  We demonstrate a novel type of 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 polarised
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 obtained by combining
two materials with sufficiently different saturation magnetization. Here we
show that the combination of NiFe and CoFeB can result in 3 GHz shifts in the
auto-oscillation frequencies. Bipolar SHNOs as such may bring enhanced
synchronisation capabilities to neuromorphic computing applications.

Applied Physics;Materials Science