Phase-locking in double-point-contact spin-transfer devices

Spin-transfer1, 2 in nanometre-scale magnetic devices results from the torque on a ferromagnet owing to its interaction with a spin-polarized current and the electrons' spin angular momentum. Experiments have detected either a reversal3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 or high-frequency (GHz) steady-state precession17, 18, 19, 20, 21, 22, 23 of the magnetization in giant magnetoresistance spin valves and magnetic tunnel junctions with current densities of more than 107 A cm-2. Spin-transfer devices may enable high-density, low-power magnetic random access memory24, 25 or direct-current-driven nanometre-sized microwave oscillators. Here we show that the magnetization oscillations induced by spin-transfer in two 80-nm-diameter giant-magnetoresistance point contacts in close proximity to each other can phase-lock into a single resonance over a frequency range from approximately 24 GHz for contact spacings of less than about 200 nm. The output power from these contact pairs with small spacing is approximately twice the total power from more widely spaced (400 nm and greater) contact pairs that undergo separate resonances, indicating that the closely spaced pairs are phase-locked with zero phase shift. Phase-locking may enable control of large arrays of coupled spin-transfer devices with increased power output for microwave oscillator application