Incommensurate spin density wave state in layered cobaltites

Magnetism of misfit layered cobaltites with a triple rocksalt-type subsystem, i.e., [A(M1-yCoy)O3]xRS[CoO2(A=Ca and Sr, M=Ti, Hg, Tl and Pb, x=0.5-0.7 and RS denotes the rocksalt-type subsystem) was investigated by μ+SR, using polycrystalline samples down to 1.8K. Weak transverse-field measurements indicated the existence of a magnetic transition below ∼150K(=Tcon). A clear muon spin oscillation was however observed only in [Ca2Pb0.4Co0.6O3]0.62RS[CoO2] below ∼50K, indicating formation of an incommensurate spin density wave (IC-SDW) state. The oscillation frequency as T approaches at 0K (fSDW(0K)) was ∼60MHz which is comparable to that in the related compound [Ca2CoO3]0.62RS[CoO2] (fSDW(0K)∼54MHz). Since the (Pb0.4Co0.6)O (or the Co-O) sheet is sandwiched by the two adjacent Ca-O sheets in the RS subsystem, the IC-SDW is most unlikely to form in the RS subsystem but in the CoO2 plane. Only a fast relaxation is apparent even at 1.8K in both [Sr2Hg0.4Co0.6O3]0.57RS[CoO2] and [Ca2Ti0.4Co0.6O3]0.57RS[CoO2], although the Co content in the RS subsystem is equivalent to that for [Ca2Pb0.4Co0.6O3]0.62RS[CoO2]. Assuming that the RS subsystem acts as a charge reservoir as in the high TC cuprates, the present result suggests that the IC-SDW order is strongly affected by the carrier concentration in the [CoO2] plane