Abstract
We study the interplay between correlation, itinerant ferromagnetism, and local moment formation on the electron doped triangular lattice of sodium cobaltates ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$. We find that strong correlation renormalizes the Stoner criterion and stabilizes the paramagnetic state for $x<{x}_{c}\ensuremath{\simeq}0.67$. 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 ${\mathrm{Co}}^{3+}$ 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, $\ensuremath{\mu}\mathrm{SR}$, and NMR experiments on the evolution of the magnetic properties in the sodium-rich phases.
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