Magnetization (M) precessions driven by ultrafast laser-induced nonthermal effects are observed in undoped yttrium iron garnet (YIG) films of (100) and (111) orientations using pump-probe time-resolved magneto-optical Kerr effect. The M precessions show a strong dependence on the polarization direction of linearly polarized pump pulses of 400 nm. In contrast, we can barely observe any M precession using circularly polarized pump pulses, which indicates that the inverse Faraday effect is negligible. For the case of linear pump polarization, a phenomenological model is introduced, based on the modulation of M via a modulation of fourth-rank susceptibility tensors by a laser pulse. This allows one to distinguish the contributions of the inverse Cotton-Mouton effect (ICME) from those of the photoinduced magnetic anisotropy (PMA). Using the formula derived from the phenomenological model, we perform the fitting of the polarization-direction-dependent precession phase and amplitude in (100)- and (111)-oriented YIG films. The fitting results reveal that the M-precession excitation originates from a combination effect of ICME and PMA, but the ICME plays the dominant role.

The probe of antiferromagnetic (AFM) spin dynamics in thin films has traditionally relied on the short x-ray pulses based on the magnetic dichroism or diffraction effect. Here, we demonstrate the direct optical probe of ultrafast laser-induced AFM spin dynamics of thin AFM CoO films using time-resolved magnetic linear dichroism effect in reflection geometry. The ultrafast laser excitation of the CoO film leads to the quench of AFM order, manifested as large polarization rotations of the reflected probe light. Far below the Néel temperature (TN), the quench of the AFM order occurs within 300 fs, which is faster than the lattice thermalization (∼1ps) via electron-lattice scattering. This AFM quench process, however, slows down near TN, where an additional slower quench process with the time constant longer than 20 ps is emergent. Such an AFM spin dynamics is dramatically different than the transient reflectivity dynamics which is nearly invariant in a wide temperature range across TN. We attribute the quench of AFM order in CoO to the charge-transfer excitation and thermal effect, but the latter mechanism only plays a significant role near TN.