FLUCTUATION MAGNETOCONDUCTIVITY IN Fe DOPED YBa2Cu3O7-δ; SIGN CHANGE FOR B AND I PARALLEL TO THE PLANES

The magnetoconductivity in Fe doped YBa 2 Cu 3 O 7-δ single crystals has been measured in the temperature region above T c where superconducting fluctuations are known to be important. For small Fe concentrations and B//c we obtain excellent agreement with accepted fluctuation theories. However, at higher Fe concentrations this description seems to break down. For B⊥c we observe an anomalous dependence on the relative orientation of current and field, both applied along the ab-planes. For B//I the magnetoconductivity is negative, as expected. For B⊥I we observe a cross-over to a positive magnetoconductivity above about T c + 10 K.