Eur. Phys. J. B 13, 245-256 (2000)
https://doi.org/10.1007/s100510050029

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

¹ Institute for Particle Physics of ETHZ, 5232 Villigen PSI, Switzerland
² Physics Department, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
³ Paul Scherrer Institute (PSI), 5232 Villigen PSI, Switzerland

Received: 20 April 1999
Published online: 15 January 2000

Abstract

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 , which arises from the nearest U-neighbors. By comparison with the bulk susceptibility it is found that and 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 and 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.