Incommensurate spin density wave state in layered cobaltites

摘要

Magnetism of misfit layered cobaltites with a triple rocksalt-type subsystem, i.e., [A(M1-yCoy)O-3](x)(RS) [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 mu+SR, using polycrystalline samples down to 1.8 K. Weak transverse-field measurements indicated the existence of a magnetic transition below similar to 150 K (= T-c(on)). A clear muon spin oscillation was however observed only in [Ca2Pb0.4Co0.6O3](0.62)(RS)[CoO2] below similar to 50K, 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 similar to 60 MHz which is comparable to that in the related compound [Ca2CoO3](0.62)(RS)[CoO2] (f(SDW)(0 K)similar to 54 MHz). 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 1.8 K in both [Sr2Hg0.4Co0.6O3](0.57)(RS)[CoO2] and [Ca2Ti0.4Co0.6O3](0.57)(RS)[CoO2], although the Co content in the RS subsystem is equivalent to that for [Ca2Pb0.4Co0.6O3](0.62)(RS)[CoO2]. 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 [CoO2] plane. (c) 2005 Elsevier B.V. All rights reserved.