Valency and spin states of substituent cations inBi2.15Sr1.85CaCu2O8+δ

Abstract

We studied the valency and spin behavior of M $=$ Mn, Fe, Co, Li, and Al in the high-temperature superconducting compound ${\mathrm{Bi}}_{2.15}{\mathrm{Sr}}_{1.85}\mathrm{Ca}{({\mathrm{Cu}}_{1\ensuremath{-}\mathrm{z}}{\mathrm{M}}_{\mathrm{z}})}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi-2212) for small values of $z$. Mn, Fe, and Co retain their magnetic moments, and our thermopower and magnetic susceptibility data imply ionization states ${\mathrm{Mn}}^{3+}$, ${\mathrm{Fe}}^{2+}$, and ${\mathrm{Co}}^{2+}$, while Li and Al are accommodated in the charge reservoir layers. Single-crystal studies show that the susceptibility of Co${}^{2+}$ ions in Bi-2212 is strongly anisotropic, with a weak anisotropy detected for ${\mathrm{Mn}}^{3+}$ and none for ${\mathrm{Fe}}^{2+}$. 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 ${T}_{c}$ 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 ${T}_{c}$ values to within 1% or 2%. This agrees with a 2D thermodynamic fluctuation argument given by Junod et al.