NEUTRON-DIFFRACTION STUDY OF MAGNETIC-ORDERING IN THE PYROCHLORE SERIES ND2MO2O7, TB2MO2O7, Y2MO2O7

Nd2Mo2O7, Y2Mo2O7, and Tb2Mo2O7 are all ordered crystalline oxides with the cubic pyrochlore structure. At 53 K, Nd2Mo2O7 is long-range ordered with a ferrimagnetic structure. A transition temperature of 95 K is determined from small-angle-neutron-scattering (SANS) data. Antiferro-magnetic structures are proposed below 20 K that are consistent with previous magnetization results. No magnetic scattering from Y2Mo2O7 was observed at low temperature, which is believed to be a result of the small moment on Mo4+. For Tb2Mo2O7 strong diffuse scattering from the large Tb3+ moments is observed below 25 K. Fourier analysis of this data yields an approximation to the radial correlation function, which gives information concerning spin-spin correlations for the first four coordination shells. The first-, third-, and fourth-neighbor correlations are ferromagnetic, but the second-neighbor correlation is strongly antiferromagnetic. SANS data show magnetic scattering only at large Q, reenforcing the view that antiferromagnetic interactions dominate. The Q-integrated magnetic SANS shows a temperature dependence similar to that for the correlation functions. The long-range order in Nd2Mo2O7 and lack of long-range order in Y2Mo2O7 and Tb2Mo2O7 can be understood, to a certain extent, in terms of Landau-theory results for this highly frustrated lattice.