Spin transfer exchange torque in ferromagnet/ferromagnet structures made of half metals with large exchange gaps

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.