  The exchange interaction between electrons and magnetic spins is considerably
enhanced near interfaces, in magnetic multilayers. As a result, a dc current
can be used to generate spin oscillations. We review theory and experimental
evidence. The s-d exchange interaction causes a rapid precession of itinerant
conduction-electron spins s around the localized spins S of magnetic electrons.
Because of the precession, the time-averaged interaction torque between s and S
vanishes. An interface between a magnetic layer and a spacer causes a local
coherence between the precession phases of differnt electrons, within 10 nm
from the interface, and restores the torque. Also, a second magnetic layer with
pinned S is used to prepare s in a specific direction. the current-induced
drive torque of s on S in the active layer may be calculated from the spin
current (Slonczewski) or from the spin imbalance Delta-mu (Berger). Spin
current and Delta-mu are proportional to each other, and can arise from
Fermi-surface translation, as well as from expansion/contraction.
