Low Energy μSR Research Articles

SRF material research using muon spin rotation and beta-detected nuclear magnetic resonance

Muon spins precess in transverse magnetic fields and emit a positron preferentially in the spin direction at the instant of decay, enabling muon spin rotation (μSR) as a precise probe of local magnetic fields in matter. μSR has been used to characterize superconducting radio-frequency (SRF) materials since 2010. At TRIUMF, a beam of 4.2 MeV μ+ is implanted at a material-dependent depth of approximately 150 μm. A dedicated spectrometer was developed to measure the field of first vortex penetration and pinning strength in SRF materials in parallel magnetic fields of up to 300 mT. A low-energy beam available at PSI implants μ+ at variable depth in the London layer allowing for direct measurements of the London penetration depth from which other material parameters relevant for SRF applications, such as the lower critical field and the superheating field, can be calculated. Beta-detected nuclear magnetic resonance (β-NMR) is a technique similar to low-energy μSR using beams of low-energy β radioactive ions. With a recent upgrade, it is capable of detecting the penetration of parallel magnetic vortices, depth resolved with nanometer resolution at applied fields of up to 200 mT. In this paper, we review the impact and capabilities of these techniques for SRF research.

Depth profiling of LE-µSR parameters with musrfit

The study of thin-film and multi-layered structures with nanometer resolution is possible with low energy µSR (LE-µSR). Modeling of the measured µSR parameters such as diamagnetic asymmetry and relaxation rate as a function of sample depth can be obtained from a series of experimental implantation energy measurements and its correlation with the simulated stopping profiles. The fitting approach assumes a sharp transition between regions with distinct properties. The fitting method, previously developed in matlab, was implemented in musrfit, a free µSR data analysis framework written in C++. The main goal is to make this fitting method widely available for energy dependent measurements and to increase the modeling possibilities within musrfit.

Thin film and surface preparation chamber for the low energy muons spectrometer

We have designed and constructed a thin film preparation chamber with base pressure of <2×10−9 mbar. Currently, the chamber is equipped with two large area evaporators (a molecular evaporator and an electron-beam evaporator), an ion sputtering gun, a thickness monitor and a substrate heater. It is designed such that it can handle large area thin film samples with a future possibility to transfer them in vacuum directly to the low energy muons (LEM) spectrometer or to other advanced characterization facilities in the Quantum Matter and Materials Center (QMMC) which will be constructed in 2024. Initial commissioning of the chamber resulted in high quality, large area and uniform molecular films of CuPc and TbPc2 on various substrate materials. We present first results from low energy µSR (LE-µSR) measurements on these films.

Low Energy Measurements in Low-Energy µSR

In the context of µSR studies on magnetic materials in the ordered state, often a strong initial depolarization is found in the zero field spectra. For transverse field measurements this is often referred to as a loss in asymmetry. In case of the low-energy µSR (LE-µSR) setup this needs a more detailed discussion since effects such as time-of-flight distribution of impinging muons, back scattering, and muon reflection will change the spectra at early times and low implantation energies (E < 3keV). These effects are well understood and reproducible allowing to correct for in any given experiment. We will discuss them and show how to correct for in experiments at low implantation energies.

Characterization of the Interfacial Defect Layer in Chalcopyrite Solar Cells by Depth‐Resolved Muon Spin Spectroscopy

AbstractAs devices become smaller and more complex, the interfaces between adjacent materials become increasingly important and are often critical to device performance. An important research goal is to improve the interface between the absorber and the window layer by inserting buffer layers to adjust the transition. Depth‐resolved studies are key for a fundamental understanding of the interface. In the present experiment, the interface between the chalcopyrite Cu(In,Ga)Se2 absorber and various buffer layers are investigated using low‐energy muon spin rotation (μSR) spectroscopy. Depth resolution in the nm range is achieved by implanting the muons with different energies so that they stop at different depths in the sample. Near the interface, a region about 50 nm wide is detected where the lattice is more distorted than further inside the absorber. The distortion is attributed to the long‐range strain field caused by defects. These measurements allow a quantification of the corresponding passivation effect of the buffer layer. Bath‐deposited cadmium sulfide provides the best defect passivation in the near interface region, in contrast to the dry‐deposited oxides, which have a much smaller effect. The experiment demonstrates the great potential of low energy μSR spectroscopy for microscopic interfacial studies of multilayer systems.

