Incommensurate spin density wave state in layered cobaltites

Abstract

Magnetism of misfit layered cobaltites with a triple rocksalt-type subsystem, i.e., [ A ( M 1 - y Co y ) O 3 ] x RS [ CoO 2 ( 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 1.8 K. Weak transverse-field measurements indicated the existence of a magnetic transition below ∼ 150 K ( = T c on ) . A clear muon spin oscillation was however observed only in [ Ca 2 Pb 0.4 Co 0.6 O 3 ] 0.62 RS [ CoO 2 ] below ∼ 50 K , indicating formation of an incommensurate spin density wave (IC-SDW) state. The oscillation frequency as T approaches at 0 K ( f SDW ( 0 K ) ) was ∼ 60 MHz which is comparable to that in the related compound [ Ca 2 CoO 3 ] 0.62 RS [ CoO 2 ] ( f SDW ( 0 K ) ∼ 54 MHz ) . Since the ( Pb 0.4 Co 0.6 ) 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 CoO 2 plane. Only a fast relaxation is apparent even at 1.8 K in both [ Sr 2 Hg 0.4 Co 0.6 O 3 ] 0.57 RS [ CoO 2 ] and [ Ca 2 Ti 0.4 Co 0.6 O 3 ] 0.57 RS [ CoO 2 ] , although the Co content in the RS subsystem is equivalent to that for [ Ca 2 Pb 0.4 Co 0.6 O 3 ] 0.62 RS [ CoO 2 ] . Assuming that the RS subsystem acts as a charge reservoir as in the high T C cuprates, the present result suggests that the IC-SDW order is strongly affected by the carrier concentration in the [ CoO 2 ] plane.