Direct Link between Low-Temperature Magnetism and High-Temperature Sodium Order in Na_xCoO₂

We prove the direct link between low-temperature (T) magnetism and high-T Na⁺ ordering in Na_xCoO₂ using the example of a so far 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 Na⁺ 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 Na⁺ Coulomb potential on the CoO₂ physics is evidenced opening new ways to experimentally revisit the Na_xCoO₂ phase diagram.