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We have measured the structure of the field cooled flux line lattice (FLL) in single crystal Nb using small angle neutron scattering. Augmented by transport and thermodynamic data, a scenario for the dramatic disordering of the FLL near the peak effect emerges. A precursor to the peak effect is an hexatic FLL as positional order is lost. As the critical current rises, the orientational and longitudinal correlation lengths also fall, leading to an amorphous array of lines at the critical current maximum. [S0031-9007(97)05112-0]

We investigated Prl 5Ceo 5Sr2NbCuzO&0 and the superconducting Eul 5Ceo 5Sr2NbCu20lo materials by several complementary experimental techniques. Oxygen-rich Prl 5Ceo, Sr2NbCu20, 0 is not superconducting and the magnetic-susceptibility data reveal two magnetic anomalies at 11 and 54 K. Both anomalies always appear together and are sensitive to oxygen concentration, whereas for oxygen-poor material both anomalies are absent. The presence of 0.5% Fe dramatically affects only the transition at 54 K and shifts it to 94 K, and Mossbauer studies indicate that this transition is related to antiferromagnetic ordering of the Cu sublattice. The peak in the susceptibility at 11 K is probably due to spin reorientation of Cu moments. While the anomalies are clearly identified by magnetic measurements, no specific-heat anomaly was observed at either temperature. A strong two-dimensional quantum spin fluctuation is assumed as the cause of the unusual behavior at TN of Cu. A high linear term of 265 mJ/mol Pr E is obtained in the specific heat of the oxygen-rich Prl, Ceo,Sr2NbCu20lo material.

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