  Using spin currents generated by fs laser pulses, we demonstrate excitation
of GHz ferromagnetic resonance and THz ferrimagnetic exchange resonances in
Co/Gd/Co/Gd multilayers by time-resolved magneto-optic Kerr effect
measurements. Varying the Gd layer thickness allows for a tuning of the
resonance spectrum by manipulating the total angular momentum and strength of
effective exchange fields between the antiferromagnetically coupled layers.
Close to the compensation point of angular momentum, a minimum in the frequency
of the exchange-dominated mode and a maximum in the frequency of the
ferromagnetic resonance mode is observed. Finally, to gain better understanding
of the excitation mechanism, we analyze the anomalous variation in the measured
exchange mode amplitude as a function of its frequency. A peak in this
amplitude in the vicinity of the compensation point of angular momentum is
explained using a macrospin model, taking nonlinear effects at finite
precession amplitudes into account.
