Fragile magnetic ordering between robust 2D-ferrimagnets in the AFe3(SeO3)2F6 (A = K, Rb, Cs) series

Abstract

Mixed anion compounds AFe₃(SeO₃)₂F₆ (A = K, Rb) have been synthesized by hydrothermal reactions. Prior results obtained for the isostructural Cs analog have revealed original 2D-ferrimagnetic (FI) blocks, antiferromagnetically (AFM) ordered around ∼120 K. Surprisingly, the whole series orders at a nearly constant T_N temperature with similar field-induced magnetization steps, despite significant changes of the interlayer thickness. The metamagnetic transition from AFM to FI on oriented single crystals and Cp measurements prove an in-plane easy-magnetic axis confirmed by neutron powder diffraction (NPD). The ferrimagnetic alignment [Fe(1)↑–Fe(2) ↓ –Fe(1)↑] in each 2D subunit is robust, but the AFM-ordering between them is broken under weak field leaving field-aligned 2D-macrospins, with weak magnetocrystalline anisotropy. DFT+U calculations validate the exchange ratio J_inter/J_intra of ∼5.10⁻³ for all compounds and the relatively high T_N value is well predicted by a modified random-phase approximation (RPA) like relation, established for Quasi-Low-Dimensional Heisenberg Antiferromagnets. However, our calculations of intralayer magnetic dipole–dipole (MDD) interactions indicate a non-neglectable values and suggest the latter to assist the in-plane spin orientation below T_N. At T_N, under field a 3D-Heisenberg model is proposed for the spin-flop AFM → FI transition from the critical behavior using modified Arrott plots.