A Higher Degree of Magnetic Symmetry Induced by Intercalation of Non-Magnetic Na into Quasi-Two-Dimensional van der Waals gapped FeOCl

A spiral spin arrangement with a magnetic unit cell 28 times the size of nuclear one has been reported for the Fe spins below T_N = 80 K in bilayered van der Waals gapped FeOCl. In this work, we used neutron magnetic diffraction and ac magnetic susceptibility to reveal a much-reduced magnetic unit cell of 4 times the size of nuclear one for the Fe spins below T_N = 119 K, when 27% of non-magnetic Na were intercalated into the van der Waals gaps of FeOCl. X-ray emission spectra and X-ray absorption edge spectra reveal charge transfers from the intercalated Na into the Fe sites that reduce the Fe³⁺ into Fe²⁺ ions, giving a significantly larger Fe-O-Fe bond angle that largely strengthens the strength of antiferromagnetic superexchange (AFMSE) coupling over the competing ferromagnetic direct exchange (FMDE) coupling between the two neighboring Fe ions, driving to a higher degree of magnetic symmetry and a significantly higher Neel temperature for the Fe spins in Na_0.27FeOCl.