  Fast spin manipulation in magnetic heterostructures, where magnetic
interactions between different materials often define the functionality of
devices, is a key issue in the development of ultrafast spintronics. Although
recently developed optical approaches such as ultrafast spin-transfer and
spin-orbit torques open new pathways to fast spin manipulation, these processes
do not fully utilize the unique possibilities offered by interfacial magnetic
coupling effects in ferromagnetic multilayer systems. Here, we experimentally
demonstrate ultrafast photo-enhanced interfacial exchange interactions in the
ferromagnetic Co$_2$FeAl/(Ga,Mn)As system at low laser fluence levels. The
excitation efficiency of Co$_2$FeAl with the (Ga,Mn)As layer is 30-40 times
higher than the case with the GaAs layer at 5 K due to a photo-enhanced
exchange coupling interaction via photoexcited charge transfer between the two
ferromagnetic layers. In addition, the coherent spin precessions persist to
room temperature, excluding the drive of photo-enhanced magnetization in the
(Ga,Mn)As layer and indicating a proximity-effect-related optical excitation
mechanism. The results highlight the importance of considering the range of
interfacial exchange interactions in ferromagnetic heterostructures and how
these magnetic coupling effects can be utilized for ultrafast, low-power spin
manipulation.
