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 TbCo 2 /FeCo 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. Strain control of magnetic states in magnetoelectric heterostructures is currently considered as a breakthrough in prospective nanoelectronic applications 1-4. TbCo 2 /FeCo heterostructures have been proposed as basic component for straintronic magnetoelectric random access memory (MELRAM) with ultralow energy consumption 4-8. The principle of operation of such devices is based on spin reorientation transitions (SRT) induced by short strain pulses generated by piezoelectric constituent elastically coupled with the structure. The switching rate between magnetic states in the MELRAM cells was estimated as fractions of a nanosecond. This rate, as well as the operating conditions of the combined magnetic/piezoelectric structure depend both on the rate of strain pulses generation in piezoelectric and on the dynamic properties of the spin system of the active magnetic layer. However, the experimental demonstration of magnetic switching in the magnetic TbCo 2 /FeCo layer has so far been performed only on a millisecond time scale. The sub-nanosecond spin dynamics was studied by ferromag-netic resonance method 9 which applicability is limited to materials with a fairly small resonance line width. The most adequate method for studying spin dynamics is the magneto-optical pump-probe technique with the use of femtosecond laser pulses 10. This technique was successfully applied for observation of ultrafast spin-reorientation transitions in materials with natural SRT induced by temperature variation 11-13. An enhancement was reported of spin precession amplitude and intensity of THz emission excited by femtosecond pulse in the vicinity of SRT temperature. The conditions for this type of SRT are determined by the natural temperature range of the SRT in the materials, which is generally below or above room temperature, which severely limits applications. Recently, a magnetooptical pump-probe technique has been used to study the dynamics in TbCo 2 /FeCo heterostructure with SRT induced by magnetic field 14. However, in those studies, the anisotropy field H A of the sample was stronger than the available magnetizing field H(H < H A). This prevented the achievement of optimal conditions for optical excitation of the spin system. The experimental results were interpreted using assumption of disruption of the uniaxial anisotropy by the femtosecond laser pulse. The similar assumption was proposed in Ref. 15 for explanation of photo-induced excitations of the spin system in a magnetic dielectric.