Simultaneous multitone microwave emission by DC-driven spintronic nano-element

TL;DR

This paper demonstrates the experimental observation and analysis of simultaneous bimodal microwave emission from spin-torque nano-oscillators, revealing tunable incommensurate frequencies through mode hybridization and four-magnon scattering.

Contribution

It reports the first clear experimental evidence of bimodal incommensurate microwave emission in STNOs with analytical and simulation support, highlighting tunability via magnetic field orientation.

Findings

Bimodal microwave emission observed in STNOs.

Emission frequencies are tunable by magnetic field tilt.

Four-magnon scattering explains the mode coupling.

Abstract

Current-induced self-sustained magnetization oscillations in spin-torque nano-oscillators (STNOs) are promising candidates for ultra-agile microwave sources or detectors. While usually STNOs behave as a monochrome source, we report here clear bimodal simultaneous emission of incommensurate microwave oscillations, where the two tones correspond to two parametrically coupled eigenmodes with tunable splitting. The emission range is crucially sensitive to the change in hybridization of the eigenmodes of free and fixed layers, for instance, through a slight tilt of the applied magnetic field from the normal of the nano-pillar. Our experimental findings are supported both analytically and by micromagnetic simulations, which ascribe the process to four-magnon scattering between a pair of radially symmetric magnon modes and a pair of magnon modes with opposite azimuthal index. Our findings open up new possibilities for cognitive telecommunications and neuromorphic systems that use frequency multiplexing to improve communication performance.