We demonstrate experimental studies of the magnetization behavior from statics to ultrafast photoinduced dynamics with high temporal resolution in ultrathin Co/garnet heterostructures with a sub-nanometer roughness at the interface. We report on modulation of spin precession in Co/garnet heterostructures with distinct frequencies and show that the excitation efficiency of these precessions strongly depends on the amplitude and the direction of external magnetic field. Furthermore, it is shown

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... at the magnetization precession in the garnet film can be manipulated by the strong magnetostatic coupling between Co and garnet layers. These findings could provide new possibilities in all-optical excitation and local spin manipulation by polarized femtosecond pulses for the application in nanodevices with high-speed switching.