Mesoscopic phase separation in Na$_x$CoO$_2$ ($0.65\leq x\leq 0.75$)

TL;DR

This study investigates mesoscopic phase separation in Na$_x$CoO$_2$ within a specific sodium concentration range, revealing coexistence of metallic and magnetic insulating regions and their temperature-dependent behavior.

Contribution

It provides experimental evidence of phase separation in Na$_x$CoO$_2$ using NMR, EPR, and magnetization measurements, highlighting the temperature at which this occurs and its relation to electron delocalization.

Findings

Phase separation occurs below T^*(x) (220-270 K).

EPR signal decreases anomalously above T^*, indicating electron delocalization.

Magnetic and metallic regions coexist without magnetic order in metallic areas.

Abstract

NMR, EPR and magnetization measurements in Na$_x$CoO$_2$ for $0.65\leq x\leq 0.75$ 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 $T^*(x)$ ($220 - 270$ 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 $T<T^*$ were localized on Co$^{4+}$ $d_{z^2}$ orbitals. It is pointed out that the in-plane antiferromagnetic coupling $J\ll T^*$ cannot be the driving force for the phase separation.