  Development of energy efficient techniques for generation of spin waves
(magnons) is important for implementation of low-dissipation spin-wave-based
logic circuits and memory elements. A promising approach to achieve this goal
is based on the injection of short strain pulses into ferromagnetic films with
a strong magnetoelastic coupling between spins and strains. Here we report
micromagnetoelastic simulations of the magnetization and strain dynamics
excited in Fe$_{81}$Ga$_{19}$ films by picosecond and nanosecond acoustic
pulses created in a GaAs substrate by a transducer subjected to an optical or
electrical impulse. The simulations performed via the numerical solution of the
coupled Landau-Lifshitz-Gilbert and elastodynamic equations show that the
injected strain pulse induces an inhomogeneous magnetization precession in the
ferromagnetic film. The precession lasts up to 1 ns and can be treated as a
superposition of magnon modes having the form of standing spin waves. For
Fe$_{81}$Ga$_{19}$ films with nanoscale thickness, up to seven (six) distinct
modes have been revealed under free-surface (pinning) magnetic boundary
conditions. Remarkably, magnon modes with frequencies over 1 THz can be excited
by acoustic pulses with an appropriate shape and duration in the films
subjected to a moderate external magnetic field. This finding shows that short
strain pulses represent a promising tool for the generation of THz spin waves
necessary for the implementation of high-speed magnonic devices.
