A. Hofer

Low-energy μSR at PSI: present and future

Abstract An overview of recent developments and of the present status concerning low-energy μSR (LE-μSR) at the Paul Scherrer Institute is given. We also discuss some schemes of possible future developments, which are able to improve the potential of the LE-μSR method.

Superparamagnetic relaxation in iron nanoclusters measured by low energy muon spin rotation

Low energy (16 keV) muons were used to probe the dynamic magnetic behaviour of iron nanoclusters embedded in a silver thin film matrix. The silver film was 500 nm thick and contained a volume fraction of 0.1% iron. Measurements were made in a field of 25 mT, applied normal to the plane of the film, in the temperature range 4.7 K to 300 K. At temperatures above 20 K thermal activation of the cluster moments was seen as a narrowing of the field distribution sensed by the implanted muons. An intrinsic cluster relaxation time of τ0 = 12 ± 4 ns and an activation energy of 51 ± 9 K were deduced from fits to the data. SQUID magnetometry of thicker (1.5 µm) but otherwise identical films on graphite substrates showed the clusters to have a volume of the order of 10-26 m3, from which a cubic anisotropy constant of K = 2.3 ± 0.4 × 105 J m-3 was calculated. Remanence measurements showed no evidence of a preferred orientation for the magnetization of the cluster assembly.

Reorientational dynamics of C60 in the solid state. An avoided level-crossing muon spin resonance study

The dynamics of the muonium (Mu) adduct to C60 in the crystalline state have been investigated using avoided level crossing muon spin resonance (ALC-μSR). The motion of C60 in the orientationally ordered phase (T < 260 K) is well described by a spherical diffusion or isotropic jump-reorientation motion with an Arrhenius activation energy of 176(1) meV and a frequency factor of 2.95(13) × 10−12 s. This is in striking contrast to the behaviour of MuC70 adducts whose reorientational motion fits well in a pseudo-static model with a temperature dependent order parameter.

CHARACTERISTICS OF CONDENSED GAS MODERATORS FOR THE GENERATION OF VERY SLOW POLARIZED MUONS

Motivated by the possibility of using condensed gas moderators to produce very slow (epithermal) polarized muons we have studied the moderator properties of rare gas solids and solid N2 thin films as a function of the growing parameters (deposition temperature and rate, heat treatment) and of the film thickness. For Ar and Kr moderators the moderation efficiency is found to depend on the growth temperature and an annealing effect is observed. The dependence can be interpreted in terms of the changes in granularity and porosity of the condensed gas layers when the growth parameters are varied. From the thickness dependence of the moderation efficiency the escape depth of the very slow muons was determined. Its large value indicates a suppression of electronic energy loss mechanisms for the epithermal muons. The time dependent decay properties of the moderators were also investigated and found to depend solely on the rest gas pressure. At a pressure of 10−10 mbar the moderation efficiency remains stable ove...

Generation of very slow polarized muons by moderation

At the Paul Scherrer Institute very slow, nearly 100% polarized, positive muons with an energy of \sim\mbox10 eV are produced by moderating a secondary beam of surface muons in a thin film of an appropriate condensed gases. These epithermal muons can be used as a source of a tertiary beam of tunable energy between \sim\mbox10 eV and \sim\mbox20 keV. Such a beam allows the μSR technique to be extended to the study of thin films and surfaces.In order to be able to perform time differential μSR experiments we have developed an ultra‐thin detector that registers the passage of keV muons and permits to trigger the experiment. The results achieved so far demonstrate that first investigations of thin film samples can be performed with the present set‐up.

Doping dependence of the magnetic penetration depth in (Yb1−xCax)(Ba1.6Sr0.4)Cu3O7−δ studied by muon spin rotation

