Long-range magnetic order in the Heisenberg pyrochlore antiferromagnets Gd2Ge2O7 and Gd2Pt2O7 synthesized under high pressure

Gd2Sn2O7 and Gd2Ti2O7 have been regarded as good experimental realizations of the classical Heisenberg pyrochlore antiferromagnet with dipolar interaction. The former was found to adopt the Palmer-Chalker state via a single, first-order transition at T-N approximate to 1K, while the latter enters a distinct, partially ordered state through two successive transitions at T-N1 approximate to 1K and T-N2 = 0.75 K. To shed more light on their distinct magnetic ground states, we have synthesized two more gadolinium-based pyrochlore oxides, Gd2Ge2O7 andGd(2)Pt(2)O(7), under highpressure conditions and performed detailed characterizations via x-ray powder diffraction, dc and ac magnetic susceptibility, and specific heat measurements down to 100 mK. We found that both compounds enter a long-range antiferromagnetically ordered state through a single, first-order transition at TN = 1.4K for Gd2Ge2O7 and TN = 1.56K for Gd2Pt2O7, with the specific heat anomaly similar to that of Gd2Sn2O7 rather than Gd2Ti2O7. Interestingly, the low-temperature magnetic specific heat values of both Gd2Ge2O7 and Gd2Pt2O7 were found to follow nicely the T-3 dependence as expected for a three-dimensional antiferromagnet with gapless spin-wave excitations. We have rationalized the enhancement of TN in terms of the reduced Gd-Gd distances for the chemically pressurized Gd2Ge2O7 and the addition of extra superexchange pathways through the empty Pt -eg orbitals for Gd2Pt2O7. Our current study has expanded the family of gadolinium-based pyrochlores and permits us to achieve a better understanding of their distinct magnetic properties in a more comprehensive perspective.