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A systematic neutron-scattering study of large near-optimally doped single crystals of the cuprate superconductor, Bi2Sr2CaCu2O8+δ, indicates that its magnetic properties are governed by localized magnetic moments, and not by itinerant quasiparticles, as widely expected. Much of what we know about the electronic states of high-temperature superconductors is due to photoemission1,2,3 and scanning tunnelling spectroscopy4,5 studies of the compound Bi2Sr2CaCu2O8+δ. The demonstration of well-defined quasiparticles in the superconducting state has encouraged many theorists to apply the conventional theory of metals, Fermi-liquid theory, to the cuprates6,7,8,9. In particular, the spin excitations observed by neutron scattering at energies below twice the superconducting gap energy are commonly believed to correspond to an excitonic state involving itinerant electrons10,11,12,13,14. Here, we present the first measurements of the magnetic spectral weight of optimally doped Bi2Sr2CaCu2O8+δ in absolute units. The lack of temperature dependence of the local spin susceptibility across the superconducting transition temperature, Tc, is incompatible with the itinerant calculations. Alternatively, the magnetic excitations could be due to local moments, as the magnetic spectrum is similar to that in La1.875Ba0.125CuO4 (ref. 15), where quasiparticles16 and local moments17 coexist.

Understanding the magnetic excitations in high-temperature (high-T(c)) copper-oxide superconductors is important because they may mediate the electron pairing for superconductivity(1,2). By determining the wavevector (Q) and energy ((h) over bar omega) dependence of the magnetic excitations, it is possible to calculate the change in the exchange energy available to the superconducting condensation energy(3-5). For the high-T(c) superconductor YBa(2)Cu(3)O(6+x), the most prominent feature in the magnetic excitations is the resonance(6-12). Suggestions that the resonance contributes a major part of the superconducting condensation(4,13) have not gained acceptance because the resonance is only a small portion of the total magnetic scattering(12-14). Here, we report an extensive mapping of magnetic excitations for YBa(2)Cu(3)O(6.95) (T(c) similar to 93 K). Absolute intensity measurements of the full spectra allow us to estimate the change in the magnetic exchange energy between the normal and superconducting states, which is about 15 times larger than the superconducting condensation energy(15,16) - more than enough to provide the driving force for high-T(c) superconductivity in YBa(2)Cu(3)O(6.95).