Dynamic spin reordering in a hybrid layered ferrimagnet with intercalated biferrocenium radicals

Molecule-based hybrid layered magnets provide an ideal platform for investigating the long-range spin-ordering process in low-dimensional magnetic systems. Within this context, a promising area of research is spin-sandwiched hybrid layered magnets. These materials offer the potential to explore how the spin, which is sandwiched between magnetic layers, is influenced by the internal magnetic fields generated by the magnetic layers. Herein, we report a layered ferrimagnet with intercalated biferrocenium ([bifc]⁺) radicals, [bifc][{Ru₂(2,3,5,6-F₄ArCO₂)₄}₂(TCNQF₂)] (1, TCNQF₂ = 2,5-difluorotetracyano-p-quinodimethane). The [{Ru₂(2,3,5,6-F₄ArCO₂)₄}₂(TCNQF₂)]^- moiety acts as a ferrimagnetic layer with S _T = 3/2, composed of a paddlewheel [Ru₂ ^II,II(2,3,5,6-F₄ArCO₂)₄] (2,3,5,6-F₄ArCO₂ ^- = 2,3,5,6-tetrafluorobenzoate) with S = 1 and 2,5-difluoro-7,7,8,8-tetracyanoquinodimethanate (TCNQF₂˙^-) units with S = 1/2 in a 2 : 1 ratio. The isostructural paramagnetic compound [bifc][{Rh₂(2,3,5,6-F₄ArCO₂)₄}₂(TCNQF₂)] (2) consisting of diamagnetic [Rh₂ ^II,II(2,3,5,6-F₄ArCO₂)₄] components was also synthesized. An investigation of the properties of 2 revealed minimal magnetic interaction between the [bifc]⁺ and TCNQF₂˙^- components. Compound 1 displayed long-range ferrimagnetic ordering at the Curie temperature of 105 K without any frequency dependence on alternating current (AC) susceptibility, due to the combination of predominant ferrimagnetic ordering within the layer and interlayer ferromagnetic dipole interactions. However, subsequent stepwise magnetic ordering involving a strong AC frequency dependence was observed upon further cooling. These dynamic behaviors are associated with the ordering of two types of anisotropic [bifc]⁺ spins between the ferrimagnetic layers, indicating that [bifc]⁺ spin ordering is sensitive to anisotropic internal magnetic fields generated by the ferrimagnetic layers.