  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
$T_1T/T_{\rm 2G}$ and linear $T_1T$, 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.

Condensed Matter