Thin Film, Near-Surface and Multi-Layer Investigations by Low-Energy μ + SR

At the Paul Scherrer Institute (PSI, Villigen, Switzerland) the beam of low-energy positive polarised muons (LE-μ +) with tunable energy between 0.5 and 30 keV allows the extension of the muon-spin-rotation technique (μSR) to studies on thin films and multi-layers (LE-μ +SR). The range of these muons in solids covers the near-surface region up to implantation depths of about 300 nm. As a sensitive local magnetic probe with a complementary observational time window to other techniques LE-μ +SR offers the unique possibility to gain new insights in these nano-scale objects. After outlining the current status of the LE-μ + beam line we demonstrate the potential of this new technique by presenting the results of recent experiments: i) the direct observation of non-local effects in a superconducting Pb film, ii) the oxygen isotope effect on the in-plane penetration depth in optimally doped \({\text{YBa}}_{2} {\text{Cu}}_{3} {\text{O}}_{{7 - \delta }} \), and iii) the first observation of the conduction electron spin polarisation in the Ag spacer of a Fe/Ag/Fe tri-layer.