Low energy muons as probes of thin films and near surface regions

Abstract

Questions involving thin films, multilayered samples, surfaces and interfaces are generally not accessible by the conventional sources delivering muons with kinetic energy ∼4 MeV. The muon as a sensitive local magnetic and spin probe with complementary observational time window to other probes and techniques is able to offer unique new insights into these objects of investigations. Low energy muons (LE-μ +) with tunable energy between ∼0.5 and 30 keV penetrate only to a depth between a few and few hundreds of nm depending on their energy. Hence they provide the desired non-destructive, non-invasive and microscopic probe for local investigations of properties near surfaces and in thin samples. The intensity of the LE-μ + source at PSI and the capability of the sample environment have been steadily evolving over the past years and the flux at the sample has now reached intensities of more than 700 μ +/s. After a summary of the most recent developments we give a brief overview of the experimental program together with some anticipated applications. The measurement of the value of local magnetic fields as a function of position below a surface on a scale of a few nm, which has provided the first direct confirmation of the London formula and has the potential to yield information crucial to understanding the details of the superconducting state, has been used to map films of non-conventional and conventional superconductors. Investigations of magnetic systems range from the first studies of magnetic properties of buried thin films to investigations of dimensional effects in a single spin glass layer and interlayer coupling in magnetic-nonmagnetic systems. The future prospects of this technique are outlined. To optimize the flux of LE-μ + at PSI a new, dedicated surface muon beam line has been designed. It is presently under construction and will be installed in 2003/2004. The predicted intensity increase by a factor of about ten will contribute to the realization of the full potential of polarized muons used as nanoprobes.