M. E. Schillaci

Muon spin relaxation studies of the spin glass AgMn (invited)

We report measurements of magnetic ion dynamics in the spin‐glass sytem AgMn (1.6 at. %) using the muon spin relaxation technique (μSR). The salient features of the theoretical μSR lineshapes in zero and finite longitudinal applied fields are reviewed. Zero‐field measurements below the glass temperature (Tg = 7.6 K) exhibit a static width a(T) which decreases with increasing temperature reminiscent of an ’’order parameter’’ behavior. At T≲Tg/2 the static width is consistent with that calculated for a random orientation of frozen Mn spins. The temperature dependence of a(T) is attributed to rapid, small‐amplitude fluctuations of the local field about its quasistatic equilibrium position. An apparent stiffening of the fluctuation frequency spectrum is observed as the applied magnetic field is increased. The measured upper limit for frequencies which significantly relax the muon spin is about 50–100 times smaller than the peak in the mode density calculated by Walstedt and Walker in Monte Carlo simulations o...

Measurement of the depolarization rate of positive muons in copper and aluminum

Abstract Positive muon spin rotation experiments for polycrystalline Cu and Al from 19K to temperatures near the melting points are reported. At low temperatures, the depolarization associated with localization of the muons at octahedral interstitial sites is seen in Cu, while in Al only slight depolarization is observed below 250K. At high temperatures, no evidence for trapping of positive muons at vacancies in thermal equilibrium is found for either metal. It is concluded that the muons either diffuse too slowly to find vacancies or, if they do find vacancies, are bound too weakly to remain trapped.

Anomalous paramagnetic‐state μSR in spin‐glass AgMn

Muon spin rotation (μSR) experiments have been carried out in the paramagnetic state (T≳Tg) of a dilute AgMn spin glass, to probe internal field distributions during the onset of impurity‐spin freezing. μ+ resonance frequencies and depolarization rates were measured in a 1.6 at.% Mn sample in transverse fields between 0.15 and 5 kOe for temperatures up to ∼3Tg. The depolarization rate Λ increased well above the value Λd expected in the absence of impurity‐spin exchange coupling as Tg was approached from above. The largest relative increase of Λ was obtained for lowest fields. The average precession frequency was constant to within 0.2% down to 1.1Tg. The data scale roughly with T/Tg and H/Tg when compared to recent μSR studies of CuMn. Although a case has been made for attributing this anomamous increase in Λ to dynamic broadening (spin‐lattice relaxation), evidence exists which argues that static inhomogeneity in the muon hyperfine field is the origin of Λ. If this is the case, the absence of muon enviro...

μSR measurement of rare‐earth moment dynamics in the HoxLu1−xRh4B4 ternary alloy system

μSR measurements have been carried out in the ternary rare‐earth system HoxLu1−xRh4B4, x = 0, 0.7, and 1, to study the effects of superconductivity and magnetic order on Ho moment dynamics in this alloy series. In nonmagnetic LuRh4B4, muon relaxation is due to nuclear dipolar fields; the decrease of the relaxation rate above ∼200 K is attributed to the onset of muon diffusion at this temperature. In HoRh4B4, the zero‐field relaxation rate Λ(T) exhibits a strong maximum well above the magnetic ordering temperature TM = 6.6 K, but no singularity at TM. Instead, the relaxation function develops a form indicative of very slow Ho moment fluctuations below ∼9 K. The high‐temperature relaxation rates are too low to be explained by a ferromagnetic mean‐field estimate of the exchange frequency; interactions of competing signs may be involved. For x = 0.7, where reentrant superconductivity is observed between TM = 4.1 K and TS = 7.7 K, a shoulder in Λ(T) is observed between TM and TS. This seems to indicate an impo...

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.

Muon diffusion and trapping studies in high purity vanadium

We present the first results of a study of the effects of varying impurity concentration on the temperature dependence of the depolarization rate of positive muons implanted into vanadium. Data are reported for the most highly purified polycrystalline sample yet measured, and the same sample subsequently doped with about 500 ppm oxygen by weight. The data for the pure sample shows a low depolarization rate (<.15 μsec−1) at all temperatures measured, showing a broad minimum centered at ∼35 K, followed by a sharp peak near 90 K and a rapid drop to negligible values at 200 K. The data is contrasted with previously published data [2] on less pure samples, and calls into question previous interpretations of the behavior of the μ+ at low temperatures in impure vanadium [1] as one-phonon-assisted tunneling.

μ+ diffusion and trapping in high purity and oxygen-doped Nb

Data are presented for the temperature dependence of the muon depolarization rate between 10 K and 120 K for three samples of niobium of varying. purity. Two samples, each containing approximately 200 ppm substitutional Ta and interstitial concentrations of 10 ppm and 560 ppm (mostly O), respectively, were studied. A third sample containing only 3 ppm Ta and an estimated 10 ppm total interstitial impurities was also measured. The results indicate that even at the lowest temperatures studied the depolarization of the muon is dominated by traps associated with impurities.

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-oxygen bonding in V2O3

A muon site search using calculated internal fields has been performed for V2O3, where purely dipolar fields allow a site determination free from covalent complications. The obtained sites are a subset of the Rodriguez and Bates sites found in α-Fe2O3 and indicate muon oxygen bond formation. The sites missing at low temperatures are consistent with the vanadium pairing mechanism for the metal-to-insulator (corundum-to-monoclinic) phase transition.

Muon spin rotation study of V2O3

Muon spin rotation (μSR) measurements have been made on V2O3 as a function of temperature and externally applied magnetic field. V2O3 is an insulating antiferromagnet below about 155 K, while above this temperature it is a high‐resistivity metal. The conduction arises from the band mechanism where the density of states is quite sensitive to temperature and doping. The local magnetic field, sensed by the positive muon, has been measured from room temperature down to 10 K, with emphasis on the transition region. Clear evidence of the onset of magnetization is provided by a jump in the muon precession frequency. From experiments using an externally applied magnetic field the directions of the internal fields at the muon stop sites are found. This information can be used to determine the muon stop site since the field at this site should be determined primarily by the magnetic dipoles of the V3+ ions. Dynamic effects are observed through the muon spin depolarization. The results obtained for V2O3 are discusse...