Anion Doping Induced Topotactic Phase Transition in Ruddlesden–Popper Cobaltites

Abstract Thin films of correlated oxides are exceptional platforms for magnetoionics due to their capacity to accommodate significant changes in charge density (i.e., oxidation states) through chemical redox reactions. Ruddlesden‐Popper oxides open new avenues for probing and controlling magnetic states, as their 2D layered structure allows for a broader spectrum of ionic modifications compared to perovskite oxides. In this study, the active control of physical properties is demonstrated in Sr₂CoO₄ thin films through the insertion of anions (N³⁻ and F⁻). A comprehensive range of nominal oxidation states is achieved on cobalt ions, thereby altering their structures and magnetic ground states. Polarized neutron reflectometry confirmed that the chemical composition and magnetization are uniformly distributed throughout the film, illustrating strong magnetoionic correlation. Furthermore, this study shows that manipulating spectral transparency in both visible‐light and infrared regions is highly dependent on the anion dopants, revealing a selective optical band switch. The fundamental insights obtained from this work will serve as a cornerstone for pioneering advancements in spintronics, optical devices, magnetoionic systems, and microelectronics.