Spin- 1 2 triangular lattice with orbital degeneracy in a metallic oxide Ag2 Ni O2

A metallic and magnetic transition metal oxide Ag2 Ni O2 is studied by means of resistivity, magnetic susceptibility, specific heat, and x-ray diffraction. The crystal structure is characterized by an alternating stacking of a Ni3+ O2 layer and a (Ag2) + layer, the former realizing a spin- 1 2 triangular lattice with eg orbital degeneracy and the latter providing itinerant electrons. It is found that the Ni O2 layer exhibits orbital ordering at Ts =260 K and antiferromagnetic spin ordering at TN =56 K. Moreover, a moderately large mass enhancement is found for the itinerant electrons, suggesting a significant contribution from the nearly localized Ni 3d state to the Ag 5s state that forms a broad band.