μ⁺SR studies of magnetism and superconductivity of the heavy fermion superconductor UPd₂Al₃

We present transverse field (TF) muon spin rotation (mu+SR) measurements on single crystalline UPd2Al3 carried out in the temperature range 25 mK-300 K at different fields between 350 Oe and 10 kOe. For the magnetic easy direction H perpendicular-to c-axis, the muon Knight shift shows a deviation from linear scaling with the bulk susceptibility at T < 40 K, the origin of which is unclear. TF-mu+SR in the T range 25 mK-2.5 K, aimed at determining the temperature and angular dependence of the penetration depth lambda, revealed unusual behaviour. While the observed increase of the mu+SR depolarisation rates below T(c), caused by the formation of the flux lattice, is field independent up to 10 kOe for H parallel-to c-axis, it drastically reduces with increasing field for H perpendicular-to c-axis. Assuming the formation of a regular flux lattice for H parallel-to c a value of lambda = (4200 +/- 1500) angstrom is deduced. For H perpendicular-to c, lambda = (4600 +/- 1500) angstrom at 350 Oe. The muon Knight shift is reduced by the superconducting transition, pointing to a reduction of the U-5f susceptibility of almost-equal-to 20% at T--> 0