  Spin torques in magnetic multilayers are produced by spin polarization $P$ of
ferromagnetic (F) layers, and increase with growing $P$. The latter, however,
cannot exceed the $P=1$ value found in half metals. We study the $P=1$ case to
find what other parameters still influence spin torques in this extreme limit.
It is found that the ratio of exchange gap to Fermi energy strongly affects the
properties of the torque. For large values of the gap the magnitude of exchange
spin torque exhibits a sharp peak at very small misalignment angles between
magnetizations. This behavior is found to be linked to a transition between
Ohmic and tunneling transport regimes through the F/F boundary.
