Evidence of a single nonmagnetic Co3+ state in the Na1CoO2 cobaltate

TL;DR

This study provides evidence that in Na1CoO2, cobalt exists predominantly in a nonmagnetic Co3+ state, confirmed through multiple spectroscopic techniques, indicating a uniform low-spin configuration in the material.

Contribution

The paper demonstrates that Na1CoO2 contains a single nonmagnetic Co3+ state, contrasting with other compositions showing mixed valence states, using combined magnetization and spectroscopic measurements.

Findings

Na1CoO2 has a very low and flat magnetic susceptibility.

Muon spin rotation indicates a predominantly nonmagnetic phase.

Na NMR confirms almost zero cobalt spin susceptibility, consistent with Co3+ state.

Abstract

Macroscopic magnetization, muon spin rotation (muSR), and NMR measurements were carried out to study magnetism in the Na1CoO2 cobaltate. Using SQUID measurements, Na1CoO2 is shown to have a bulk magnetic susceptibility much lower and flatter than that of NaxCoO2 with x=0.7-0.9. In fact, muSR yields a signal of mostly nonmagnetic origin, which is attributed to the x=1 phase. The intrinsic cobalt spin susceptibility corresponding to this x=1 phase is measured using Na NMR. It is indeed found to be almost zero, in agreement with a low-spin 3+ charge state of all cobalt atoms. This single state of Co ions in CoO2 planes is confirmed by Co NMR, whose determination of cobalt shift and quadrupolar parameters allows us to give a reference value of the Co3+ orbital shift in cobaltates.