Abstract
We studied the valency and spin behavior of M Mn, Fe, Co, Li, and Al in the high-temperature superconducting compound (Bi-2212) for small values of . Mn, Fe, and Co retain their magnetic moments, and our thermopower and magnetic susceptibility data imply ionization states , , and , while Li and Al are accommodated in the charge reservoir layers. Single-crystal studies show that the susceptibility of Co ions in Bi-2212 is strongly anisotropic, with a weak anisotropy detected for and none for . 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 by Co compared with Fe for underdoped and optimally doped samples, show that the -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 values to within 1% or 2%. This agrees with a 2D thermodynamic fluctuation argument given by Junod et al.
2 More- Received 5 December 2008
DOI:https://doi.org/10.1103/PhysRevB.84.144503
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