Evidence for Local Moment Magnetism in Superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$

We have used inelastic neutron scattering to measure the dynamic spin susceptibility in optimally-doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (T$_{c}$ = 91 K). Four crystals with a total mass of 19 g were measured on the MAPS spectrometer at ISIS for temperatures of 10 K and 100 K. We have been able to identify the magnetic excitations in the energy range 20-90 meV. The magnetic nature of the scattering has been confirmed with spin-polarization analysis on IN22 at the ILL. While we see temperature-dependent changes for energies around 40 meV that are consistent with earlier studies, we find that the Q-integrated signal shows a much weaker variation with temperature. The absolute magnetic cross section is quite comparable to that of spin fluctuations in stripe ordered La$_{1.875}$Ba$_{0.125}$CuO$_{4}$. As the magnetism in the latter system has been shown to have a dominant contribution from local moments [1], we argue that the same must be true for Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. [1] M. Huecker et al., Phys. Rev. B (accepted); cond-mat/0503417v3.