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).

Abstract

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.

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).

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