Effect of Surface Roughness on the Magnetic Field Profile in the Meissner State of a Superconductor

It is well known that the London model (valid for a hard type II superconductor) predicts an externally applied magnetic field decays exponentially as a function of depth into the superconductor on a length scale λ. Direct measurements of the field profile using low energy μSR on high-Tc superconductors, such as YBa2Cu3 O x measure deviations from a simple exponential decay. In particular, there is a short length scale δ close to the surface where the magnetic field does not decay. It has been proposed that this is due to surface roughness, which leads to a suppression of the order parameter near the surface. A model of surface roughness has been studied for the case of a sinusoidally modulated surface roughness on an isotropic superconductor showing that in some cases the profiles resulting from surface roughness may be qualitatively similar to the dead layer phenomena in that the field magnitude decay rate may be slowed near the surface relative to a flat interface but that for modest roughness, the quantitative value of the length over which the field decay is slowed is much smaller than the experiments measure. In this paper, we extend this work in two directions: firstly, by using atomic force microscopy data of YBa2Cu3 O x crystals, we predict the expected field profiles within the context of the Isotropic London model of superconductivity given their actual surface geometry; and secondly, we consider how surface roughness could affect experimental values for λ and δ. The main finding is that roughness within an isotropic model does not produce the dead layers found in experiments on YBa2Cu3 O x . However, we suggest that roughness in a highly anisotropic superconductor could account for the observed dead layer.

Geant4 simulation of the PSI LEM beam line: energy loss and muonium formation in thin foils and the impact of unmoderated muons on the μSR spectrometer

The PSI low-energy μSR spectrometer is an instrument dedicated to muon spin rotation and relaxation measurements. Knowledge of the muon beam parameters such as spatial, kinetic energy and arrival-time distributions at the sample position are important ingredients to analyze the μSR spectra. We present here the measured energy losses in the thin carbon foil of the muon start detector deduced from time-of-flight measurements. Muonium formation in the thin carbon foil (10 nm thickness) of the muon start detector also affect the measurable decay asymmetry and therefore need to be accounted for. Muonium formation and energy losses in the start detector, whose relevance increase with decreasing muon implantation energy (<10 keV), have been implemented in Geant4 Monte Carlo simulation to reproduce the measured time-of-flight spectra. Simulated and measured time-of-flight and beam spot agrees only if a small fraction of so called ``unmoderated'' muons which contaminate the mono-energetic muon beam of the μSR spectrometer is introduced. Moreover the sensitivity of the beam size and related upstream-downstream asymmetry for a specially shaped ``nose'' sample plate has been studied for various beam line settings, which is of relevance for the study of thermal muonium emission into vacuum from mesoporous silica at cryogenic temperatures.

Simulation of TF-μSR histograms in germanium in the presence of cyclic charge state transitions of muonium

It has recently been shown that at T < 270 K a persistent inversion from n- to p-type can be induced by illumination in a 200-nm-thick surface layer of lowly-doped commercial germanium wafers [1]. The presence of photo-generated holes is detected by the appearance of a fast relaxing component in TF-μSR measurements, caused by cyclic muonium charge state transitions Mu−T + h+ ⇋ Mu0T. For a quantitative determination of the photo-induced hole carrier concentration we use a Monte-Carlo simulation to generate muon decay histograms for different hole capture rates (forward reaction), and ionization rates (reverse reaction due to thermal activation) [2]. The hole carrier concentration is determined by comparing simulated and experimental relaxation rates. These results have been used to estimate the photo-induced hole concentration in low-energy μSR experiments [1].

β-Detected NMR Search for Magnetic Phase Separation in Epitaxial GaAs:Mn

Abstract To test for the microscopic magnetic phase separation in the dilute magnetic semiconductor Ga1− x Mn x As sug-gested by low energy muon spin rotation measurements [1] , we present a detailed analysis of the amplitudes of the 8 Li β-detected nuclear magnetic resonance in an epitaxially grown thin film of x = 5.4% Mn doped GaAs on a semi-insulating GaAs substrate with magnetic transition temperature TC =72 K. The spectrum at 100 K corresponds to 73% of the full room temperature amplitude, and at 60 K to about 62%. The 11% loss of signal through the magnetic tran-sition is much smaller than that ∼ 50% found by low energy μ SR [1] , and may be entirely due to an amplitude change intrinsic to GaAs. This lack of evidence for phase separation is, however, consistent with the full volume fraction magnetism found by a second low energy μSR measurement on a different sample using weak transverse field [2] .

Low-energy μSR Investigations of Photo-induced Effects on a nm Scale

Abstract Photogeneratedcharge carriersopenthefascinating possibilitytotunecharge carrierconcentrationsinsitu,which provides a convenient way to change physical properties of a system. Low-energy μ SR is ideally suited for this kind of studies due to the perfect overlap of muon stopping distribution and the region of highest photon absorption close to the surface.We presenta descriptionof the currentexperimental setup and the resultsofa prototypeexperimentin Si. These results demonstratethevery promising prospectofextendingLE- μ SR to the investigation of photo-induced effects on a nanometer scale near surfaces, in thin films and heterostructures in general.

