  We investigate a W-Ta alloying route to reduce the auto-oscillation current
densities and the power consumption of nano-constriction based spin Hall nano
oscillators. Using spin-torque ferromagnetic resonance (ST-FMR) measurements on
microbars of W$_{\text{100-x}}$Ta$_{\text{x}}$(5 nm)/CoFeB(t)/MgO stacks with t
= 1.4, 1.8, and 2.0 nm, we measure a substantial improvement in both the
spin-orbit torque efficiency and the spin Hall conductivity. We demonstrate a
34\% reduction in threshold auto-oscillation current density, which translates
into a 64\% reduction in power consumption as compared to pure W-based SHNOs.
Our work demonstrates the promising aspects of W-Ta alloying for the
energy-efficient operation of emerging spintronic devices.
