Incommensurate Magnetic Fluctuations inYBa2Cu3O6.6

We use inelastic neutron scattering to demonstrate that the low-frequency magnetic fluctuations in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$ ( $T_c=62.7\mathrm{}$ K) change from commensurate to incommensurate on cooling with the incommensurability first appearing at temperatures above $T_c$. For the energies studied, the susceptibility at incommensurate positions increases on cooling below $T_c$, accompanied by a suppression of the spin fluctuations at the commensurate points. These results suggest that incommensurate spin fluctuations may be a common feature for all cuprate superconductors.