Magnetic properties of Yb2Mo2O7 and Gd2Mo2O7 from rare earth Mossbauer measurements

Using Yb-170 and Gd-155 Mossbauer measurements down to similar to0.03 K, we have examined the semiconducting pyrochlore Yb2Mo2O7 where the Mo intra-sublattice interaction is anti-ferromagnetic and the metallic pyrochlore Gd2Mo2O7 where this interaction is ferromagnetic. Additional information was obtained from susceptibility, magnetisation and Yb-172 perturbed angular correlation measurements. The microscopic measurements evidence lattice disorder which is important in Yb2Mo2O7 and modest in Gd2Mo2O7. Magnetic irreversibilities occur at 17 K in Yb2Mo2O7 and at 75 K in Gd2Mo2O7 and below these temperatures the rare earths carry magnetic moments which are induced through couplings with the Mo sublattice. In Gd2Mo2O7, we observe the steady state Gd hyperfine populations at 0.027 K are out of thermal equilibrium, indicating that Gd and Mo spin fluctuations persist at very low temperatures. Frustration is thus operative in this essentially isotropic pyrochlore where the dominant Mo intra-sublattice interaction is ferromagnetic.