Frustrated magnetism in the double perovskite L a2LiOs O6: A comparison with L a2LiRu O6

The frustrated double perovskite La2LiOsO6, based on Os5+(5d3,t23) is studied using magnetization, elastic neutron scattering, heat capacity, and muon spin relaxation (μSR) techniques and compared with isostructural (P21/n)La2LiRuO6,Ru5+(4d3,t23). While previous studies of La2LiOsO6 showed a broad susceptibility maximum (χmax) near 40 K, heat capacity data indicate a sharp peak at 30 K, similar to La2LiRuO6 with χmax∼30K and a heat capacity peak at 24 K. Significant differences between the two materials are seen in powder neutron diffraction where the magnetic structure is described by k=(1/21/20) for La2LiOsO6, while La2LiRuO6 has been reported with k=(000), structure for face centered lattices. For the k=(1/21/20) structure, one has antiferromagnetic layers stacked antiferromagnetically, while for k=(000) structure, ferromagnetic layers are stacked antiferromagnetically. In spite of these differences, both can be considered as type I fcc antiferromagnetic structures. For La2LiOsO6, the magnetic structure is best described in terms of linear combinations of basis vectors belonging to irreducible representations Γ2 and Γ4. The combinations Γ2-Γ4 and Γ2+Γ4 could not be distinguished from refinement of the data. In all cases, the Os5+ moments lie in the yz plane with the largest component along y. The total moment is 1.81(4) μB. For La2LiRuO6, the Ru5+ moments are reported to lie in the xz plane. In addition, while neutron diffraction, μSR and NMR data indicate a unique TN=24K for La2LiRuO6, the situation for La2LiOsO6 is more complex, with heat capacity, neutron diffraction, and μSR indicating two ordering events at 30 and 37 K, similar to the cases of cubic Ba2YRuO6 and monoclinic Sr2YRuO6.