  Generation of spin imbalance in nonmagnetic semiconductors is crucial for the
functioning of many spintronic devices. An attractive design of spin injectors
into semiconductors is based on a spin pumping from a precessing ferromagnet,
typically excited by a microwave magnetic field leading to a high power
consumption of the device. Here we describe theoretically a spin injector with
greatly reduced energy losses, in which the magnetic dynamics is excited by an
elastic wave injected into a ferromagnet-semiconductor heterostructure. To
demonstrate the efficient functioning of such an injector, we perform
micromagnetoelastic simulations of the coupled elastic and magnetic dynamics in
Ni films and Ni/GaAs bilayers. For thick Ni films, it is shown that a
monochromatic acoustic wave generates a spin wave with the same frequency and
wavelength, which propagates over distances of several micrometers at the
excitation frequencies close to the frequency of ferromagnetic resonance. The
simulations of Ni/GaAs bilayers with Ni thicknesses comparable to the
wavelength of the injected acoustic wave demonstrate the development of a
steady-state magnetization precession at the Ni/GaAs interface. The amplitude
of such a precession has a maximum at Ni thickness amounting to three quarters
of the wavelength of the elastic wave, which is explained by an analytical
model. Using simulation data obtained for the magnetization precession at the
Ni/GaAs interface, we evaluate the spin current pumped into GaAs and calculate
the spin accumulation in it by solving the spin diffusion equation. Then the
electrical signals resulting from the spin flow and the inverse spin Hall
effect are determined via the numerical solution of the Laplace's equation. It
is shown that amplitudes of these ac signals are large enough for experimental
measurement, which indicates an efficient acoustically driven spin pumping into
GaAs.
