Spin-1/2 Triangular Lattice with Orbital Degeneracy in a Metallic Oxide Ag2NiO2

A novel metallic and magnetic transition metal oxide Ag2NiO2 is studied by means of resistivity, magnetic susceptibility, specific heat and X-ray diffraction. The crystal structure is characterized by 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 NiO2 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.