  This paper analyzes theoretically the signal propagation in spin transport by
modulating the current passing through magnetic multilayers. Using a
macroscopic description of spin transport based on the dynamical Boltzmann
equation, we show that time-dependent spin transport possesses a wave-like
character that leads to modifications of pure spin-diffusion dynamics. In
particular, the wave-like characteristics allow one to extract a finite spin
signal-propagation velocity.
