Spin correlations and cobalt charge states: A new phase diagram of sodium cobaltates

Abstract

Using 23Na NMR measurements on sodium cobaltates at intermediate dopings (0.44<=x<=0.62), we establish the qualitative change of behavior of the local magnetic susceptibility at x*=0.63-0.65, from a low x Pauli-like regime to the high x Curie-Weiss regime. For 0.5<=x<=0.62, the presence of a maximum T* in the temperature dependence of the susceptibility shows the existence of an x-dependent energy scale. T_1 relaxation measurements establish the predominantly antiferromagnetic character of spin correlations for x<x*. This contradicts the commonly assumed uncorrelated Pauli behavior in this x range and is at odds with the observed ferromagnetic correlations for x>x*. It is suggested that at a given x the ferromagnetic correlations might dominate the antiferromagnetic ones above T*. From 59Co NMR data, it is shown that moving towards higher x away from x=0.5 results in the progressive appearance of nonmagnetic Co3+ sites, breaking the homogeneity of Co states encountered for x<=0.5. The main features of the NMR-detected 59Co quadrupolar effects, together with indications from the powder x-ray diffraction data, lead us to sketch a possible structural origin for the Co3+ sites. In light of this ensemble of new experimental observations, a new phase diagram is proposed, taking into account the systematic presence of correlations and their x-dependence.

12 pages, 11 figures

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Cited by in corpus (11)

• Na ordering imprints a metallic kagome lattice onto the Co planes of Na2/3CoO2
• Correlation effects on the doped triangular lattice in view of the physics of sodium-rich Na${}_x$CoO${}_2$
• Magnetic fluctuations and specific heat in NaxCoO2 near a Lifshitz topological transition
• Evolution of Co charge disproportionation with Na order in Na${}_x$CoO${}_2$
• Enhanced Seebeck coefficient by a filling-induced Lifshitz transition in KxRhO2
• From Hubbard bands to spin-polaron excitations in the doped Mott material Na${}_x$CoO${}_2$
• NMR in strongly correlated materials
• Magnetic correlations in subsystems of the misfit [Ca${}_2$CoO${}_3$]${}_{0.62}$[CoO${}_2$] cobaltate
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• Transition from a phase-segregated state to single-phase incommensurate sodium ordering in Na_xCoO_2 with x \approx 0.53
• Temperature-driven sodium-ion dynamical-to-static crossover in the zig-zag ordered phase of Na${}_{0.5}$CoO${}_2$