Coherent excitation of spin waves in synthetic antiferromagnets by subpicosecond spin-transfer-torque

The subpicosecond spin-transfer-torque (STT) related to the laser-induced spin current is an effective approach to exciting spin waves in a noncollinear magnetic configuration. However, its validity on the excitation of spin waves in a synthetic antiferromagnet is still far from conclusive. Here, we investigate the impact of subpicosecond STT on the coherent excitation of spin waves in CoFeB/Ru/CoFeB. After pulsed-laser exposure, we observe both the precessional signals of acoustic mode and optical mode and find that their precession amplitudes are magnetic configuration-dependent. Not as the common explanation in the previous reports that the spin waves are triggered by the transient modulation of effective field, we attribute the initial stimulation of magnetization precession mainly to the subpicosecond STT. This mechanism is further confirmed by the field-dependent precession amplitudes. Our results pave a way for the practice of subpicosecond STT in the potential application at terahertz range.