Phase locking and frequency doubling in spin transfer torque oscillators with two coupled free layers

We report measurements of spin-torque-driven oscillations in magnetic multilayer devices containing two in-plane-oriented free layers designed to have significant coupling between them. They are driven to oscillate by spin-transfer torque from two perpendicularly oriented polarizers. For both measured devices and micromagnetic simulations, we find that the oscillations in the two free layers are phase locked, resulting in a frequency doubling and large output signals. The simulations suggest that the oscillations are due to spatially nonuniform dynamics characterized by coupled large-amplitude motion of the two free layers.