Stability of a magnetic bilayer in the presence of spin transfer torque

Magnetoelectronic devices usually incorporate at least two magnetic layers, a "fixed" reference layer and a "free" active layer. We analyze the stabile configurations of such a bilayer in the presence of spin transfer torque acting on both layers. In the approximation of uniaxial anisotropy, we derive analytical stability conditions dependent on the parameters of the bilayer as well as the applied field and current. We illustrate our results with stability diagrams for several simple cases. We also determine the conditions for stable dynamical current-induced states. Our analysis provides criteria for the design of magnetic nanodevices with optimized response to electric current.