Static magnetic order inNa0.75CoO2detected by muon spin rotation and relaxation

The nature of the magnetic transition of the Na-rich thermoelectric ${\mathrm{Na}}_{0.75}{\mathrm{CoO}}_2$ at 22 K was studied by positive muon-spin-rotation and relaxation $\left({\mu }^{+}\mathrm{SR}\right)$ spectroscopy, using a polycrystalline sample in the temperature range between 300 and 2.5 K. Zero-field $\mu \mathrm{SR}$ measurements indicated the existence of a static internal magnetic field at temperatures below 22 K ${(=T}_{\mathrm{m}}).\mathrm{}$ The observed muon-spin precession signal below $T_{\mathrm{m}}$ consisted of three components with different precession frequencies, corresponding to three inequivalent ${\mu }^{+}$ sites in the ${\mathrm{Na}}_{0.75}{\mathrm{CoO}}_2$ lattice. The total volume fraction of the three components was estimated as $\sim 21\%$ at 2.5 K; thus, this magnetic transition was not induced by impurities but is an intrinsic change in the magnetism of the sample, although the sample was magnetically inhomogeneous otherwise. On the other hand, a similar experiment on a ${\mathrm{Na}}_{0.65}{\mathrm{CoO}}_2$ sample exhibited no magnetic transition down to 2.5 K; which indicates that the average valence of the Co ions is responsible for inducing the magnetic transition at 22 K.