Itinerant and localized magnetism on the triangular lattice: Sodium-rich phases of Na_xCoO₂

We study the interplay between correlation, itinerant ferromagnetism, and local moment formation on the electron doped triangular lattice of sodium cobaltates Na_xCoO₂. We find that strong correlation renormalizes the Stoner criterion and stabilizes the paramagnetic state for x<x_c≃0.67. For x>x_c, ferromagnetic (FM) order emerges. The enhanced Na dopant potential fluctuations play a crucial role in the sodium-rich phases and lead to an inhomogeneous FM state, exhibiting nonmagnetic Co³⁺ patches, antiferromagnetic (AF) correlated regions, and FM clusters with AF domains. Hole doping the band insulator at x=1 leads to the formation of local moments near the Na vacancies and AF correlated magnetic clusters. We explain recent observations by neutron, μSR, and NMR experiments on the evolution of the magnetic properties in the sodium-rich phases.