Magnetism of layered cobalt oxides investigated by muon spin rotation and relaxation

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.

Thermoelectric transport in the layered Ca3Co4–xRhxO9 single crystals

“…Hsieh et al have shown that the decrease of T/S with lowering temperature originates from the decrease of the carrier concentration n according to Eq. (2) [18], which is also supported from the results of the Hall coefficient measurements [38], and proposed that this phenomenon is ascribed to an opening of the pseudogap caused by the short-range order of SDW below 100 K [19]. We also note that such a reduction of T/S below a pseudogap temperature is observed in other layered cobalt oxide [39] and the Kondo semiconductor CeNiSn [40].…”

“…The anomalous charge transport of this compound is clearly seen in the non-monotonic temperature variations of the electrical resistivity and the Seebeck coefficient [7,[12][13][14][15], but an underlying origin for these behaviors remains unclear. For instance, below T ∼ 60 K, the resistivity increases with decreasing temperature like an insulator while the Seebeck coefficient shows a metallic behavior, which has been discussed in terms of the variable range hopping [16], quantum criticality [17], and pseudogap opening [18] associated with a spin-density-wave (SDW) formation [19]. At higher temperature, the resistivity shows a Fermiliquid behavior varying as ρ(T ) = ρ 0 + AT 2 up to the coherence temperature T * ∼ 140 K, above which an incoherent bad-metal state is realized.…”

Electron correlation in the two-dimensional triangle lattice ofNaxCoO2

“…However, the internal magnetic field of the ordered state, ν µ (0 K), is found to be ∼3 MHz for Na [16,18] The muon locates probably ∼0.1 nm away from the oxygen ions, and there is no space for it in the CoO 6 octahedra in the [CoO 2 ] plane as in the case for the high-T c cuprates [25]. This discrepancy is difficult to explain only by differences in the µ + site experiencing the SDW field.…”

“…Furthermore, the µ + SR result suggested that the ordered phase below T m could be either a ferrimagnetic (FR) or a commensurate (C) spin density wave (SDW) state. [15] On the other hand, the related compound, pure and doped [Ca 2 CoO 3 ] RS 0.62 [CoO 2 ] (RS denotes the rocksalttype susbsystem), exhibited a transition to an incommensurate (IC) SDW state below ∼100 K. [16] Recent µ + SR experiment using both single crystals and c-aligned polycrystals suggested that long-range IC-SDW order forms below ∼ 30 K (≡ T SDW ), while a short-range order appears below 100 K (≡ T on c ). [17,18] Also, the IC-SDW was found to be induced by the ordering of the Co spins in the CoO 2 planes.…”

Two dimensionality in quasi-one-dimensional cobalt oxides

“…[1,2,3,4,5] Several researchers reconfirmed later the existence of long-range magnetic order in the layered cobaltites by not only µ + SR [6] but also neutron diffraction experiments. [7] These cobaltites share a common structural component as a conduction path, i.e., the CoO 2 planes, in which a two-dimensional-triangular lattice of Co ions is formed by a network of edge-sharing CoO 6 octahedra.…”