E. Karlsson

Hyperfine fields at Ce in GdAl2 and DyAl2

The local fields at140Ce in the cubic intermetallic compounds GdAl2 and DyAl2 have been measured with the DPAC method. At our lowest temperatures we obtainBeff (30K)=54(2) T for GdAl2 andBeff(12.5K)=27(1) T for DyAl2 which are considerably lower than the hyperfine field of the free Ce3+ ion (183 T). The (Ce)GdAl2 field is quantitatively explained by a cubic crystal field splitting of 4f states but for DyAl2 additional effects are discussed.

Temperature and magnetic field dependence of the muonic Knight shift in antimony

The bound state of an electron and a muon (muonium) gives rise to a hyperfine field of 164.6 kG in vacuum. In semiconductors some of the muons are found to form muonium. In metals, however, the electronic relaxation time is much shorter than the Larmor precession period, and only a very small (and temperaturedependent) fraction of the free mu~nium field should be expected, if bound states exist. Such bound states have been searched for in Cu, AI, Zn, and C [I] but have not been observed. The conclusion has been that bound electrons in these metals are either absent or are extremely loosely bound. Unbound electrons in non-magnetic metals are expected to produce Knight shifts of the order of i0-i00 ppm at the muon sites. The theories predict an increasing shift with a decreasing density of states (large screening radius rs). In the semimetals As, Sb, and Bi the electron densities are very low (2 • i0 z0 , 5 • I019 and 3 • 1017 cm -3 as compared to i022-I02s cm -3 for ordinary metals). The corresponding screening radius r s for an interstitial charge is estimated from the free electron model to be 4, 7, and 20 ~ for As, Sb, and Bi, respectively, compared to ~ 0.6 ~ for Cu and AI. It was considered worth while to start measurements with muons in Sb, As, and 5i in order to look for effects from the local electronic structure. Some preliminary results were reported in ref. 2. The experiments were performed in a beam of polarized positive muons from the 600 MeV Synchro-cyclotron at CERN. Low temperatures were normally obtained with a continuous-flow 4He cryostat, but for temperatures below 2K a 3He-4He dilution cryostat was employed. Muon precession frequencies were studied in polycrystalline antimony in the temperature range 2 K-300 K in an applied field of 0.4 kG. Data for applied fields of 0.8 kG and 2.1 kG were also obtained at a few temperatures. A few experimental points were also run with a polycrystalline sample of arsenic. The samples were of 6N purity. The data are shown in Fig. I.

Initial or thermally controlled impurity trapping of muons in niobium

We have studied muon depolarization in some very well characterized samples of niobium (pure Nb with <1 ppm impurities and Nb doped with 15 at.ppm N and 53 at.ppm Ta, respectively). This has allowed us to separate the influence of substitutional and interstitial impurities on theμSR linewidthσ. The purest sample shows a low but non-zero linewidth from 0.1 to 70 K. Ta-doping increases the width strongly below 20 K. while N-doping gives a broad maximum between 30–70 Kand a considerable width below 20 K.Conventional two-trap models cannot explain the occurrence of a linewidth significantly lower that that predicted for staticμ+and constant over a wide temperature range. A consistent explanation of these three observations can however be obtained from the following model: In pure Nb only a fraction of the muons is self-trapped thermally; the other muons do not form small polarons but remain in a propagating metastable nonlocalized state. Impurities can catalyse further initial polaron formation, decreasing the metastable fraction. This process causes temperature-independent plateaus inσ up to the detrapping temperature. The muons localized at shallow traps (Ta induced) can diffuse at higher temperatures and be trapped again at deeper traps (associated with the N-impurities).

On muon localization in doped aluminium samples

The magnetic field dependence of the transverseμSR linewidth has been studied in anAlAg single crystal. The muon site is found to be tetrahedral at 22 K and octahedral at 0.05 K. Both the site change and the magnitudes of the electric field gradients are similar to previous observations onAlMn, although these two impurities create different strain fields in Al.

Mobility of muons in Cu below 2 K

We have performed transverse fieldμSR experiments on several different samples of copper in the temperature range below 2 K, including isotope separated Cu and impurity doped polycrystalline Cu.We do not observe any strong effect of the isotope composition of the sample. A63Cu and a natural Cu sample of identical purity both yield 0.16μs−1 for the low-temperature plateau, while an increased linewidth in the65Cu case may be related to the strong effects of Fe impurities.Careful transverse field measurements on large single crystals at 0.08 K reveal non-Gaussian lineshapes in accordance with the picture of diffusing muons at this temperature. This allows us to reject several of the existing models for muon behaviour in copper below 2 K.

μSR study of spin correlations in rare earth aluminum intermetallics

Abstract A systematic μSR study of Laves phase intermetallic compounds REAl 2 (RE = Ce, Pr, Nd, Gd, Dy, Ho, Er, Tm) has been undertaken. The damping factor which reflects the motion of RE spins rises quickly near the magnetic transition temperature. This is due to the formation of spin correlations in the paramagnetic region.

Trapping of Positive Muons in Dilute Aluminium Alloys

The precession of positive muons in AIMnx samples with x ranging from 5 to 1300 ppm was studied. All the samples show a maximum in the µSR linewidth around 17 K, with a linewidth depending on the Mn concentration. In samples of AIMgx, AILix and AIAgx with x = 40 – 120 ppm the maximum occurs at different temperatures 17 – 45 K. This suggests that the muons are trapped in a strain field characteristic for each kind of impurity atom.

MUON DIFFUSION IN PURIFIED AND DOPED IRON

We have performed positive muon (~+SR) experiments from ~ K to room temperature in two polycrystalline iron samples under zero external field. The two samples were purified by zone melting and one of them was doped with Si. The linewidth % for the two samples shows a monotonic temperature dependence from 45 K to room temperature, in contrast to recently published results which show a peak in the 120-150 K region. We interpret our results below 40 K as due to coherent diffusion.

Positive Muons in Metal Physics

Positive muons have been used extensively during the last five years as probes of local fields, relaxation processes, diffusion, etc., in solidstate physics [1,2]. A new spectroscopy, called Muon Spin Rotation (μSR) (in analogy with NMR and ESR) has been developed, which is based on the spin interaction of the muons. The properties of the μ+ listed in Table 1 will help to elucidate the possibilities of μSR as compared with other techniques.