Large-amplitude easy-plane spin-orbit torque oscillators driven by out-of-plane spin current: A micromagnetic study

Spin torque oscillators generate a periodic output signal from a nonperiodic input, making them promising candidates for applications like microwave communications and neuromorphic computing. However, traditional spin torque oscillators suffer from a limited precessional cone angle and thermal stability, as well as a need for an applied bias magnetic field. Here, we use micromagnetic simulations to demonstrate a spin torque oscillator that relies on spin-orbit effects in ferromagnets to overcome these limitations. The key mechanism behind this oscillator is the generation of an out-of-plane spin current, in which both the spin flow and the spin orientation are out of plane. The torque from this spin current enables easy-plane coherent magnetic precession with a large cone angle and high thermal stability over a micron-scale lateral area. Moreover, the precession occurs about an internal field in the free layer, thereby eliminating the need for an external bias field. We find that the ratio of the unconventional out-of-plane spin current to the conventional spin-Hall spin current can be as low as $4%$ and still result in bias-field-free, room-temperature, self-sustained oscillations. Our results are fundamentally important in demonstrating that a small ratio of unconventional to conventional spin currents critically affects magnetization dynamics. Our findings also provide a theoretical proof of concept of a spintronic device with promising applications.