Two dimensionality in quasi-one-dimensional cobalt oxides

Magnetism of quasi-one-dimensional (1D) cobalt oxides An+2Con+1O3n+3 (A=Ca, Sr and Ba, n=1-5 and ∞) was investigated by μ+SR using polycrystalline samples, at temperatures from 300 K down to 1.8 K. The wTF-μ+SR experiments showed the existence of a magnetic transition in all six samples investigated. The onset temperature of the transition (Tcon) was found to decrease with n; that is, 100±25, 90±10, 85±10, 65±1050±10, and 15±1K for n=1-5, and ∞, respectively. In particular, for the samples with n=2-5, Tcon was detected only by the present μ+SR measurements. A muon spin oscillation was clearly observed in both Ca3Co2O6(n=1) and BaCoO3(n=∞), whereas only a fast relaxation is apparent even at 1.8 K in the other four samples (n=2-5). Taking together with the fact that the paramagnetic Curie temperature ranges from -150 to -200K 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 F interaction, appears below Tcon for all compounds with n=1-5; but quasi-static 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.