No abstract

Muon spin rotation-relaxation (SR) spectroscopy has been used to investigate the magnetic properties of polycrystalline Ca 3Ϫx M x Co 4 O 9 (xр0.5,M ϭSr,Y, and Bi͒ and Na 0.7 CoO 2 samples in the temperature range between 2.5 and 300 K. It was found that Ca 3 Co 4 O 9 exhibits a magnetic transition at around T c ϭ100 K; at lower temperatures, two types of relaxation were observed using a weak transverse field SR technique with Hϭ104 Oe: one with a fast relaxation rate F ϳ10 s Ϫ1 and the other with a slow S ϳ0.1 s Ϫ1 . Zero-field SR measurements suggest the existence of an incommensurate spin-density-wave ͑SDW͒ state below T c ͑i.e., T c ϭT 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 SDW , while the substitution of Sr for Ca did not affect T SDW . This indicates that the SDW transition depends strongly on the average valence of the Co ions. The related material Na 0.7 CoO 2 showed no magnetic transitions below 30 K. Considering the difference between the crystal structures of Ca 3 Co 4 O 9 and Na 0.7 CoO 2 , we suggest that Co ions in the rocksalt-type layers of Ca 3 Co 4 O 9 are likely to play a significant role in inducing the SDW transition around 100 K.