Field-free exchange-spring spin-Hall nano-oscillator

Spin–orbit-torque-driven spin-Hall nano-oscillators (SHNOs) are promising spintronic microwave emitters and magnon sources, offering gigahertz frequencies, high-quality factors, and nanoscale scalability. However, conventional SHNOs require strong external magnetic fields to stabilize auto-oscillation and maintain high-frequency output. This dependence on an external biasing field hinders their integration into compact, scalable electronic systems. In this work, we investigate an experimentally feasible field-free SHNO based on a nanoconstricted L10-FePt(110)/[Co/Pt]n/Pt exchange-spring heterostructure through micromagnetic simulations. This architecture leverages the interlayer exchange field between the ultrahard in-plane magnet L10-FePt(110) and the perpendicular magnet [Co/Pt]n to stabilize auto-oscillation, eliminating the need for external magnetic fields. With a driving current from 1.8 to 4.2 mA, the device exhibits a widely tunable output frequency of 25–40 GHz, corresponding to a linear tuning rate of 5.60 GHz mA−1. The field-free SHNO sustains a single spin wave mode with monochromatic magnons propagating at 1030 m s−1. The magnon wave vector, tunable from 0.08 to 0.13 rad nm−1, scales linearly with the applied current, demonstrating direct electrical control over magnon momentum in a field-free regime. Our work establishes a precedent for field-free SHNOs, delivering high-spectral-purity microwave emission and on-demand magnon generation.