Abstract We report transverse-field muon spin rotation measurements on the 123 system (Yb 0.7 Ca 0.3 )(Ba 1.6 Sr 0.4 )Cu 3 O 7-δ for various oxygen and calcium contents. Ca substitution creates additional holes in the CuO 2 planes, with increasing Ca content the filling of the CuO chains therefore occurs further in the overdoped regime. By varying the oxygen content in the system with a Ca content of x =0.3 we investigated within a single system the doping dependence of the low-temperature depolarization rate σ 0 and thus of the magnetic penetration depth from the under- to the heavily overdoped regime. In the heavily overdoped region σ 0 is reduced with T c despite an increased normal-state carrier concentration, as previously reported for Tl 2 Ba 2 CuO 6+ δ . Around optimal doping ( T c = T c,max ) we observe no “plateau” in σ 0 vs. T c like that reported for YBa 2 Cu 3 O 7−δ , where optimal doping nearly coincides with the filling of the CuO chains. A ‘plateau’ around optimum doping is however restored in a series of optimally doped samples ( T c = T c,max ) with progressively lower Ca content. These results are discussed in terms of induced superconductivity on well-ordered, nearly oxygen-filled CuO chains.

Development of a beam of very slow polarized muons

During the last few decades, a variety of methods has been developed which makes use of polarized positive muons as a microscopic probe of the magnetic properties of condensed matter (muon spin rotation, relaxation, resonance,μSR). Until now, available beams for μSR studies have delivered 100% polarized muons with energies in the MeV range, resulting in a deep penetration of the muons into the sample material under investigation. This presently limits the applications of theμSR technique to the study of the bulk characteristics of matter. To be able to control the implantation depth, a very low energy beam of polarized muons is being developed at the Paul Scherrer Institute. Very slow polarized muons (kinetic energy ∼ 10 eV, polarization ∼ 90%) are obtained from the moderation of a high energy muon beam in a thin film of an appropriate condensed gas. These muons can be used as a source for a beam of tunable energy between a few tens of eV and some tens of keV. Implantation depths in the range of few to a few hundreds of nanometers can thus be achieved by varying the energy.

First μ+SR studies on thin films with a new beam of low energy positive muons at energies below 20 keV

At the Paul Scherrer Institute slow positive muons (μ+) with nearly 100% polarization and an energy of about 10 eV are generated by moderation of an intense secondary beam of surface muons in an appropriate condensed gas layer. These epithermal muons are used as a source of a tertiary beam of tunable energy between 10 eV and 20 keV. The range of these muons in solids is up to 100 nm which allows the extension of the μ+SR techniques (muon spin rotation, relaxation, resonance) to the study of thin films. A basic requirement for the proper interpretation of μ+SR results on thin films and multi-layers is the knowledge of the depth distribution of muons in matter. To date, no data are available concerning this topic. Therefore, we investigated the penetration depth of μ+ with energies between 8 keV and 16 keV in Cu/SiO2 samples. The experimental data are in agreement with simulated predictions. Additionally, we present two examples of first applications of low energy μ+ in μ+SR investigations. We measured the magnetic field distribution inside a 500-nm thin High-TC superconductor (YBa2Cu3O7-δ), as well as the depth dependence of the field distribution near the surface. In another experiment a 500-nm thin sample of Fe-nanoclusters (diameter 2.4(4) nm), embedded in an Ag matrix with a volume concentration of 0.1%, was investigated with transverse field μ+SR.

Muon Spin Rotation and Relaxation Experiments on Thin Films

The recent development at the Paul Scherrer Institute of a beam of low energy muons allows depth dependent muon spin rotation and relaxation investigations in thin samples, multilayers and near surface regions (low energy μSR, LE-μSR). After a brief overview of the LE-μSR method, some representative experiments performed with this technique will be presented. The first direct determination of the field profile just below the surface of a high-temperature superconductor in the Meissner phase illustrates the power and sensitivity of low energy muons as near-surface probe and is an example of general application to depth profiling of magnetic fields. The evolution of the flux line lattice distribution across the surface of a YBa2Cu3O7 film in the vortex phase has been investigated by implanting muons on both sides of a normal-superconducting boundary. A determination of the relaxation time and energy barrier to thermal activation in iron nanoclusters, embedded in a silver thin film matrix (500nm), demonstrates the use of slow muons to measure the properties of samples that cannot be made thick enough for the use of conventional μSR. Other experiments investigated the magnetic properties of thin Cr(001) layers at thicknesses above and below the collapse of the spin density wave.

Measurements of the penetration depth of an YBa2Cu3O7−δ thin film with low-energy muons

We review how novel μSR measurements of the magnetic field distribution near the surface of a superconducting film of YBa2Cu3O7−δ in both the mixed state and the Meissner state are now possible using low-energy muons.