Mesoscopic phase separation in Na${}_x$CoO${}_2$ ($0.65\le x\le 0.75$)

Abstract

NMR, EPR and magnetization measurements in Na${}_x$CoO${}_2$ for $0.65\le x\le 0.75$ are presented. While the EPR signal arises from Co${}^{4+}$ magnetic moments ordering at $T_c\simeq 26$ 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 $T^{\ast }(x)$ ($220-270$ K). Above $T^{\ast }$ an anomalous decrease in the intensity of the EPR signal is observed and associated with the delocalization of the electrons which for $T<T^{\ast }$ were localized on Co${}^{4+}$ $d_{z^2}$ orbitals. It is pointed out that the in-plane antiferromagnetic coupling $J\ll T^{\ast }$ cannot be the driving force for the phase separation.

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