Critical current measurements in YBa2(Cu1−xFex)3O7−δ

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Abstract
The influence of Fe substituted for Cu in YBa2Cu3O7−δ on the microstructure and the critical properties has been studied. In the concentration regime 0≤x≤0.05, the microstructure changes drastically from a twin structure for x≤0.01 to a microdomain structure for x≥0.02. The average spacing of the twin or domain boundaries varies from ∼90 nm for x=0 to ∼5 nm for x≥0.02, thus providing an ideal system to test the role of boundary pinning. Large intragranular pinning forces are observed for x≤0.01 followed by a sudden drop to values ∼20 times smaller for x≥0.02. However, although d/a0 (twin‐spacing/flux‐line lattice parameter) changes drastically from d/a0>1 for x=0 to d/a0x≥0.02, the same field dependence of Fp is observed for all Fe concentrations, i.e., Fp∼B2c2b(1−b)2. It is, therefore, concluded that the twin boundaries are not significantly contributing to the flux pinning and that the decrease in Fp for 0.01≤x≤0.02 is related to the decrease of the upper critical field Bc2, which is caused by a strong reduction of the electron density of states at the Fermi surface.