Itinerant and localized magnetism on the triangular lattice: sodium rich phases of Na$_x$CoO$_2$

TL;DR

This paper investigates how electron correlation and doping influence magnetic phases in sodium cobaltates, revealing a transition from paramagnetic to ferromagnetic states and inhomogeneous magnetic textures in sodium-rich compositions.

Contribution

It provides a detailed theoretical analysis of the magnetic phase diagram of Na$_x$CoO$_2$, highlighting the role of dopant potential fluctuations and correlation effects in stabilizing various magnetic states.

Findings

Paramagnetic state stabilized for x<x_c due to correlation effects.

Emergence of ferromagnetic order for x>x_c.

Inhomogeneous magnetic states with coexisting FM, AF, and nonmagnetic regions.

Abstract

We study the interplay between correlation, itinerant ferromagnetism and local moment formation on the electron doped triangular lattice of sodium cobaltates Na$_x$CoO$_2$. We find that strong correlation renormalizes the Stoner criterion and stabilizes the paramagnetic state for $x<x_c\simeq0.67$. 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 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, $\mu$SR, and NMR experiments on the evolution of the magnetic properties in the sodium rich phases.