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 mu m 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 a decreases linearly with tHf by up to 40%. The weaker anisotropic magnetoresistance (AMR similar or equal to 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 zeta(SH) is independent of the NiFe thickness, remaining unaffected (zeta(SH) = 0.115) up to t(Hf) = 0.4 nm and then decreasing linearly for higher t(Hf). The different trends of alpha and zeta(SH) suggest that there is an optimum Hf thickness (similar or equal to 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 theta(SH) = 0.22 by calculating the transparency of the interface. Published by AIP Publishing.