Current-induced instability of a composite free layer with antiferromagnetic interlayer coupling

Stability conditions for a spin valve with a composite free layer consisting of two antiferromagnetically coupled films-known as synthetic antiferromagnet or ferrimagnet-is studied theoretically by means of the linearized coupled Landau-Lifshitz-Gilbert equations. The Lyapunov and Routh-Hurwitz methods have been used to examine stability of the free layer subject to external in-plane magnetic field and electric current flowing perpendicularly to the layers' plane. A simple formula for the critical current density, valid for a variety of composite free layer structures, also has been derived. The analytical results are compared with those obtained from numerical simulations in which we took into account the spin transfer torque in the diffusive spin-dependent transport model. An excellent agreement between the analytical and numerical results has been achieved.