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.

• Received 18 May 2016
• Revised 6 September 2016

DOI:https://doi.org/10.1103/PhysRevB.94.140402