Muon-spin rotation study of magnetism in Na _x CoO ₂ single crystals with 0.78 ⩽ x ⩽ 0.97

Abstract

By muon spin rotation we investigated the magnetic properties of a series of highly Na-doped Na_<em>x</em>CoO₂ single crystals with 0.78(1) ≤ <em>x </em>≤ 0.97(1). Our data provide evidence for an intrinsically inhomogeneous magnetic state which can be described in terms of hole-doping (Na vacancy)-induced magnetic clusters that percolate at 1<em>-x</em> ≳ 0.04 until they yield a bulk magnetic state near <em>x</em> = 0.78. Evidence for a strong (likely geometrical) frustration of the magnetic order is obtained from the anomalous doping dependence of the spin fluctuation rate (above the ordering temperature) which is strongly enhanced at <em>x</em> = 0.78 as compared to <em>x</em> = 0.97.