Tuning the interactions in the spin-ice materials Dy₂Ge_2−xSi_xO₇ by silicon substitution

We report that the lattice constant of Dy₂Ge_2−xSi_xO₇ (x = 0, 0.02, 0.08, 0.125) can be systematically reduced by substituting the nonmagnetic germanium ion in the cubic pyrochlore oxide with silicon. A multianvil high-pressure synthesis was performed up to 16 GPa and 1100 °C to obtain polycrystalline samples in a solid-state reaction. Measurements of magnetization, ac susceptibility, and heat capacity reveal the typical signatures of a spin-ice phase. From the temperature shift of the peaks, observed in the temperature-dependent heat capacity, we deduce an increase in the strength of the exchange interaction. In conclusion, the reduced lattice constant leads to a changed ratio of the competing exchange and dipolar interaction. This puts the new spin-ice compounds closer towards the phase boundary of a short-range spin-ice arrangement and antiferromagnetic long-range order consistent with an observed reduction in the energy scale of monopole excitations.