S.A. Dodds

Observation of the anisotropic spin-glass transition and transverse spin ordering in pseudo-brookite through muon spin relaxation

Zero-field longitudinal muon-spin-relaxation (μSR) experiments have been performed on single crystals of pseudo-brookite (Fe2−xTil+xO5; x=0.25), an anisotropic spin-glass system. The spinglass temperature (Tg) is determined to be 44.0±0.5K. Above Tg, a distinct exponential muon-spin-relaxation rate (λ) is observed, while below Tg a square-root exponential decay is seen, indicating fast spin fluctuations in the ‘frozen’ state. Near 8K, a maximum in λ is observed, which is due to transverse spin ordering at these low temperatures. Even near Tg, λ is very low (<1 μs−1), likely due to a well-defined muon-oxygen state in the single crystals. The sharp λ-increase (with decreasing temperatures) above Tg allows a comparison between spinfreezing models like the Vogel-Fulcher law and a power law. The results of these initial measurements indicate that dynamic (and static) magnetism in oxide spin glasses can be directly monitored through μSR.

Effects of superconductivity on rare‐earth ion dynamics in (HoxLu1−x)Rh4B4

Zero‐field muon spin‐lattice relaxation rates have been measured in magnetically diluted HoxLu1−xRh4B4 ternary compounds, primarily for x=0.02. For temperatures below ∼11 K a characteristic two‐component structure of the muon depolarization function is observed, which is consistent with slow (quasistatic) Ho3+‐moment fluctuations. We have reported similar behavior for x=0.7, which indicates that the slow fluctuations are due to crystal‐field isolation of the Ho3+ ground state. The observed relaxation rates in this temperature regime depend little on x, which is consistent with conduction‐electron (Korringa) exchange scattering as the dominant mechanism for the fluctuations. The observed temperature dependence of muon spin‐lattice relaxation in the superconducting state is not presently understood.

Muon spin relaxation in spin glass PdMn

Muon spin relaxation (μSR) rates have been measured in transverse, longitudinal, and zero applied fields for the spin glass PdMn (7 at. %), and are compared with a previous study of the disordered ferromagnet PdMn (2 at. %). The calculated paramagnetic state transverse field relaxation rate for noninteracting spins is much larger than the observed rate in spin glass PdMn, but is in good agreement with ferromagnetic PdMn. The zero field relaxation rate shows a sharp cusp at Tg=5 K. An applied longitudinal field of 5 kG is insufficient to suppress this cusp in spin glass PdMn, but will suppress a similar cusp in ferromagnetic PdMn. Below Tg, a distribution of quasistatic local fields is observed in zero field, which has the same temperature dependence for both samples. Comparisons with model calculations are discussed.

Resolved nuclear hyperfine structure of muonium in CuCl by means of muon level-crossing resonance

Datailed muon level-crossing resonance measurements of Mu1 and Mu11 centres in single crystals of CuCl are presented. The hyperfine and nuclear hyperfine parameters of the closest two shells of nuclei are remarkably similar for the two centres, indicating that both are located at the same tetrahedral interstitial site with four Cu nearest neighbours and six Cl next-nearest neighbours. About 30% of the total unpaired-electron spin density is located on the muon, about 60% on the four nearest neighbours and the rest on the six next-nearest neighbours, with nothing observable for any other shell.

The effects of large oscillating fields on spin precession in magnetic resonance

Magnetic resonance of a spin system which is acted upon by a large near-resonance oscillating magnetic field transverse to a static field has been studied experimentally and theoretically for many years. The technique of DEMUR (Double Electron Muon Resonance) has many advantages for such studies. This paper will describe the results of an experiment to study the precession of the muonium triplet near magnetic resonance using DEMUR.

Beam chopper development at LAMPF

In order to reduce pileup limitations on μSR data rates, a fast chopper for surface muon beams was built and tested at LAMPF. The system allowed one muon at a time to be stopped in a μSR sample in the following way: A surface beam from the LAMPF Stopped Muon Channel was focused through a crossed-field beam separator and onto a chopper slit. With the separator E and B fields adjusted properly, the beam could pass through the slit. The beam to the μSR sample was turned on or off (chopped) rapidly by switching the high voltage applied to the separator plates on or off within approximately 500 ns; with the E field off, the B field deflected the beam, dumping it near the slit. We demonstrated that, with improved electronics, we will be able to stop a single muon in a μSR sample as frequently as once every 20 μs and that data rates for the system can be a factor of five higher than is attainable with unchopped beams. The observed positron contamination of the beam was less than five percent, and the ratio of the muon rate with beam on to the rate with beam off was 1540.

Muon spin relaxation study of exchange coupling in dilute AgMn alloys

Abstract We have observed for the first time a conduction-electron-mediated component of the interaction between muons and local moments in a dilute magnetic alloys, Ag Mn. Comparison of the amplitude of this component and neutron quasielastic linewidth data yields some disagreement with a covalent-mixing exchange model.

Ho‐ion dynamics in HoxLu1−xRh4B4

New μSR measurements of Ho‐ion dynamics in HoxLu1−xRh4B4 are reported for x=0.02 and 0.005. These compounds are superconducting below Tc≂11 K. The measured concentration dependence of the Ho3+ spin‐lattice relaxation rate Γ0 is consistent with single‐ion, conduction electron scattering (the Korringa mechanism) as the dominant process. Above about 10 K, Γ0 is activated with an energy of 66 K, about 1/2 of the overall crystal‐field splitting. Below about 10 K (in the superconducting state) a more gradual temperature dependence is observed, as reported previously for x=0.70 and x=0.02. This temperature dependence is not consistent with the usual exponential decay in the superconducting state. Using a model of conduction‐electron‐exchange scattering which includes the full crystal‐field potentials, we show that a reasonably quantitative understanding of the overall temperature dependence can be achieved.

Muon Diffusion in Noble Metals

We have measured diffusion-induced muon depolarization in dilute AgGd and AgEr in the temperature range 200–700 K and have thereby determined the muon diffusion parameters in Ag. The diffusion parameters for μ+ in Cu, Ag, and Au are compared with those of hydrogen. For Ag and Au, the μ+ parameters are similar to those of hydrogen, whereas for Cu, the μ+ parameters are much smaller. Lattice-activated tunneling and over-barrier hopping are investigated with computational models.

Large scale inhomogeneities in granular metals

We present direct evidence for large-scale inhomogeneities in granular metal films. Using Rutherford backscattering data and measurements of the superconducting critical fields, we show that granular materials (Al:Al2Te3 and Al:Al2O3) are deposited in layers. Each film contains 3–5 layers approximately 400 A thick. The layer thicknesses are significantly less than the film thicknesses and are of the order of the coherence length. This layering has significant consequences when using such films to investigate dimensionally dependent theories.