The effect of pressure on the spin density wave transition in the layered cobaltites [Ca_2CoO_3]_(0.62)[CoO_2] and [Ca_2Co_(4/3)Cu_(2/3)O_4]_(0.62)[CoO_2]

摘要

In order to investigate the pressure effect on the magnetism in the layered cobaltites, positive muon spin rotation and relaxation μ~+ SR experiments have been carried out up to 1.3 GPa using c-aligned polycrystalline samples of [Ca_2CoO_3]_(0.62)[CoO_2] and [Ca_2Co_(4/3)Cu_(2/3)O_4]_(0.62)[CoO_2]. A transverse field μ~+ SR experiment indicates that the transition temperature to an incommensurate spin density wave IC-SDW state is independent of hydrostatic pressure up to 1.3 GPa for the both compounds. Furthermore, there are no changes in the spontanious muon precession frequency in zero field at 5 K even under 1.3 GPa. These results strongly suggest that the IC-SDW exists not in the rocksalt-type block ([Ca_2CoO_3] and/or [Ca_2Co_(4/3)Cu_(2/3)O_4]) but in the CoO_2 plane.