Coresonant Enhancement of Spin-Torque Critical Currents in Spin Valves with a Synthetic-Ferrimagnet Free Layer

It is experimentally shown that the critical current for onset of spin torque instability in current-perpendicular-to-plane spin valves can be strongly increased using synthetic-ferrimagnet free layers of form FM(1)/Ru/FM(2) (FM=ferrromagnet). However, this increase occurs for only one polarity of bias current. A two-macrospin model is shown to reproduce the observations. The model suggests that this phenomenon is related to a polarity-dependent, spin torque-induced coresonance between the two natural dynamic modes of the FM(1)/FM(2) couple. The coresonance condition facilitates energy transfer out of the spin-torque destabilized mode into the other stable mode whose effective damping (or line width) is strongly enhanced by spin torques.