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A neutron small-angle scattering study of the flux-line lattice in heavily twinned YBa2Cu3O72x is presented. It is found that the diffraction signal disappears at temperatures well below Tc , associated with a melting of the flux lattice. The shape of the melting line is consistent with both a Lindemann criterion and the scaling expected for a vortex-glass transition with the superconducting parameters from the three-dimensional XY model. The influence of twin planes on the structure of the vortex lattice and its melting is studied by applying the field at different angles to the c axis. The results are compared with recent specific heat measurements on similar crystals. @S0163-1829~98!03022-7#

We review some of the assumptions made in the use of muon spin rotation in superconductors: i.e. that the muons are implanted at random positions in the flux lattice, remain static after implantation and do not appreciably affect the properties of the surrounding superconductor; also that the flux lines are straight and static, and that the observed muon rotation frequency spectrum reflects the microscopic distribution of field values. We shall show how evidence for and against the truth of these assumptions in particular cases may be obtained from the μSR results themselves or by comparison with other measurements, and how this in turn may lead to deeper understanding of flux line structure and motion in superconductors.

The magnetic ordering in single crystals of PrBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} has been investigated by elastic neutron scattering over the full range of temperatures for reduced and oxygenated crystals. The crystals were grown in alumina crucibles and therefore contained dissolved aluminum on the Cu(1) site. Both aluminum and oxygen contents were analyzed in detail in order to establish their effects on the magnetic ordering. Our crystals exhibited Pr ordering and the two types of antiferromagnetic Cu ordering frequently reported in related compounds, but our results differ in several respects from previous studies. We observed three-dimensional (3D) collinear ordering of the Pr moments in the oxygenated crystal, with a finite correlation length of (34{plus_minus}3) A parallel to the {ital c} axis, but in contrast to earlier neutron scattering results, which tentatively placed the Pr moment as being parallel to the {ital c} axis, we find the moment to be aligned well away from the {ital c} axis, in agreement with recent {sup 170}Yb{sup 3+} M{umlt o}ssbauer spectroscopy results. Ridges of scattering indicative of 2D magnetic ordering were seen in both oxygenated and reduced crystals, though we believe different magnetic moments are responsible in each case. The Pr N{acute e}el temperaturesmore » were suppressed compared with those reported for nominally pure samples. We have observed a very small 3D-ordered moment on the Cu(1) site in the second type of Cu antiferromagnetic ordering (in the reduced crystal), but an ordered Cu(1) moment may not necessarily be a characteristic of this structure. Our results are discussed with reference to previous works, and in the light of the Al substitution. {copyright} {ital 1996 The American Physical Society.}« less

We have used the technique of polarized-neutron reflectometry to study the magnetic-induction profile just beneath the surface of a 1-\ensuremath{\mu}m-thick film of lead with the magnetic field applied parallel to the surface. The sample was maintained at a temperature of 1.5 K throughout the experiment, and the applied field H was varied from below ${\mathit{H}}_{\mathit{c}}$, the bulk critical field, up to the critical field for surface superconductivity, ${\mathit{H}}_{\mathit{c}3}$. From the measurements with the lead film in the bulk superconducting phase (H${\mathit{H}}_{\mathit{c}}$), we found that the spin-dependent reflectivity profiles are consistent with an exponential decay of magnetic induction with a penetration depth \ensuremath{\lambda}=39\ifmmode\pm\else\textpm\fi{}1 nm. At higher applied fields (${\mathit{H}}_{\mathit{c}}$H${\mathit{H}}_{\mathit{c}3}$) we obtained information on the diamagnetism in the surface superconducting layer. We discuss the sensitivity of the measurements to the form of the magnetic-induction profile, in particular to nonlocal effects, and show that a good description of the surface diamagnetism, over the whole range of applied fields, can be achieved with the local Ginzburg-Landau theory if the Ginzburg-Landau parameter \ensuremath{\kappa} is allowed to vary with the applied field.

Small-angle neutron diffraction measurements from the flux lattice in a single crystal of the cubic Laves' phase superconductor, CeRu2, are reported. The mixed state is described in terms of aligned rigid bundles of vortices. The bundle diameters decrease above 1 2 H c 2 (consistent with collective weak pinning theory) and become comparable with the penetration length at a field at which a ‘peak effect’ is seen in magnetisation measurements. A clear memory of field histories that pass through the ‘peak effect’ region is also found; however, some of the induced disorder can be removed by subsequently cycling the field.

We have studied the magnetic ordering of the Cu and Pr ions in PrBa 2 Cu 3 O 6+x by neutron diffraction on single crystals with different oxygen contents. Two types of Cu ordering were observed, qualitatively similar to the anti-ferromagnetic phases reported in some studies of YBa 2 Cu 3 O 6+x . A third magnetic structure was observed below 15K, which we believe corresponds to the magnetic ordering of the Pr sub-lattice.