Itinerant-localized duality picture of spin fluctuations in high-T c cuprates. II. A unified phenomenology

We present a phenomenology of spin fluctuations in the normal metallic state of high-T_c, cuprates where the dynamical spin susceptibility resulting from the modemode couplings of the itinerant fermions and the localized spins consists of two parts: the coherent and the incoherent ones. The long-standing paradoxes on the experiments of the neutron scattering and the NMR, concerning the magnetic correlation length and the longitudinal and the transverse NMR relaxation rates, are resolved and the nature of the spin gap for these quantities is clarified.