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Nuclear magnetic resonace ͑NMR͒, EPR and magnetization measurements in Na x CoO 2 for 0.65ഛ x ഛ 0.75 are presented. While the EPR signal arises from Co 4+ magnetic moments ordering at T c Ӎ 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 * ͑x͒ ͑220-270 K͒. 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 Ͻ T * were localized on Co 4+ d z 2 orbitals. It is pointed out that the in-plane antiferromagnetic coupling J Ӷ T * cannot be the driving force for the phase separation. PACS number(s): 76.60. Ϫk, 76.30.Ϫv, 71.27.ϩa II. EXPERIMENTAL RESULTS A. Sample preparationNa x CoO 2 (x = 0.65, 0.70, and 0.75) samples were prepared following the "rapid heat-up" method. 12,13 A stoichiometric mixture of 99.99% purity Co 3 O 4 and Na 2 CO 3 was thoroughly ground, and placed in a furnace and preheated at 750°C for 12 hours. The obtained samples were reground and annealed for 15 hours at 850°C in air, followed by slow cooling to room temperature. X-ray powder diffraction measurements confirmed all samples to be single phase of hex-PHYSICAL REVIEW B

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