Precession coupled spin current in spin torque driven magnetic tunnel junctions

A spin-torque switchable magnetic tunnel junction contains two ferromagnetic electrodes across a barrier that supports spin-polarized tunnel current. The spin-torque induced magnetic switching of its more agile, or "free"layer provides the "write"mechanism. Often the dynamics of the non-switching "reference"layer is also important. Here, we illustrate such dynamics involving both the free and the reference layers by using an exchange-coupled two-macrospin-moment numerical model, described by a set of Landau-Lifshitz-Gilbert (LLG) equations, together with a stochastic Langevin-field for finite temperature. Damping-like spin-transfer torque is included for both moments. In steady-state, the coupled precession is shown to reduce effective spin-current delivered to the free layer due to a precessional resonant spin-current back flow. This back-flow of spin current preferentially affects the parallel state dynamics. It is not directly related to the reference layer's thermal stability, nor its spin-torque switching threshold, as determined by the total anisotropy energy and magnetic volume. Rather, the spin-current reduction relates primarily to the matching of precession frequency between the free- and the reference-layer. Therefore, a desirable materials choice is to avoid anisotropy fields giving the free and the reference layer similar dynamic frequencies, so as to prevent such resonance-related spin-current loss.