Nanowire spin Hall oscillators: nanowire width dependence

We present experimental studies of auto-oscillatory magnetization dynamics in nanowire spin Hall oscillators (SHOs) as a function of the nanowire width. The oscillators schematically shown in Fig. 1 consist of a long Pt(7 nm)/Py(5 nm)/AlOx (2 nm) nanowire on a sapphire substrate with two Cr/Au leads attached to the wire [1]. The 2 μm gap between the leads defines the active region of the oscillator. A 750 Oe saturating magnetic field is applied in the plane of the sample at 85 deg with respect to the nanowire axis, and direct current generating anti-damping spin torque is applied between the leads. Figure 2 shows microwave emission spectra for four oscillator devices with the active region width ranging from 0.17 μm to 1.07 μm. All devices show onset of auto-oscillations at similar critical current densities. For the 0.17 μm wide nanowire SHO, auto-oscillatory modes arising from the bulk and edge eigenmodes of the nanowire are clearly seen in the emission spectra. For SHO devices based on wider wires, the bulk auto-oscillatory modes dominate the emission spectrum due to the larger wire volume occupied by the bulk modes. The maximum integrated power generated is similar for all four SHO devices (~ 10^-10 W). Our work demonstrates robust operation of nanowire-based SHOs over a wide range of nanowire widths and presents an example of a spin torque oscillator with the active area extended into the um^2 domain.