Robust Mutual Synchronization in Long Spin Hall Nano-oscillator Chains

Mutual synchronizationof N serially connectedspintronic nano-oscillators boosts their coherence by N and peak power by N (2). Increasing thenumber of synchronized nano-oscillators in chains holds significancefor improved signal quality and emerging applications such as oscillatorbased unconventional computing. We successfully fabricate spin Hallnano-oscillator chains with up to 50 serially connected nanoconstrictionsusing W/NiFe, W/CoFeB/MgO, and NiFe/Pt stacks. Our experiments demonstraterobust and complete mutual synchronization of 21 nanoconstrictionsat an operating frequency of 10 GHz, achieving line widths 79,000. As the number of mutually synchronizedoscillators increases, we observe a quadratic increase in peak power,resulting in 400-fold higher peak power in long chains compared toindividual nanoconstrictions. While chains longer than 21 nanoconstrictionsalso achieve complete mutual synchronization, it is less robust, andtheir signal quality does not improve significantly, as they tendto break into partially synchronized states.