Tunable spin structure in full-magnetic Aurivillius layered-perovskites oxyfluorides

Structurally and magnetically, the  Aurivillius oxyfluorides have recently emerged as intriguing modifications of their well-known parent layered ferroelectric perovskite-oxides. Indeed, starting from their modular bidimensional (2D) formula [Bi₂O₂]_fluorite[A_n−1M_nO_3n+1]_perovskite, the full substitution of  O^2- for F^– anions in the perovskite slabs allows the stabilization of n = 1 members with the “ideal formula” [Bi₂O₂][M²⁺F₄] containing paramagnetic 3d ⁿ transition M²⁺ metal ions (M = Fe, Co, Ni, and Mn) [1-4]. Their structural features range from disordered average cells to anionic-ordered polar supercells, while their magnetic structures evolve from colinear to canted magnetic spin orientations, depending on the electronic configuration of the metal ion. Altogether, the combination of a canted spin structure and a polar nuclear superstructure may drive multiferroicity, as experimentally verified for the M = Fe case [1]. Although the O/F contrast is poor in most diffraction techniques, the use of combined X-ray, electron and neutron-diffraction has been primordial to unravel several ambiguities related to the cationic and anionic full-content, in the context of possible Bi and M vacancies (vac) and O/F/vac mixed positions.

Magnetically, the spin-orientation of the M²⁺ ions (refined by low temperature neutron powder diffraction@ ILL, D1B, λ = 2.52 Å) is governed by the spin-orbit coupling (SOC) responsible for variable magnetocrystalline anisotropy in the M= Mn, Fe, Co, and Ni compounds. Most intriguingly the combination of mixed Fe/M transition metals in new Aurivillius oxyfluorides has evidenced that the major spin contributions fully order at various Néel temperatures, in spite of the cationic random distribution [5].  It is far from the spin-glassy situation expected in such disordered systems.  The refined antiferromagnetic structures are shown in Fig. 1, including those of the single M²⁺ ions cases. It appears that in most Fe/M mixed cases the original Fe magnetocrystalline anisotropy is lost, where the spins align along the c-axis. Keeping in mind that this solution is the one adopted for the Heisenberg Mn²⁺ spins (d⁵, L = 0, i.e. no SOC influence), a plausible scenario would consist of the orbital component M_SOC not ordering (due to their competition between their specific axial origin in the disordered Fe/M mixed phases) while the spin order is only governed by M_spin. Such features may be enhanced by the 2D character of the weakly interacting perovskite slabs, which confine a cooperative set of coplanar magnetic Fe‑Fe, Fe‑M and M‑M interactions. The overall antiferromagnetic ordering would be probably lost within 3D counterpart materials.