  In 1996, Berger and Slonczewski independently predicted that a large enough
spin-polarized dc current density sent perpendicularly through a ferromagnetic
layer could produce magnetic excitations (spin-waves) or reversal of
magnetization (switching). In the past few years, both current-driven switching
and current-driven excitation of spin-waves have been observed. The switching
is of potential technological interest for direct 'writing' of magnetic random
access memory (MRAM) or magnetic media. The spin-wave generation could provide
a new source of dc generated microwave radiation. We describe what has been
learned experimentally about these two related phenomena, and some models being
tested to explain these observations.
