1. Electromagnetic Technology Division, National Institute of Standards and Technology, Boulder, Colorado 80305, USA
2. Hitachi San Jose Research Center, San Jose, California 95120, USA

Correspondence to: Shehzaad Kaka¹ Correspondence and requests for materials should be addressed to S.K. (Email: shehzu21@gmail.com).

The spin torque^{1, 2} effect that occurs in nanometre-scale magnetic multilayer devices can be used to generate steady-state microwave signals in response to a d.c. electrical current^{3, 4, 5, 6, 7, 8}. This establishes a new functionality for magneto-electronic structures that are more commonly used as magnetic field sensors and magnetic memory elements⁹. The microwave power emitted from a single spin torque nano-oscillator (STNO) is at present typically less than 1 nW. To achieve a more useful power level (on the order of microwatts), a device could consist of an array of phase coherent STNOs, in a manner analogous to arrays of Josephson junctions and larger semiconductor oscillators^{10, 11, 12}. Here we show that two STNOs in close proximity mutually phase-lock—that is, they synchronize, which is a general tendency of interacting nonlinear oscillator systems^{13, 14, 15}. The phase-locked state is distinct, characterized by a sudden narrowing of signal linewidth and an increase in power due to the coherence of the individual oscillators. Arrays of phase-locked STNOs could be used as nanometre-scale reference oscillators. Furthermore, phase control of array elements (phased array) could lead to nanometre-scale directional transmitters and receivers for wireless communications.

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