Static magnetic order in Na_0.75CoO₂ detected by muon spin rotation and relaxation

The nature of the magnetic transition of the Na-rich thermoelectric Na_0.75CoO₂ at 22 K was studied by positive muon-spin-rotation and relaxation (μ⁺SR) spectroscopy, using a polycrystalline sample in the temperature range between 300 and 2.5 K. Zero-field μSR 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 μ⁺ sites in the Na_0.75CoO₂ lattice. The total volume fraction of the three components was estimated as 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 Na_0.65CoO₂ 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.