Nuclear magnetic field in Na0.7CoO2 detected with μ−SR

Abstract

The internal magnetic field in a sodium battery compound, i.e., the paramagnet ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_{2}$, was investigated with negative muon spin rotation and relaxation (${\ensuremath{\mu}}^{\ensuremath{-}}\mathrm{SR}$), and the result was compared with the results previously obtained with ${\ensuremath{\mu}}^{+}\mathrm{SR}$. The majority of implanted ${\ensuremath{\mu}}^{\ensuremath{-}}$ is captured on an oxygen nucleus, while ${\ensuremath{\mu}}^{+}$ locates an interstitial site. Therefore, a ${\ensuremath{\mu}}^{\ifmmode\pm\else\textpm\fi{}}\mathrm{SR}$ work provides information on the internal magnetic field, which is formed by nuclear magnetic moments of $^{23}\mathrm{Na}$ and $^{59}\mathrm{Co}$, from the two different viewpoints. Besides a slight decrease in the field distribution width ($\mathrm{\ensuremath{\Delta}}$) around 300 K, the nuclear magnetic field detected with ${\ensuremath{\mu}}^{\ensuremath{-}}\mathrm{SR}$ was found to be almost static and temperature independent up to 400 K, even though Na ions are known to start to diffuse above 290 K based on ${\ensuremath{\mu}}^{+}\mathrm{SR}$, Na-NMR, neutron scattering, and electrochemical measurements. Such a discrepancy is caused by the fact that the Na contribution to $\mathrm{\ensuremath{\Delta}}$ is only about 3% at the O site whereas it is about 13% at the interstitial site, where the ${\ensuremath{\mu}}^{+}$ is presumably located.