Mesoscopic phase separation in NaxCoO2 (0.65 LT = x LT = 0.75)

Abstract

Nuclear magnetic resonace (NMR), EPR and magnetization measurements in NaxCoO2 for 0.65less than or equal toxless than or equal to0.75 are presented. While the EPR signal arises from Co4+ magnetic moments ordering at T(c)similar or equal to26 K, Co-59 NMR signal originates from cobalt nuclei in metallic regions with no long range magnetic order and characterized by a generalized susceptibility typical of strongly correlated metallic systems. This phase separation in metallic and magnetic insulating regions is argued to occur below T-*(x) (220-270 K). Above T-* an anomalous decrease in the intensity of the EPR signal is observed and associated with the delocalization of the electrons which for T LT T-* were localized on Co4+ d(z)(2) orbitals. It is pointed out that the in-plane antiferromagnetic coupling JT(*) cannot be the driving force for the phase separation.