Injection locking at zero field in hybrid spin-valves

Recent experiments point out the possibility of achieving microwave oscillations (oscillation frequency around 6GHz) with zero external fi eld in spin valves composed of two in-plane free layers and two out-of-plane polarizers e.g. the structure is [Co 0.5/Pt 2]4/Co 0.6/Cu 2/Co 4/Cu 4/Co 4/Cu 2/[Co 0.2/Ni 0.8]8 where the number after each element is the film thickness in nm.[1] Micromagnetic simulations show that the origin of the dynamical behavior is due to nucleations and rotation of an edge vortex [2] coupled with a propagating domain wall in each free layer. The dynamics of the two free layers are phase locked each other giving rise to a frequency doubling. In other words, the oscillation frequency of the giant-magnetoresistive (GMR) signal is two times the oscillation of the magnetization. Here we present a micromagnetic study[3] of the complex spinvalves with different shape anisotropy in a symmetric case (both polarizers are Co/Ni) or in an asymmetric case (polarizers are Co/Ni and Co/Pt). We also study the injection locking response to a microwave current.