| We report thermodynamic measurements on the cooperative paramagnet Tb 2Ti2O7 and the stuffed spin ices Ho2(Ho xTi2-x)O7-x/2 and Dy2(DyxTi 2-x)O7-x/2, where 0 ≤ x ≤ 0.67. For Tb2Ti 2O7, AC susceptibility data taken down to T = 1.8 K and in applied magnetic fields up to H = 9 T 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 Lu 3+ and complementary measurements on pure and diluted Dy2Ti 2O7 strongly suggest that the relaxation is associated with dipolar spin correlations, representing unusual cooperative behavior in a paramagnetic system.;For the stuffed spin ices Ho2(HoxTi2-x)O 7-x/2 and Dy2(DyxTi2-x)O7-x/2 , 0 ≤ x ≤ 0.67, magnetization data show an increasingly antiferromagnetic effective interaction with increased stuffing x, with Ising like single spin ground states. Although heat capacity measurements down to T = 0.4 K yield the expected residual entropy for x = 0, surprisingly, despite the changing magnetic interactions, the total entropy per spin in (HoxTi 2-x)O7-x/2 remains at the spin ice value for all x, while the entropy per spin in Dy2(DyxTi2-x)O 7-x/2 approaches the Ising value of S = Rln2 for x ≥ 0.3. AC susceptibility measurements confirm different low temperature states in Ho2.67Ti 1.33O6.67 and Dy2.67Ti1.33O6.67 , showing a disordered spin freezing in Ho2.67Ti1.33 O6.67, and a partial freezing in Dy2.67Ti 1.33O6.67 coexisting with behavior seen in some cooperative paramagnets and spin liquids. |
