Using thin film pillars similar to 100 nm in diameter, containing two Co la
yers of different thicknesses separated by a Cu spacer, we examine the proc
ess by which the scattering from the ferromagnetic layers of spin-polarized
currents flowing perpendicular to the layers causes controlled reversal of
the moment direction in the thin Co layer. The well-defined geometry permi
ts a quantitative analysis of this spin-transfer effect, allowing tests of
competing theories for the mechanism and also new insight concerning magnet
ic damping. When large magnetic fields are applied, the spin-polarized curr
ent no longer fully reverses the magnetic moment, but instead stimulates sp
in-wave excitations.