  We report measurements of the ac susceptibility of the cooperative paramagnet
Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation
maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of
this peak, suggesting very slow spin relaxations are occurring. Measurements on
samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on
pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated
with dipolar spin correlations, representing unusual cooperative behavior in a
paramagnetic system.
