  Femtosecond laser pulses can induce ultrafast demagnetization as well as
generate bursts of hot electron spin currents. In trilayer spin valves
consisting of two metallic ferromagnetic layers separated by a nonmagnetic one,
hot electron spin currents excited by an ultrashort laser pulse propagate from
the first ferromagnetic layer through the spacer reaching the second magnetic
layer. When the magnetizations of the two magnetic layers are noncollinear,
this spin current exerts a torque on magnetic moments in the second
ferromagnet. Since this torque is acting only within the sub-ps timescale, it
excites coherent high-frequency magnons as recently demonstrated in
experiments. Here, we calculate the temporal shape of the hot electron spin
currents using the superdiffusive transport model and simulate the response of
the magnetic system to the resulting ultrashort spin-transfer torque pulse by
means of atomistic spin-dynamics simulations. Our results confirm that the
acting spin-current pulse is short enough to excite magnons with frequencies
beyond 1 THz, a frequency range out of reach for current induced spin-transfer
torques. We demonstrate the formation of thickness dependent standing spin
waves during the first picoseconds after laser excitation. In addition, we vary
the penetration depth of the spin-transfer torque to reveal its influence on
the excited magnons. Our simulations clearly show a suppression effect of
magnons with short wavelengths already for penetration depths in the range of 1
nm confirming experimental findings reporting penetration depths below $2\,
{\rm nm}$.
