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

Abstract

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 lm 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 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 n SH is independent of the NiFe thickness, remaining unaffected (n SH ¼ 0.115) up to t Hf ¼ 0.4 nm and then decreasing linearly for higher t Hf. The different trends of a and n 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 h SH ¼ 0.22 by calculating the transparency of the interface.

Current‐Induced Spin–Orbit Torques for Spintronic Applications

“…[78,81,[98][99][100][101][102][103] The enhancement in the SOT efficiency has also been demonstrated by the modification of NM/FM interface, which increases REEinduced spin current and associated SOT, and/or improves the spin transmission through the interface. [47,60,79,104] The second issue is the necessity of an in-plane external field for the SOT-induced deterministic switching of perpendicular magnetization. This is because the SOT alone cannot switch magnetization that is aligned perpendicular to the spin polarization.…”

“…There have been many studies to understand interface contributions to the SOT and utilize it to improve SOT efficiency. [41,47,60,73,79,104,127,[142][143][144] For example, the introduction of an interfacial layer with weak SOC can increase SOT efficiency by improving the interface transmission. [47,60,73,104,127] Note that SOT efficiency is represented by effective spin Hall angle, θ SH, eff = T int θ SH , which includes the spin transparency of the interface T int .…”

“…Similar results for the enhancement of SOT efficiency by interface modification have been reported in Pt(W)/CoFeB/MgO and Pt/ NiFe structures, in which a Hf layer is inserted at the Pt(W)/ CoFeB(NiFe) interface. [47,73,104,127] Another approach to modify the interface and improve SOT efficiency is through oxygen manipulation. Figure 3e shows the SOT-induced magnetization switching in Pt (2 nm)/ CoFeB (0.8 nm)/MgO (2 nm)/SiO 2 (t nm) stacks, [98] in which the oxygen level of the structures is controlled by the top SiO 2 layer thickness t. It is found that the switching polarities of the samples with t = 0-1.5 nm are opposite to those with t > 1.8.…”

Current Modulation of Nanoconstriction Spin-Hall Nano-Oscillators

“…The properties and quality of the interfaces to Reproduced from Ref. [138] with the permission of AIP Publishing.…”

“…(a) Spin Hall efficiency versus Hf thickness; (b) the normalized calculated threshold current density. Reproduced from Ref [138]. with the permission of AIP Publishing.…”