Appearance of a two-dimensional antiferromagnetic order in quasi-one-dimensional cobalt oxides

By means of muon spin rotation and relaxation (μ⁺SR) techniques, we have investigated the magnetism of quasi-one-dimensional (1D) cobalt oxides A_n+2Co_n+1O_3n+3 (A=Ca, Sr, and Ba; n=1, 2, 3, 5, and ∞), in which the 1D CoO₃ chain is surrounded by six equally spaced chains forming a triangular lattice in the ab plane, using polycrystalline samples, from room temperature down to 1.8 K. For the compounds with n=1–5, transverse field μ⁺SR experiments showed the existence of a magnetic transition below ∼100 K, although there were no clear anomalies in the susceptibility-vs-T curve. The onset temperature of the transition (T_c^on) was found to decrease with n; from 100 K for n=1 to 60 K for n=5. A damped muon spin oscillation was observed only in the sample with n=1 (Ca₃Co₂O₆), whereas only a fast relaxation obtained even at 1.8 K in the other three samples. Combining the fact that the paramagnetic Curie temperature ranges from −150 to −200 K for the compound with n=2 and 3, the μ⁺SR result indicates that a two-dimensional (2D) short-range antiferromagnetic (AF) order, which has been thought to be unlikely to exist at high T due to a relatively strong 1D ferromagnetic (F) interaction, appears below T_c^on for all compounds with n=1–5; but quasistatic long-range AF order formed only in Ca₃Co₂O₆, below 25 K. For BaCoO₃ (n=∞), as T decreased from 300 K, 1D F order appeared below 53 K, and a sharp 2D AF transition occurred at 15 K.