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 Mössbauer, 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 Mössbauer 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 Mössbauer data also indicates that the transition at 0.75 K involves a small change in the magnetic structure.