Structural, Magnetic, and Superconducting Properties of Single Crystals of Yttrium BARIUM(2) COPPER(3-X) Iron(x) OXYGEN(7-DELTA)

Measurements of the magnetic properties, superconducting properties, and the structural features of rm YBa_2 Cu_{3-x} Fe_ x O_ {7-delta} were done on single crystals with concentrations of iron x = 0.06, 0.09, 0.15, and 0.24. The magnetic measurement analysis in a Curie-Weiss fashion gave an effective magnetic moment corresponding to a low spin state (t^5_ {2g}) of iron(III). The lack of magnetic anisotropy between the ab plane and the c direction, and the uniform antiferromagnetic ordering seen with increasing iron concentrations support the existence of iron clusters. Superconducting measurements in these single crystals showed the magnitude of lower critical field (H _{rm c1}) in both directions to be similar to that obtained in untwinned rm YBa_2 Cu_{3-x} Fe_ x O_ {7-delta} and also showed a decrease in superconducting anisotropy with increased iron concentration. Similarly, upper critical field (H_{rm c2 }) showed a decrease in anisotropy between the two directions, and a change in the mass ratio, and derived Ginzberg-Landau superconducting quantities. All of these effects on H_{rm c1 } and H_{rm c2} are also consistent with a cluster model of Fe. The lattice parameters of the X-ray analysis of the single crystals showed an apparent orthorhombic-tetragonal transition at an iron concentration of x = 0.15. The structural analysis showed that the coupling distances between the CuO planes and chains are responsible for the change in superconducting anisotropy with increasing iron concentration.