Spin-transfer^{1, 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 reversal^{3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16} or high-frequency (GHz) steady-state precession^{17, 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 memory^{24, 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 <10 to >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 applications.

1. Technology Solutions Organization, Freescale Semiconductor Inc., Chandler, Arizona 85224, USA

Correspondence to: F. B. Mancoff¹ Correspondence and requests for materials should be addressed to F.B.M. (Email: fred.mancoff@freescale.com).

Received 31 May 2005 | Accepted 13 July 2005

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