Orbital order and partial electronic delocalization in a triangular magnetic metal Ag₂MnO₂

Magnetic and electrical properties of Ag₂MnO₂ were examined by elastic and inelastic neutron-scattering measurements and by density-functional calculations. The spins of the triangular antiferromagnet metal Ag₂MnO₂ are found to freeze into a gapless short-range collinear state below 50 K because of a ferro-orbital ordering and spin-orbit coupling of the high-spin Mn³⁺ ions. The decrease in the spin-spin correlation lengths of Ag₂ MnO₂ in the order, ξ_b ≫ ξ_a ≫ ξ_c, is explained by the spin-exchange interactions calculated for the ferro-orbital ordered state. The electronic states around the Fermi level have significant contributions from the spin-polarized Mn3d and O2p states, which makes electron-electron scattering dominate over electron-phonon scattering at low temperatures leading to the ρ∝ T² behavior below 50 K.