μSR Results Research Articles

Spin dynamics in the spin glass phase of Y(Mn1−xAlx)2

Zero field μSR has been used to probe spin dynamics in the concentrated spin glass Y(Mn0.9Al0.1)2. The spectra follow a stretched exponential form, Gz(t)=exp(−(λt)β), with β reaching 1/3 as the freezing temperature, Tg=60K, is approached. The evolution of Gz(t) with temperature is suggestive of the behaviour of the spin correlation function found by Ogielski in Monte Carlo simulations of the dynamics of the Ising spin glass model. The μSR results are correlated with results previously obtained by inelastic neutron scattering.

Magnetism in hydrogenated YBa2Cu3O7 a comparison of μSR and NMR results

Hydrogenated YBa2Cu3O7 with [H]/cell=2.0 was studied by X-ray diffraction, μSR and1H-NMR. For the first time, the hydride phase (16% larger c-axis) known from thin film experiments was seen in a bulk sample. A comparison of the μSR and NMR results shows that the 15 mT site (2 MHz in μSR) is the stable position for the proton and muon. The three other muon sites (4 MHz line and two exponentially decaying components) are metastable.

μ+SR study of LaNi2As2, URh2Si2 and CeRh2Si2

Muon spin relaxation experiments have been carried out in the paramagnetic and magnetically ordered states of URh2Si2 and CeRh2Si2. As the magnetic structure of these compounds is well known, these measurements can help to characterise their magnetic properties probed by μSR and to understand the μSR results of the heavy fermion compounds of the same crystallographic family. Our measurements show that the muons occupy two different crystallographic sites. The spectra of URh2Si2 and CeRh2Si2 in the magnetically ordered states are very different, probably reflecting their different magnetic structures. The spectra obtained on CeRh2Si2 are similar to the published spectra of the heavy fermion compound CeCu2.1 Si2. Muon spin rotation measurements on LaNi2As2 indicate that the muon is diffusing at 150 K.

Low-Temperature Electron Spin Susceptibility in YBa2Cu3O7-x

EPR studies were performed in order to clarify the oxygern-deficiency dependent magnetism in YBa 2 Cu 3 O 7- x . A giant EPR signal emerges at low temperatures in this compound. This is not due to impurity phases, but is indicative of the presence of electrons free from superconducting condensation. With decreasing temperature, the spin susceptibility increases more rapidly than a Curie law and the magnitude correlates with the oxygen deficiency. We consider that our results are related to disorder-induced electron localizations associated with the randomness caused by oxygen nonstoichiometry. This idea is discussed along with recent µSR, NMR, and specific heat results.

The ion–molecule reactivity of the positive muon molecular ions HeMu+ and NeMu+

Thermal (300 K) ion–molecule reaction rates are measured, using the µSR (muon spin rotation) technique, for the muonated rare gas molecular ions HeMu+ and NeMu+ reacting with NO, O2, N2O, NH3, CF4, C2H4, TMS, and CH3NO2. In almost every case (excepting O2), both charge transfer (ke) and muon transfer (kµ) contribute to the reaction rate. Reaction is believed to occur from ro-vibrational excited states, [HeMu+]* and [NeMu+]*, due to the poor efficiency of He and Ne moderators for collisional deactivation. The total experimental rate constants, kexp = kµ + ke, are generally in excellent agreement with total capture rates predicted by the simple ADO theory, regardless of the degree of internal excitation. Comparisons with literature values for corresponding protonated ion reaction rates with O2 and C2H4 reveal little or no isotope effect, although it is noted that these reactions are dominated by proton transfer, in contrast to the µSR results.

Internal magnetic fields in the high-temperature superconductor YBa2Cu3O7−δ from muon spin rotation experiments

Muon spin rotation (μSR) measurements of the internal magnetic fields in sintered YBa2Cu3O7−δ samples are reported. Pronounced differences between field-cooled (FC) and zero-field-cooled (ZFC) μSR signals, hysteresis effects and flux trapping are observed. The μSR results are compared with the results of macroscopic experiments (susceptibility, magnetization, resistivity) and with predictions obtained from models of weakly-coupled superconducting regions.

Pion decay site spectroscopy in copper

With the aim of obtaining direct information on the sites occupied by positive pions (π+), channelling experiments have been performed on the decay muons (μ+) from π+ implanted into high-purity copper crystals. At 100 K virtually exclusive occupancy of octahedral interstitial sites (0) is found in good accordance with earlier μSR results. At room temperature a significant fraction of the implanted pions decay on metastable tetrahedral interstitial sites (T). The data suggest strongly that above about 100 K both μ+ and π+ diffuse by phonon-assisted tunnelling between stable 0 sites and metastable T sites. The enthalpy difference between these two sites is estimated to be about 50 meV for μ+ and of the same order of magnitude for π+. These values are much smaller than the available theoretical predictions.