Magnetic order in the double pyrochlore Tb₂Ru₂O ₇

Polycrystalline Tb₂Ru₂O₇ has been studied using dc susceptibility, specific heat and neutron scattering techniques. The high temperature paramagnetic state is dominated by the single ion character of Tb³⁺ and very similar to that of the well-studied spin liquid Tb ₂Ti₂O₇. However, both the Ru⁴⁺ and Tb³⁺ sublattices order, at about 110K and 3.5K, respectively. Although the Tb sublattice does not fully order until 3.5K, it is polarized in the presence of the internal field generated by the Ru⁴⁺ sublattice and possesses a significant moment at 7K. Magnetic entropy measurements suggest that four levels exist in the first 30K and inelastic neutron scattering investigations revealed two more levels at 10 and 14meV. As the magnetic sublattices order, the excitations are perturbed from that measured in the paramagnetic state. These data are compared to data for other terbium based and double pyrochlores.