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 the nuclear one has been reported for Fe spins below TN = 80 K in bilayered van der Waals gapped FeOCl. In this work, we employ neutron magnetic diffraction and ac magnetic susceptibility to reveal a much smaller magnetic unit cell only 4 times the size of the nuclear one for Fe spins below TN = 119 K, upon intercalation of 27% non-magnetic Na ions into the van der Waals gaps of FeOCl. X-ray emission spectra and X-ray absorption edge spectra reveal a charge transfer from the intercalated Na ions to the Fe sites, which partially reduces the Fe3+ into Fe2+ ions. The reduction results in a significantly increased Fe-O-Fe bond angle, which strongly enhances the antiferromagnetic superexchange (AFMSE) coupling relative to the competing ferromagnetic direct exchange (FMDE) coupling between neighboring Fe ions, thereby driving to a higher degree of magnetic symmetry and a substantially higher Neel temperature for the Fe spins in Na0.27FeOCl.