Coupled Spin Ordering in the Ln(2)LiRuO(6) Double Perovskites

Synchrotron X-ray, neutron powder diffraction, and magnetic susceptibility measurements have been used to characterize the Ln(2)LiRuO(6) (Ln = Pr, Nd, Sm, Eu, Gd, and Tb) double perovskites. All compositions have the monoclinic P2(1)/n superstructure with a fully ordered arrangement of Li+ and Ru5+ cations. As the Ln-size decreases, the lattice volume decreases due to the increased tilting of the octahedra, but the Li-O and Ru-O distances remain constant. Antiferromagnetic transitions with Neel temperatures in the 20-35 K range were observed for all compositions, except for Gd2LiRuO6, which does not order down to 5 K. Nd2LiRuO6 shows a field-induced ferrimagnetic transition at similar to 4 Tesla. The Ln = Pr, Nd, and Tb materials have type I magnetic structures with coupled spin ordering on the Ln and Ru sublattices. Critical fits reveal that the Ln(3+) sublattice magnetization shows mean field behaviour (beta approximate to 0.5), whereas the Ru5+ spin order is consistent with the three-dimensional Heisenberg model (beta = 0.31-0.35).