disturbance introduced by the \(\) affects the CEF-Hamiltonian in an expected manner, suggesting strongly that a CEF-picture implying a rather local 5f-electron wave function is indeed valid. Reanalyzing older data on UPd2Al3 we arrive at the same conclusion. A necessary condition for extracting the local susceptibility was the knowledge of the \(\)-site, this information was derived from the analysis of the TF-relaxation rates. At low temperatures we found about 30% of the implanted \(\) 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.
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