Spin-liquid dynamics of pyrochlore magnets Gd2Ti2O7 and Gd2Sn2O7

The spin-liquid phase of two highly frustrated pyrochlore magnets Gd2Ti2O7 and Gd2Sn2O7 is probed using electron spin resonance in the temperature range 1.3 - 30 K. The deviation of the absorption line from the paramagnetic position \nu =\gamma H observed in both compounds below the Curie-Weiss temperature \Theta_CW ~ 10 K, suggests an opening up of a gap in the excitation spectra. On cooling to 1.3 K (which is above the ordering transition T_N ~ 1.0 K) the resonance spectrum is transformed into a wide band of excitations with the gap amounting to \Delta ~ 26 GHz (1.2 K) in Gd2Ti2O7 and 18 GHz (0.8 K) in Gd2Sn2O7. The gaps increase linearly with the external magnetic field. For Gd2Ti2O7 this branch co-exists with an additional nearly paramagnetic line absent in Gd2Sn2O7. These low lying excitations with gaps, which are preformed in the spin-liquid state, may be interpreted as collective spin modes split by the single-ion anisotropy.