Static magnetic order in Na0.75CoO2 detected by muon spin rotation and relaxation -: art. no. 214420

The nature of the magnetic transition of the Na-rich thermoelectric Na0.75CoO2 at 22 K was studied by positive muon-spin-rotation and relaxation (mu(+)SR) spectroscopy, using a polycrystalline sample in the temperature range between 300 and 2.5 K. Zero-field muSR measurements indicated the existence of a static internal magnetic field at temperatures below 22 K (=T-m). The observed muon-spin precession signal below T-m consisted of three components with different precession frequencies, corresponding to three inequivalent mu(+) sites in the Na0.75CoO2 lattice. The total volume fraction of the three components was estimated as similar to21% 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 Na0.65CoO2 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.