Low temperature magnetic properties of geometrically frustrated Gd2Sn2O7 and Gd2Ti2O7

We have examined the low temperature magnetic properties of the geometrically frustrated antiferromagnetic pyrochlores Gd2Sn2O7 and Gd2Ti2O7 using specific heat, 155Gd M?ssbauer, magnetic susceptibility and magnetization measurements. For Gd2Sn2O7, the specific heat evidences a single, strongly first order magnetic transition near 1.0?K; in Gd2Ti2O7, we confirm the presence of both the transition near 1.0?K and the second transition near 0.75?K. Below 1?K, magnetic irreversibilities are present in both compounds, but their signature (the difference between the FC and ZFC branches) is more marked in Gd2Sn2O7. At 0.03?K in each compound, the M?ssbauer data show that the four Gd3+ of a tetrahedron carry moments of equal sizes and on a frequency scale of 120 ? 106?s?1 each is oriented perpendicular to the local direction. In Gd2Ti2O7, the M?ssbauer data also indicates that the transition at 0.75?K involves a small change in the magnetic structure.