S-D CHARGE-TRANSFER AND MAGNETIC-ORDERING IN BI-2212 AND PB-DOPED BI-2212 THROUGH MOSSBAUER-SPECTROSCOPY

Fe-57-doped Bi-2212 and Bi, Pb-2212 samples were prepared by the matri x method. Their phase purity was checked by thermoanalytical technique s and by X-ray diffraction. Mossbauer spectra show the characteristic of Fe(3+) in a pyramidal coordination. The Mossbauer parameters, isome r shift (IS) and quadrupole splitting (QS), of the Pb-doped samples we re always slightly less than those of Pb-free samples. The temperature dependence of IS and QS were measured down to 15 K. As the temperatur e is decreased, both IS and QS change rapidly, show a pronounced disto rtion around 100-50 K, and then rapidly decrease. The drop was much mo re drastic in Pb-free nitrogen annealed samples. This is interpreted i n terms of s-d charge transfer and charge delocalization as the temper ature is reduced. The second-order Doppler shift (SOD) is calculated f rom the Menil's expression using the literature value of Debye tempera ture theta(D) = 237 K. Pb-free nitrogen annealed sample shows the magn etic ordering around 15 K with a field of 40.4 T. A region of interest (ROI) thermal scan is performed to measure the Neel ordering temperat ure T-N which is found to be 17 K. The orientation of the Fe-spins wit h respect to the EFG axis is deduced.