Effects of Vortex Pinning on the Temperature Dependence of the Magnetic Field Distributions in Superconductors

The temperature and applied-magnetic-field dependence of the second moments of the magnetic-field distributions as measured by mu SR for YBCO and BSCCO have been fit for four different intrinsic-field-distribution models (d-wave, 2-fluid, empirical, and BCS). It is found that if a pinning potential becomes important at about 20 K, all of the models can fit the data reasonably well. The fits and the associated fitting parameters are presented

Characterization of superconducting and magnetic materials with muon spin rotation and neutron scattering. Progress report, March 1996--August 1997 and final report, June 1988--August 1997

This report represents the culmination of over nine years of research activity in the study of superconducting and magnetically ordered materials using the muon spin rotation ({mu}SR) and neutron scattering techniques. Because all the activities that took place up until March 1996 have been covered in previous annual reports, this final report includes only a brief overview of activities prior to that date, and concentrates on the period from March 1996 through August 1997. The primary activity undertaken in this project has been studies of high-temperature superconductors and their close chemical relatives with the {mu}SR technique. These experiments extend from early work done with a relatively primitive muon beam at the AGS of Brookhaven National Laboratory and large polycrystalline samples of the earliest known high-{Tc} materials to studies of very small high-purity single crystals of the best high-{Tc} materials currently available using the highest quality surface muon beams and specially-designed low-background spectrometers at the Tri-University Meson Facility (TRIUMF) in the past three years. During the period since the last annual report five {mu}SR experiments were done at TRIUMF with DOE support. A study of single-crystal high-temperature superconductors was done in July 1996. A study of the quasicrystal materials Gd{sub 8}Mg{sub 42}Zn{sub 50} and Tb{sub 8}Mg{sub 42}Zn{sub 50} was done by D.R. Noakes in collaboration with G.M. Kalvius of the Technical University of Munich and R. Waeppling of Uppsala University during the first week of December 1996. During the second week of December 1996 a study of the cryocrystals CH{sub 4} and CF{sub 4} was done by D.R. Noakes in collaboration with S. Storchak of Moscow State University and J.H. Brewer of the University of British Columbia. A study of high-{Tc} superconductors was done at TRIUMF during the third week of December 1996 by C.E. Stronach and D.R. Noakes

Field dependence on texture in HTc's

International audienc

A μ SR magnetic study of UNiGe

We have carried out SR spectroscopy on polycrystalline UNiGe between 2 K and 100 K. The existence of two magnetic transitions at TN=51 K and T1=41.5 K is confirmed. The SR spectra clearly reveal that the magnetic state between 51 K and 41.5 K is an incommensurate spin structure ruling out a spin-slip structure which had been considered an alternative. Below 41.5 K the spectra are compatible with simple antiferromagnetic order. The local field for T→0 is Bμ=170 mT, a comparatively low value, indicating a rather small uranium ordered moment. When going from the commensurate to the incommensurate structures at T1 a sudden reduction in local field by 23% occurs reflecting an equal change in ordered moment. The transition at T1 is sharp, but TN extends over roughly 5 K. The antiferromagnetic spin structure exhibits persistent spin fluctuations in the limit T→0, implying the presence of some additional spin interactions which tries to suppress long-range magnetic order

Texture effect on vortex-state TF-µ+SR in Bi-2223 high-Tc materials

International audienceThe influence of texture in Bi‐2223 ingots on the temperature dependence of the μ+ spin relaxation rate has been investigated. Texture was induced during cold‐isostatic‐pressing by applying an additional uniaxial load which tends to align single grain c axes along the stress direction, ž. Small plates were cut with faces either ‖ or ⊥ ž and μSR was measured in a 15 mT transverse field (TF) during cooling from room temperature to 6 K. While for highly‐textured material, low‐temperature limits are substantially smaller than expected, the large shift (55%) observed when switching from ž⊥TF to ž‖TF clearly indicates that anisotropy has developed. Relaxation rates for the weakly‐textured sample lie in between. To a lesser extent, sintering also enhanced the low‐temperature relaxation, but did not affect TC significantly. All samples showed a weak onset of depolarization between 60 K and 90 K, most likely due to the presence of a Bi‐2212 impurity phase. This impurity phase might cause the transition‐temperature smearing in the 100–110 K range in the weakly textured (and less compacted) material. Thus the sharpness of the relaxation drop might be relevant for assessment of material quality and be used as a criterion in the improvement of the production process

Muon spin relaxation in CeCu/sub 2/Si/sub 2/ and muon Knight shift in various heavy-fermion systems

Positive muon spin precession has been observed in varius heavy-fermion systems in the transverse external magnetic field. In the superconductor CeCu/sub 2.1/Si/sub 2/, the relaxation rate of muon spins increases rapidly with decreasing temperature below T/sub c/. This is interpreted as the results of the inhomogeneous fields due to the imperfect penetration of the external field into the type-II superconducting state. The magnetic-field penetration depth lambda is derived from the observed muon spin relaxation rate. lambda is about 1200 A at T approx. 0.5T/sub c/, and the temperature dependence of lambda is consistent with the relation expected for a BCS superconductor. We have also measured the muon Knight shift K/sub ..mu../ in the normal (or paramagnetic) state of various heavy-fermion systems. K/sub ..mu../ is large and negative (about -1000 approx. -3000 ppM at T = 10 K) for CeCu/sub 2/Si/sub 2/, UPt/sub 3/ and CeAl/sub 3/, while more complicated signals are measured in CePb/sub 3/ and CeB/sub 6/. The negative muon Knight shift in the non-magnetic heavy-fermion systems is discussed in terms of the Kondo-coupling between the conduction- and f-electrons