Colossal Electron-Antiferromagnetic-Magnon Coupling Induced Band Renormalization that Drives a Stoner Ferromagnetism*

The interactions between electrons and antiferromagnetic magnons (AFMMs) are important for a large class of correlated materials. For example, they are the most plausible pairing glues in high-temperature superconductors, such as cuprates and iron pnictides. However, unlike electron-phonon interactions (EPIs), clear-cut observations regarding how electron-AFMM interactions (EAIs) affect the band structure are still lacking. Consequently, critical information on the EAI, such as its renormalization strength and doping dependence, remains elusive. Here we directly observe that EAI induces a kink structure in the band dispersion in Ba_1-xK_xMn₂As₂, and subsequently unveil several key characteristics of EAI. We found that the coupling constant of EAI can be as large as 6. Such a colossal renormalization from electron-boson interaction drives the system to the Stoner criteria, giving the intriguing ferromagnetic state in Ba_1-xK_xMn₂As₂. Our results expand the current knowledge of EAI, which may facilitate the further understanding of many correlated materials with similar interactions, such as high-temperature superconductors, where EAI plays a critical role.