Spin dynamics in the axion insulator candidate EuIn2As2

We report the investigation of spin dynamics in the antiferromagnetic metal EuIn2As2 with the Zintl phase by using time-resolved magneto-optical Kerr effect (TR-MOKE) and transient reflectivity spectroscopy. In TR-MOKE measurement, we observe a spin precession mode with a frequency of about 18 GHz below 1.4 T and a new emerging coherent acoustic phonon mode near 35 GHz at a higher field where the latter shows a field-independent behavior. With temperature increasing, the spin precession disappears above Neel temperature TN, whereas the acoustic phonon persists up to 25 K due to existence of magnetic-field-induced polarized paramagnetism. In contrast, the quasiparticle dynamics of EuIn2As2 does not show dramatic change in the vicinity of TN. By comparing the spin and quasiparticle dynamics, we attribute the appearance of an acoustic phonon in TR-MOKE to photoinduced transient perturbation of magneto-optical coefficient. Our paper provides an in-depth investigation on the dynamical magnetic properties and broadens the understanding of the EuIn2As2 system.