Agenda

Résumé : Manganese-based compounds have long served as prototypical systems for magnetoelectricity with improper ferroelectricity. Their multiple stable valence states and high magnetic moments (typical of 3d ions) make them ideal candidates for Type II multiferroicity, where ferroelectricity arises from magnetic ordering. The RMnO₃ and RMn₂O₅ families (with R being a rare-earth element) exemplify this behavior, exhibiting strong magnetoelectric coupling. However, their functional properties are generally limited to low temperatures, typically below 50 K [1]. To overcome this limitation, recent research has turned to iron-based compounds, which offer similar advantages—multiple valence states and large magnetic moments—along with higher magnetic ordering temperatures (often above 100 K), due to stronger magnetic exchange interactions [2].
In this context, we have investigated the quasi-one-dimensional iron spin ladder systems BaFe₂X₃ (X = Se, S). Using a combination of infrared spectroscopy, X-ray and neutron diffraction, and spectroscopy, we have mapped the complex pressure–temperature phase diagram of BaFe₂X₃. At ambient pressure, BaFe₂Se₃ is polar at room temperature, and upon cooling below 250 K, a magnetic transition induces a multiferroic state [3,4]. The underlying magnetic structure exhibits significant frustration, driven by competing magnetic interactions and anisotropy [5,6]. Above 4 GPa, we observe the emergence of a new magnetic phase coinciding with a structural transition [7], yet without significant changes in the local electronic structure or magnetic moment. The system’s metallicity increases in two distinct stages: first at 4 GPa and then again at higher pressures, eventually plateauing prior to the onset of superconductivity. Notably, the superconducting phase is associated with a non-centrosymmetric crystal structure [9], providing new insight into possible pairing mechanisms.
A parallel, ongoing study on BaFe₂S₃ further explores magneto-elastic coupling in this system, highlighting the intricate interplay between magnetism, structure, and superconductivity [10].

[1] S. Chattopadhyay, V. Balédent et al. Physical Review B 93, 104406 (2016)
[2] S. M. Disseler at al. Phys. Rev. Lett. 114, 217602 (2015)
[3] W. Zheng, V. Balédent et al. Rapid Com. Physical Review B 101, 020101(R) (2020)
[4] M. J. Weseloh, V. Balédent et al. Journal of Physics : Condensed Matter 34, 255402 (2022)
[5] W. G. Zheng, V. Balédent et al. Nature Communication Physics 5, 268 (2022)
[6] W.G. Zheng, V. Balédent et al. Physical Review B 107 024423 (2023)
[7] A. Roll, V. Balédent et al. Physical Review B 108, 014416 (2023)
[8] W.G. Zheng, V. Balédent et al. Physical Review B 109, 184428 (2024)
[9] S. Deng, V. Balédent et al. submitted to Phys. Rev Lett. (2025)
[10] Y. Oubaid, V. Balédent et al. submitted to Frontiers in Physics (2025)