Abstract
Insulating antiferromagnets are anticipated as the main protagonists of ultrafast spintronics, with their intrinsic terahertz dynamics and their ability to transport spin information over long distances. However, ultrafast transfer of spin angular momentum to an antiferromagnetic insulator remains to be demonstrated. Here, studying the picosecond and subpicosecond dynamics of ferromagnetic metal/antiferromagnetic insulator bilayers, we evidence the generation of coherent terahertz excitations in the antiferromagnet combined with a modulation of the demagnetization behavior in the ferromagnet. We thus demonstrate that magnetic information can indeed be propagated into antiferromagnetic spin waves at picosecond timescales, thereby opening an avenue toward ultrafast manipulation of magnetic information.