Mesoscopic phase separation in NaxCoO2 Ñ0.65ˇ xˇ 0.75Ö

Nuclear magnetic resonace sNMRd, EPR and magnetization measurements in NaxCoO2 for 0.65 x 0.75 are presented. While the EPR signal arises from Co 4+ magnetic moments ordering at Tc . 26 K, 59 Co 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 * sx ds 220‐ 270 Kd. 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, T * were localized on Co 4+ dz2 orbitals. It is pointed out that the in-plane antiferromagnetic coupling J ! T * cannot be the driving force for the phase separation.