  By muon spin rotation we investigated the magnetic properties of a series of
highly Na doped NaxCoO2 single crystals with 0.78(1)<x<0.97(1). Our data
provide evidence for an intrinsically inhomogeneous magnetic state which can be
described in tems of hole doping (Na vacancy) induced magnetic clusters that
percolate at 1-x>0.04 until they yield a bulk magnetic state near x=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 x=0.78 as
compared to x=0.97.
