  Magnetic nanodevices usually include a free layer whose configuration can be
changed by spin-polarized current via the spin transfer effect, and a fixed
reference layer. Here, we demonstrate that the roles of the free and the
reference layers interchange over a small range of their relative thicknesses.
Precession of both layers can be induced by spin transfer in symmetric devices,
but the dynamics of one of the layers is rapidly suppressed in asymmetric
devices. We interpret our results in terms of the dynamical coupling between
magnetic layers due to spin transfer.
