  We prove the direct link between low temperature magnetism and high
temperature sodium ordering in NaxCoO2 using the example of a heretofore
unreported magnetic transition at 8 K which involves a weak ferromagnetic
moment. The 8 K feature is characterized in detail and its dependence on a
diffusive sodium rearrangement around 200 K is demonstrated. Applying muons as
local probes this process is shown to result in a reversible phase separation
into distinct magnetic phases that can be controlled by specific cooling
protocols. Thus the impact of ordered sodium Coulomb potential on the CoO2
physics is evidenced opening new ways to experimentally revisit the NaxCoO2
phase diagram.
