Mesoscopic phase separation inNaxCoO2(0.65⩽x⩽0.75)

Abstract

Nuclear magnetic resonace $(\mathrm{NMR})$, $\mathrm{EPR}$ and magnetization measurements in ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$ for $0.65\ensuremath{\leqslant}x\ensuremath{\leqslant}0.75$ are presented. While the $\mathrm{EPR}$ signal arises from ${\mathrm{Co}}^{4+}$ magnetic moments ordering at ${T}_{\mathrm{c}}\ensuremath{\simeq}26\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $^{59}\mathrm{Co}$ $\mathrm{NMR}$ 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 ${T}^{*}(x)$ $(220--270\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Above ${T}^{*}$ an anomalous decrease in the intensity of the $\mathrm{EPR}$ signal is observed and associated with the delocalization of the electrons which for $T<{T}^{*}$ were localized on ${\mathrm{Co}}^{4+}\phantom{\rule{0.3em}{0ex}}{d}_{{z}^{2}}$ orbitals. It is pointed out that the in-plane antiferromagnetic coupling $J⪡{T}^{*}$ cannot be the driving force for the phase separation.