Bipolar Spin Hall Nano-Oscillators

We demonstrate a spin Hall nano-oscillator (SHNO) that allows for efficient tuning of magnetic auto-oscillations over an extended range of gigahertz frequencies, using bipolar direct currents at constant magnetic fields. This is achieved by stacking two distinct ferromagnetic layers with a platinum interlayer. In this device, the orientation of the spin polarized electrons accumulated at the top and bottom interfaces of the platinum layer is switched upon changing the polarity of the direct current. As a result, the effective anti-damping required to drive large amplitude auto-oscillations can appear either at the top or bottom magnetic layer. Tuning of the auto-oscillation frequencies by several gigahertz can be achieved by combining two materials with sufficiently different saturation magnetizations. Here, we show that the combination of NiFe and CoFeB can result in 3GHz shifts in the auto-oscillation frequencies. Bipolar SHNOs as such may bring enhanced synchronization capabilities to neuromorphic computing applications.