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

Abstract

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 applications.