M. Birke

Implantation studies of keV positive muons in thin metallic layers

The implantation of low energy polarized positive muons (μ+) with energies between 0.5 and 30 keV in thin (<100 nm) Al, Cu, Ag, and Au films sputtered onto quartz glass substrates has been studied by using a muon spin rotation technique. The fraction of muons stopped in the metal was determined by measuring the energy dependence of the μ+ decay asymmetry which is essentially proportional to the fraction of μ+ stopping in the metal layer. From this quantity also the backscattering probability from the metallic layer can be obtained. The results are compared with predictions of implantation profiles of muons obtained with the TRIM.SP and SRIM2000 Monte Carlo codes. The applicability of these simulations to predict the interaction of low energy muons in matter and observed differences with the experimental data are discussed.

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.

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.

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.

Spin dynamics in the quantum spin system KCu5V3O13

The complex Oxo-Cu-vanadate KCu 5 V 3 O 13 with 5 independent Cu-sites (s=) per unit cell has an exchange topology described as a ladder of spin tetrahedra or triangles. Magnetic susceptibility measurements on single crystals show a steplike anomaly at 213 K and an antiferromagnetic transition at 7.5 K. ZF pSR reveals only nuclear relaxation above 10 K and two spontaneous precession signals in the long range ordered regime. Below 3.7 K, a spin reorientation leads to a broad frequency distribution typical for an incommensurate spin structure.

The interplay of charge order and magnetism in the one-dimensional quantum spin system Sr14Cu24O41

Abstract We report muon Knight shift measurements on single crystals of Sr14Cu24O41 and Ca9La5Cu24O41 to determine the muon stopping site in the isostructural compounds of the (Sr,Ca,La)14Cu24O41 series. A site close to the ladder oxygen (0, 0.29, 0) is found. In addition, the magnetic correlations in the system Sr14Cu24O41 have been studied by zero field μ+SR between room temperature and 2 K . Quasi-static muon spin relaxation is found below 10 K , whereas thermodynamic and neutron scattering reveal a singlet ground state.

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.

μ+SR on La 2 − x−yRExSryCuO4

Abstract Magnetic order is found in La 1.85 − xNdxSr0.15CuO4 with 0.3 ⩽ x ⩽ 0.6 and in La 1.8 − yEu0.2SryCuO4 with 0.12 ⩽ y ⩽ 0.18. The onset temperature Tc depends on the Sr concentration but no dependence on the Nd concentration is detectable. The spectra of the Nd doped samples show certain deviations from the spectra of the Eu doped samples which are caused by the interactions of the Nd spins with the Cu spins. These interactions are clearly visible below 5 K.