Muon spin rotation-relaxation $\left(\mu \mathrm{SR}\right)$ spectroscopy has been used to investigate the magnetic properties of polycrystalline ${\mathrm{Ca}}_{3-x}{\mathrm{M}}_x{\mathrm{Co}}_4{\mathrm{O}}_9$ $(x<~0.5,M=\mathrm{Sr},\mathrm{Y},$ and Bi) and ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ samples in the temperature range between 2.5 and 300 K. It was found that ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ exhibits a magnetic transition at around $T_c=100\mathrm{}\mathrm{K};$ at lower temperatures, two types of relaxation were observed using a weak transverse field $\mu \mathrm{SR}$ technique with $H=104\mathrm{}\mathrm{Oe}:$ one with a fast relaxation rate ${\lambda }_F\sim 10\mu {\mathrm{s}}^{-1}$ and the other with a slow ${\lambda }_S\sim 0.1\mu {\mathrm{s}}^{-1}.$ Zero-field $\mu \mathrm{SR}$ measurements suggest the existence of an incommensurate spin-density-wave (SDW) state below $T_c$ (i.e., $T_c{=T}_{\mathrm{SDW}}),$ although a ferrimagnetic $M-H$ loop was observed by a dc susceptibility measurement below 19 K. The substitution of Y or Bi for Ca increased $T_{\mathrm{SDW}},$ while the substitution of Sr for Ca did not affect $T_{\mathrm{SDW}}.$ This indicates that the SDW transition depends strongly on the average valence of the Co ions. The related material ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ showed no magnetic transitions below 30 K. Considering the difference between the crystal structures of ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ and ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2,$ we suggest that Co ions in the rocksalt-type layers of ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ are likely to play a significant role in inducing the SDW transition around 100 K.

• Received 30 April 2002

DOI:https://doi.org/10.1103/PhysRevB.66.134413