Static magnetic order in Na${}_{0.75}$CoO${}_2$ detected by muon spin rotation and relaxation

Abstract

The nature of the magnetic transition of the Na-rich thermoelectric Na${}_{0.75}$CoO${}_2$ at 22K 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 $\mu$SR measurements indicated the existence of a static internal magnetic field at temperatures below 22 K (= $T_{\mathrm{m}}$). The observed muon spin precession signal below $T_{\mathrm{m}}$ consisted of three components with different precession frequencies, corresponding to three inequivalent muon${}^{+}$ sites in the Na${}_{0.75}$CoO${}_2$ lattice. The total volume fraction of the three components was estimated as $\sim$21% at 2.5 K; thus, this magnetic transition was not induced by impurities but is an intrinsic change in the magnetism of the sample, although the sample was magnetically inhomogeneous otherwise. On the other hand, a similar experiment on a Na${}_{0.65}$CoO${}_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.

5 pages, 4 figures, Phys. Rev. B 68 (2003) in press