  The spin-transfer effect is investigated for the vortex state of a magnetic
nanodot. A spin current is shown to act similarly to an effective magnetic
field perpendicular to the nanodot. Then a vortex with magnetization (polarity)
parallel to the current polarization is energetically favorable. Following a
simple energy analysis and using direct spin--lattice simulations, we predict
the polarity switching of a vortex. For magnetic storage devices, an electric
current is more effective to switch the polarity of a vortex in a nanodot than
the magnetic field.
