Co3Sn2S2 has drawn much attention from multiple aspects, including a magnetic Weyl semimetal candidate, a quantum anomalous Hall effect testbed, and a non-Bravais Kagome-lattice magnet. All these interesting phenomena critically require a comprehensive understanding on the magnetic structure of Co3Sn2S2, which is, however, heatedly debated in literature and remains elusive. Here, we report our experimental efforts using femto-second laser based optical techniques to address the magnetism and fluctuations in this system. Using electric quadrupole second harmonic generation (EQ SHG), we show the presence of two magnetic phases – one onset at TC,1 ~ 175K for the Ising-type ferromagnetic order for the out-of-plane spin component, and the other onset at TC,2 ~ 120K for the XY-type all-in-all-out antiferromagnetic order for the in-plane spin component. This study on Co3Sn2S2 demonstrates the possibility of using nonlinear optics to probe centrosymmetric noncollinear antiferromagnetic orders.
