Iron spin fluctuations in 57Fe-doped high-Tc superconductors (Bi,Pb)2Sr2Can-1CunO2n+4 with n=2,3

Abstract

Bi-based superconductors of 2212 and 2223 type were doped with 1-3% 57Fe. 57Fe Mossbauer spectra were taken between 293 K and 2 K. In all samples indications for a slowing down of Fe spin fluctuations were found. In the 2223 superconductor doped with 3% Fe, a magnetically split spectrum with a hyperfine field of 40.5 T is measured at T=2 K. The Fe spins are aligned nearly parallel to the electric field gradient which is positive and lies close to the c axis. The Fe valence is +3 in all cases. The RT spectra are strongly dependent on the substitution of Pb for Bi. The spectra of both the Pb-containing 2212 and 2223 samples show one well-defined doublet with a quadrupole splitting value of 1.12 mm s-1. Without Pb substitution, the spectra of the 2212 system consist of two doublets with quadrupole splittings of about 0.6 mm s-1 and 1.7 mm s-1, with their relative intensities depending on the Fe concentration.