We report a systematic study on the low-temperature thermal conductivity (kappa) of R_2Ti_2O_7 (R = Gd and Er) single crystals with different directions of magnetic field and heat current. It is found that the magnetic excitations mainly act as phono
n scatterers rather than heat carriers, although these two materials have long-range magnetic orders at low temperatures. The low-T kappa(H) isotherms of both compounds show rather complicated behaviors and have good correspondences with the magnetic transitions, where the kappa(H) curves show drastic dip- or step-like changes. In comparison, the field dependencies of kappa are more complicated in Gd_2Ti_2O_7, due to the complexity of its low-T phase diagram and field-induced magnetic transitions. These results demonstrate the significant coupling between spins and phonons in these materials and the ability of heat-transport properties probing the magnetic transitions.
