Slow spin relaxation in a highly polarized cooperative paramagnet.

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 $\ensuremath{\chi}(T)$ and also an unexpected frequency dependence of this peak at low frequencies ($l1\text{ }\text{ }\mathrm{Hz}$), 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.