Evolution of ferromagnetic clustering in Pr0.5Ca0.5MnO3 and its effect on the critical temperature of YBa2Cu3O7 thin film

We report neutron-scattering studies on static magnetic orders and spin excitations in the Fe-based chalcogenide system Fe 1+␦ Se x Te 1−x with different Fe and Se compositions. Short-range static magnetic order with an in-plane wave vector near the ͑0.5,0͒ ͑using the two-Fe unit cell͒, together with strong low-energy magnetic excitations is found in all nonsuperconducting samples for Se doping up to 45%. When the static order disappears and bulk superconductivity emerges, the spectral weight of the magnetic excitations shifts to the region of reciprocal space near the in-plane wave vector ͑0.5, 0.5͒, corresponding to "collinear" spin correlations. Our results suggest that there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in this system. Excess Fe appears to be important for stabilizing the magnetic order that competes with superconductivity.

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Abstract. The as-grown single crystal Fe 1.08 Te 0.55 Se 0.45 with the tetragonal PbO-type structure is non-superconducting owing to the excess Fe. Superconductivity is induced after oxygen annealing with an onset and zero resistance transition temperature around 14.5 K and 11.5 K, respectively. The oxygen doping is evidenced by electron energy loss spectroscopy, and accompanied by improved homogeneity in the remaining PbO-type phase as well as an increase in the L 3 /L 2 intensity ratio of the Fe-L 2,3 edge, indicating an increase in Fe valence. Local phase transformation from the tetragonal PbO-type phase to the hexagonal NiAs-type phase is also observed after oxygen annealing.2