Valency and spin states of substituent cations in Bi2.15Sr1.85CaCu2O8+δ.

We studied the valency and spin behavior of M = Mn, Fe, Co, Li, and Al in the high-temperature superconducting compound Bi2.15 Sr1.85Ca(Cu1-zMz)2O8+δ (Bi-2212) for small values of z. Mn, Fe, aud Co retain their magnetic moments, and our thermopower and magnetic susceptibility data imply ionization states Mn3+, Fe-2+, and Co2+, while Li and Al are accommodated in the charge reservoir layers. Single-crystal studies show that the susceptibility of Co2+ ions in Bi-2212 is strongly anisotropic, with a weak anisotropy detected for Mn3+ and none for Fe2+. Fits to a pseudogap formula for a pure Bi-2212 crystal suggest that the spin susceptibility of the host compound is more anisotropic than previously realized. Data in the superconducting state allow us to compare the pair-breaking properties of the different impurities. Several aspects of the data, including the stronger suppression of the superconducting transition temperature Tc by Co compared with Fe for underdoped and optimally doped samples, show that the d-level structure of the magnetic ions and multiorbital effects are important. We also find that the temperatures of the magnetization crossing points are equal to the low-field Tc values to within 1% or 2%. This agrees with a 2D thermodynamic fluctuation argument given by Junod et al. [ABSTRACT FROM AUTHOR]