  Two geometrically frustrated pyrochlore stannates, undergoing long range
magnetic order below 1K, were investigated at very low temperature. Anomalies
in the behaviour of hyperfine quantities are found, by 155Gd Mossbauer
spectroscopy in Gd2Sn2O7 and by low temperature specific heat measurements in
Tb2Sn2O7. They are interpreted in terms of fluctuations of the correlated Gd or
Tb spins, using a model two-level system (the nuclear spins) submitted to a
randomly fluctuating (hyperfine) field.
