Spin-transfer torque for two half metals in direct contact

We study an F/F double layer with 100% spin polarized ferromagnets (half metals) in direct contact. The absence of a normal spacer between the F-layers forces electron spins to rotate instantaneously from one magnetization direction to another upon crossing the interface. This process is made possible by the emergence of evanescent waves in both F-layers. Using Stoner model for ferromagnets and considering the transmission-reflection problem that is matched to the electron distribution functions in the bulk of the layers, we calculate the equilibrium exchange torque and the current-induced torques (Slonczewski and field-like). The torques exhibit highly anisotropic angular dependence controlled by the ratio of Stoner gap and Fermi energy in ferromagnets.