Studies of successive transitions in K0.5CoO2

Abstract Successive phase transitions observed in K 0.5 CoO 2 at T c1 (∼60 K) and T c2 (∼20 K) have been studied comparatively with those of Na 0.5 CoO 2 by 59 Co-NMR. For K 0.5 CoO 2 , the T dependence of the longitudinal relaxation rate divided by T , 1/ T 1 T , has sharp peaks at T c1 and T c2 , and the internal magnetic fields H int of two distinct Co sites exhibit small anomalies at T c2 , suggesting that the antiferromagnetic orderings take place at both temperatures of T c1 and T c2 . These results shed light on the mechanism of transition at T c2 in Na 0.5 CoO 2 .