Magnetic Ordering of Fe in Na-intercalated FeOCl

Neutron diffraction and ac magnetic susceptibility measurements have been performed to study the magnetic ordering of the Fe ions in sodium-intercalated FeOCl. The Na-intercalated compound crystallizes into an orthorhombic structure with space group Pmnm, as the parent compound does. The Fe ions in FeOCl are trivalent, whereas that of the Na-intercalated compound become divalent. Na-intercalation results in the appearance of a peak at around 22 K in the temperature dependence of the in-phase component of the ac magnetic susceptibility. This peak shifts to a lower temperature under an applied magnetic field, indicating an antiferromagnetic character for these magnetic responses. Neutron magnetic diffraction experiments reveal that three-dimensional long-range ordering of the Fe spins develops below 90 K, with a non-collinear magnetic structure. Both ferromagnetic and antiferromagnetic components are observed, indicating the existence of competition between the antiferromagnetic Fe-O-Fe couplings and the ferromagnetic Fe-Fe couplings.