D. W. Cooke

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.

Magnetic ordering in (Y1−xPrx)Ba2Cu3O7 as evidenced by muon spin relaxation

Using the zero‐field‐muon‐spin‐relaxation (μSR) technique clear evidence has been found for antiferromagnetic ordering of Cu moments within the CuO planes of (Y1−xPrx)Ba2Cu3O7. The Neel temperatures are approximately 285, 220, 35, 30, and 20 K for x=1, 0.8, 0.6, 0.58, and 0.54, respectively. For x=0.50 we observe a fast‐relaxing component of the muon polarization in addition to a long‐time tail, reminiscent of spin‐glass behavior. This region of the phase diagram (0.5≤x≤0.54) corresponds to the existence of both superconductivity and magnetism. The fully developed local magnetic field for x>0.54 is found to be ∼16 mT, but decreases to ∼12 mT at T=17 K for the x=1 sample, presumably due to the onset of Pr‐ion ordering. Magnetic ordering also occurs in PrBa2Cu3O6; the Neel temperature is ∼325 K.

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.

Nature of the spin state in TmNi2B2C

Abstract We had observed earlier that in the quaternary borocarbide magnetic superconductor TmNi2B2C ( T c =11 K , T N =1.5 K ) quasistatic magnetic correlations persist well above TN (up to ∼20xa0K), which is rather unusual. To further probe the nature of its spin state, we used magnetically oriented powder samples of TmNi2B2C and the μ-SR technique. We observe that the quasistatic internal field has a well defined direction. The spontaneous muon precession signal has three times the amplitude when the c-axis alignment is perpendicular to the initial muon beam polarization than when parallel. The transverse configuration enhances the oscillatory signal. This has enabled us to show that the quasistatic magnetic correlations persist even up to 50xa0K. Furthermore, we find a peak around 7xa0G in the field dependence of the longitudinal relaxation, which we attribute to the presence of a level crossing resonance arising from quadrupole levels of the boron nuclei.

Interplay of magnetism and superconductivity in thulium and lutetium nickel-borocarbides

Abstract Implanted muon spectroscopy shows that magnetic order coexists with superconductivity in TmNi2B2C. The muon response indicates that large amplitude fluctuations of the internal field are superimposed on a relatively small static component. The static field exhibits an unusual temperature dependence which shows no interruption at the superconducting transition and may represent evolution of a staggered or spiral arrangement of Tm moments. The dynamic component changes its spectral density at the superconducting transition and the question arises as to whether this is associated with Ni moment formation and fluctuation. In LuNi2B2C, where the rare earth ion is nonmagnetic, no signature of static order is seen. An intriguing change in the muon response does occur below about 4 K in this material; this seems suggestive of Ni moment fluctuation, although the evidence is not entirely conclusive.

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...

A new polaronic order-disorder phase transition in magnetite as observed through μSR

Recent μSR measurements on the Mott-Wigner glass, magnetite, as a function of temperature and external magnetic field have shown the existence of two inequivalent magnetic sites below TA=247K. These data are being interpreted in terms of the onset or destruction of local order manifested as local atomic correlations (molecular polarons).

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.

PROGRESS IN PION-DECAY CHANNELLING: REFRACTORY bcc METALS AT HIGH AND LOW TEMPERATURES

By means of π+/μ+ channelling, positive pions (π+) implanted intoTa, Mo, andW are investigated up to high temperatures. A striking observation is that the channelling effect disappears in a rather narrow temperature interval centred at 0.26 (Ta) to 0.51 (W) of the melting temperature. From studies of π+ trapping by oxygen atoms inTa estimates for the low-temperature π+ diffusivity inTa [Dπ(23K)=1.4·10−10±0.3 m2s−1,Dπ(47K)=5.7·10−10±0.3 m2s−1] as well as for the binding enthalpy of π+ to 0 atoms (HB=7·10−2 eV) have been obtained. The diffusion data are in reasonable agreement with the theory of phonon-assisted tunnelling.