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.

Textured bulk ceramics have been prepared of various cobaltites including the common CdI 2 -type CoO 2 layer (i.e., [Ca 2 (Co 0.65 Cu 0.35 ) 2 O 4 ] 0.624 [CoO 2 ]: CCO-4-layer, [Bi 2 M 22x O 4 ] p [CoO 2 ] (M 2 ~Sr 2 : BSCO, M 2 ~Ca 1 Sr 1 : B(SC)CO, M 2 ~Ca 2 : BCCO)) by using b-Co(OH) 2 (CdI 2 -type) platelets as reactive templates, and their thermoelectric (TE) properties have been examined. All the specimens were found to be highly c-axis aligned ceramics. Especially for CCO-4-layer, B(SC)CO and BCCO, the present study is the first report on the fabrication of textured ceramics. Our textured ceramic specimens exhibited larger values of electrical conductivity (s) than the ceramics prepared by a conventional solid-state reaction (SSR specimens): e.g., s ab ~0.46 6 10 4 S m 21 for the textured specimen and s SSR ~y0.26 6 10 4 S m 21 at 1060 K for the SSR specimen for BCCO. This study indicated that our preparation method using b-Co(OH) 2 templates is a versatile and effective technique to fabricate cobaltites, including CoO 2 layers, with a high degree of orientation and improved TE properties.

The nature of the magnetic transition of the Na-rich thermoelectric Na0.75CoO2 at 22K 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 (= Tm). The observed muon spin precession signal below Tm consisted of three components with different precession frequencies, corresponding to three inequivalent muon + sites in the Na0.75CoO2 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 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. PACS numbers: 76.75.+i, 72.15.Jf

Plate-like particles of Ca 3 Co 4 O 9 have been prepared in which Bi and Na were partially substituted for Ca to improve the thermoelectric performance. Successfully fabricated dense and highly textured ceramics have been obtained by combining the templated grain growth technique (TGG) with hot-pressing (HP). Ca 3 Co 4 O 9 single crystals, which have alternating layers of Co-O and Ca-Co-O in the direction of the c-axis, show high electrical conductivity along the layer compared with that across the layer. Hence, a highly oriented TGG 1 HP specimen showed high electrical conductivity compared with the specimen sintered under uniaxial pressure (UP 1 PLS). The improved electrical conductivity with high Seebeck coefficient of the highly oriented specimen (TGG 1 HP) gave a high thermoelectric power factor of 5.9 6 10 24 W m 21 K 22 at 1073 K. The figures-of-merit at 773 K and 1073 K were calculated to be 8.54 6 10 25 K 21 (ZT ~0.066 at 773 K) and 1.69 6 10 24 K 21 (ZT ~0.18 at 1073 K), respectively. These values are quite high among Ca-Co-O polycrystalline systems reported so far.

A positive muon spin rotation and relaxation (µ + SR) experiment on [Ca2CoO3]0.62[CoO2], (i.e., Ca3Co4O9, a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at ∼ 100 K and 400 -600 K; the former is a transition from a paramagnetic state to an incommensurate (IC) spin density wave (SDW) state. The anisotropic behavior of zero-field µ + SR spectra at 5 K suggests that the IC-SDW propagates in the a-b plane, with oscillating moments directed along the c-axis; also the IC-SDW is found to exist not in the [Ca2CoO3] subsystem but in the [CoO2] subsystem. In addition, it is found that the long-range IC-SDW order completes below ∼ 30 K, whereas the short-range order appears below 100 K. The latter transition is interpreted as a gradual change in the spin state of Co ions above 400 K. These two magnetic transitions detected by µ + SR are found to correlate closely with the transport properties of [Ca2CoO3]0.62 [CoO2].