We propose a unified magnetic phase diagram of cuprate superconductors. A new
feature of this phase diagram is a broad intermediate doping region of
quantum-critical, z=1, behavior, characterized by temperature independent
T1T/T2G and linear T1T, where the spin waves are not completely
absorbed by the electron-hole continuum. The spin gap in the moderately doped
materials is related to the suppression of the low-energy spectral weight in
the quantum disordered, z=1, regime. The crossover to the z=2 regime, where
T_1T/T_{\rm 2G}^2 \simeq \mbox{const}, occurs only in the fully doped
materials.Comment: 14 pages, REVTeX v2.1, PostScript file for 3 figures attached,
UIUC-P-93-06-04