Exploring Terahertz-Scale Exchange Resonances in Synthetic Ferrimagnets with Ultrashort Optically Induced Spin Currents

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.