23Na NMR Evidence for Charge Order and Anomalous Magnetism in NaxCoO2

TL;DR

This study uses 23Na NMR and magnetic susceptibility to reveal charge order and unusual magnetic behavior in NaxCoO2, showing a dominant phase at x≈0.70 with distinct Na and Co charge arrangements and temperature-dependent magnetic properties.

Contribution

It provides direct NMR evidence of charge order and magnetic anomalies in NaxCoO2, clarifying the composition and magnetic behavior of different phases.

Findings

Dominant phase at x≈0.70 with charge order

Enhanced local susceptibility below 100 K

Weaker Pauli-like magnetism at x≈0.35

Abstract

We have compared by $^{23}$Na NMR and magnetic susceptibility the magnetic properties of oriented powder samples of Na$_{x}$CoO$_{2}$ compounds. The NMR spectra are found identical for various samples with nominal $0.50\leq x\leq 0.70,$ which establishes that the dominant phase has a single composition $x\approx 0.70$. Three Na sites are identified from their single valued quadrupole effects and magnetic shifts which implies a very definite order of the Na$^{+}$ ions and of the Co charges in the CoO$_{2}$ planes. The local susceptibility of the magnetic cobalt sites displays a large enhancement below 100 K with respect to the usually found high $T$ Curie-Weiss law. A very distinct much weaker Pauli like magnetism is detected for $x\approx 0.35$, the parent of the superconducting hydrated compound.