We report measurements of the ac susceptibility of the cooperative paramagnet ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ in a strong magnetic field. Our data show the expected saturation maximum in $\chi (T)$ and also an unexpected frequency dependence of this peak at low frequencies ($<1\mathrm{Hz}$), suggesting very slow spin relaxations are occurring. Measurements on samples diluted with nonmagnetic ${\mathrm{Y}}^{3+}$ or ${\mathrm{Lu}}^{3+}$ and complementary measurements on pure and diluted ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ strongly suggest that the relaxation is associated with dipolar spin correlations, representing unusual cooperative behavior in a paramagnetic system.

• Received 14 July 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.027216