Two dimensionality in quasi-one-dimensional cobalt oxides

Magnetism of quasi-one-dimensional (1D) cobalt oxides A n + 2 Co n + 1 O 3 n + 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 ( T c on ) was found to decrease with n; that is, 100 ± 25 , 90 ± 10 , 85 ± 10 , 65 ± 10 50 ± 10 , and 15 ± 1 K for n = 1 –5, and ∞ , respectively. In particular, for the samples with n = 2 –5, T c on was detected only by the present μ + SR measurements. A muon spin oscillation was clearly observed in both Ca 3Co 2O 6 ( n = 1 ) and BaCoO 3 ( 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 - 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 F interaction, appears below T c on for all compounds with n = 1 –5; but quasi-static long-range AF order formed only in Ca 3Co 2O 6, below 25 K. For BaCoO 3 ( n = ∞ ) , as T decreased from 300 K, 1D F order appeared below 53 K, and a sharp 2D AF transition occurred at 15 K.