Anomalous Critical Behavior of Magnetic Dynamics in Kagome Antiferromagnet FeSn

The Kagome lattice antiferromagnet (AFM) FeSn has emerged as a frontier system in condensed matter physics for exploring the exotic coupling phenomena between its topological electronic structure and magnetic order. However, its magneto-optical (MO) response and the critical behavior of magnetic dynamics near the Néel temperature remain to be explored. In this study, the magnetic dynamics of FeSn was investigated by utilizing femtosecond time-resolved MO spectroscopy within a wide temperature range from 20 to 450 K. The observed MO responses in FeSn, with typical characteristics of the Voigt effect, were attributed to the Berry curvature enhanced by the Dirac nodes. Near the Néel temperature of FeSn, it was observed that the demagnetization time τD exhibited a critical behavior opposite to that of traditional ferromagnets or AFMs, while the magnetic recovery times τR1 and τR2 showed typical critical slowdown phenomena. The obtained critical exponents of −0.19 (τD), 0.24 (τR1), and 0.25 (τR2) significantly deviated from the expected values of the XXZ model, revealing the influence of the competition between the intralayer ferromagnetic and interlayer antiferromagnetic couplings on the critical behavior in FeSn. Our work provides experimental evidence for unveiling the critical dynamics and its correlation with the unique spin coupling in FeSn, which is of fundamental importance for designing spintronic devices based on Kagome AFMs.