  When a ferromagnet is deposited on the surface of a topological insulator the
topologically protected surface state develops a gap and becomes a
2-dimensional quantum Hall liquid. We demonstrate that the Hall current in such
a liquid, induced by an external electric field, can have a large effect on the
magnetization dynamics of the ferromagnet by changing the effective anisotropy
field. This change is dissipationless and may be substantial even in weakly
spin-orbit coupled ferromagnets. We study the possibility of dissipationless
current-induced magnetization reversal in monolayer-thin, insulating
ferromagnets with a soft perpendicular anisotropy and discuss possible
applications of this effect.