Low energy studies of semiconductor interfaces

Low Energy μSR (LEM) experiments have been performed on bilayers consisting of 100 nm thick heavily doped films of n or p-type GaAs each deposited on semi-insulating (SI) GaAs. The diamagnetic fraction seen in SI-GaAs depends on the conductivity type of the film. The results show that LEM experiments can be used to investigate surface/interface processes in semiconductors.

Lower contrast volumes with minor protocol changes enhance 64-channel CTA imaging in peripheral vascular disease

A new data acquisition system (DAQ) for bulk μSR and low-energy μSR (LE-μSR) has been developed at PSI. It is based on commercial and in-house VME modules, and on the MIDAS DAQ software library. The system is able to cope with the different needs of the various PSI μSR spectrometers, which was not possible with the existing CAMAC and ORTEC's pTA-clock based DAQ systems. The VME clock is a 64-channel CAEN V1190 TDC, using the CERN HPTDC chip, with programmable time resolution of 25, 100, 200 or 800 ps. The TDC onboard memory is continuously read through the VME bus by standard PC's or dedicated servers, using a 1-Gbit/s SIS3100/1100 VME–PCI interface. Detector rates are independently monitored using a 32-channel SIS3820 scaler module. In-house developed modules comprise an 8-channel constant fraction discriminator CFD950, a CD950 clock divider, an 8-channel linear fan-out SP950, a 16-channel NIM-ECL level converter LC950, and a programmable coincidence module FC950. All modules feature a superior performance compared to commercially available devices which allows their use also in the planned high-field μSR spectrometer where a time resolution of about 100 ps is envisaged. A special external hardware logic is no longer required, since the system can deal with a rate total of 5 MHz which is sufficient for μSR spectrometers with high event rate in their active veto systems.

Antiferromagnetic transition in epitaxial strained La 2CuO 4 thin films

Low-energy μSR experiments were carried out on epitaxial strained La 2CuO 4 thin films of a thickness of only 12 nm . We could determine magnetic transitions at T N =174(3) K for La 2CuO 4 on SrLaAlO 4 and T N =168(2) K for La 2CuO 4 on SrTiO 3.

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.

Superparamagnetism in heterogeneous AgFe thin films: A low energy μSR study

This paper describes a study of superparamagnetism in iron nanoclusters using low energy μSR, including for the first time zero field measurements, which yield an intrinsic nanocluster superparamagnetic relaxation rate of ν0=67±15 MHz and an energy barrier of ΔE=37±4 K. TEM micrographs of the embedded clusters show there to be a range of shapes and also sizes, which is bourne out by SQUID magnetometry measurements.

Low-energy μSR at PSI: present and future

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.

A low-energy muon study of thermal activation in single-domain iron particles

We present the first μSR study of the dynamic magnetic behaviour of iron clusters supported in a non-magnetic thin film matrix. Thin film samples afford the possibility of very tight control over the cluster size, but are too thin for study with conventional μSR owing to the relatively long (of order 0.5 mm) stopping range of 4 MeV muons. In this study, the thin film samples were a 500 nm thick, silver matrix containing iron clusters with an equivalent iron concentration of 0.1% by volume. SQUID magnetometry showed a mean cluster diameter of 2.8 nm. Transverse low-energy μSR measurements in a field of 25 mT, applied normal to the plane of the film, were made in the temperature range 4.7–300 K. At low temperatures, a relaxation rate of 0.4 μs −1 was observed and interpreted as due to a static Lorentzian distribution of dipolar fields with half-width 0.5 mT. At high temperatures, a motional narrowing of the field distribution, due to fluctuations in the orientation of the cluster moments, was observed. The fluctuations were found to follow an Arrhenius activation law, from which an activation energy of 51±9 K and an intrinsic cluster relaxation time of 12±4 ns were deduced.

Low energy muons as probes of thin films and surfaces

Polarized muons with kinetic energies of a few eV (epithermal μ+) can be generated by slowing down energetic muons in appropriate moderators consisting of a thin layer of a van der Waals gas frozen on a substrate. The availability of polarized muons with kinetic energies in the eV to several keV range opens the possibility to extend the μSR technique to the study of thin films and surfaces (low energy μSR, LE-μSR). We summarize the characteristics of the very slow polarized muons and of a low energy beam based on the moderation technique. We discuss the implantation of muons in thin film samples and the potential and limitations of LE-μSR. As an example first results obtained by implanting slow μ+ in a sample consisting of a Ni film deposited on Ag are presented.