Improving the magnetodynamical properties of NiFe/Pt bilayers through Hf dusting

We investigate the effect of hafnium (Hf) dusting on the magnetodynamical properties of NiFe/Pt bilayers using spin-torque-induced ferromagnetic resonance measurements on 6 μm wide microstrips on high-resistive Si substrates. Based on two series of NiFe(t_NiFe)/Hf(t_Hf)/Pt(5) stacks, we first demonstrate that the zero-current magnetodynamic properties of the devices benefit from Hf dusting: (i) the effective magnetization of the NiFe layer increases by 4%–8% with Hf present and (ii) the damping α decreases linearly with t_Hf by up to 40%. The weaker anisotropic magnetoresistance (AMR ≃ 0.3%–0.4%) of the 3 nm NiFe series is largely unaffected by the Hf, while the stronger AMR of the 5 nm NiFe series drops from 0.7% to 0.43% with increasing t_Hf. We find that the spin Hall efficiency ξ_SH is independent of the NiFe thickness, remaining unaffected (ξ_SH = 0.115) up to t_Hf = 0.4 nm and then decreasing linearly for higher t_Hf. The different trends of α and ξ_SH suggest that there is an optimum Hf thickness (≃0.4 nm) for which the threshold current for auto-oscillation should have a minimum, while the much lower damping should improve mutual synchronization. Our results also indicate that the spin-orbit torque is entirely damping-like with no field-like torque component. Finally, the internal spin Hall angle of Pt is estimated to be θ_SH = 0.22 by calculating the transparency of the interface.