  Using a combination of low-temperature nanocalorimetry and x-ray diffraction
we identify three temperature regimes characterized by distinct Na ordering
patterns (low temperature up to 290 K, intermediate 290-340 K, and high above
340 K). Through freezing-in of these patterns we establish the two key roles
sodium intercalation plays in the formation of the magnetic ground state:
supplying the proper electron count for in-plane ferromagnetic interaction and
through the 3D sodium ordering providing the interplane antiferromagnetic
exchange path.
