Intermetallic heterostructures of rare-earth and transition metals exhibit physical properties prospective for various applications. These structures combine giant magnetostriction, controllable magnetic anisotropy, magneto-optical activity and allow spin reorientation transitions (SRT) induced by magnetic field at room temperature. Here, we present the results of a study of spin dynamics induced by ultrafast optical excitation in the [Formula: see text] heterostructure. The time dependence of the light polarization rotation excited by a pump optical pulse with a duration of 35 fs was measured in the total range of the SRT created by external DC magnetic field. We found hysteretic dependence of the polarization rotation on magnetizing field that is specific for spin dynamics near SRT. Enhancement of the rotation is observed in the critical points of the SRT and near the points of magnetization switch from metastable to stable spin states. In the time-domain, two characteristic delays of 20 ps and 200 ps were found, corresponding to the maximum deviation of the light polarization after excitation. The first is explained by the precession motion of spins out of the plane of the structure. The latter is accounted for the spin in-plane deviation from its initial position and thermal relaxation of the anisotropy.