Abstract
Anomalous decrease in effective damping parameter αeff in sputtered Ni81Fe19 (Py) thin films in contact with a very thin β-Ta layer without necessitating the flow of DC-current is observed. This reduction in αeff, which is also referred to as anti-damping effect, is found to be critically dependent on the thickness of β-Ta layer; αeff being highest, i.e., 0.0093 ± 0.0003 for bare Ni81Fe19(18 nm)/SiO2/Si compared to the smallest value of 0.0077 ± 0.0001 for β-Ta(6 nm)/Py(18 nm)/SiO2/Si. This anomalous anti-damping effect is understood in terms of interfacial Rashba effect associated with the formation of a thin protective Ta2O5 barrier layer and also the spin pumping induced non-equilibrium diffusive spin-accumulation effect in β-Ta layer near the Ta/Py interface which induces additional spin orbit torque (SOT) on the moments in Py leading to reduction in . The fitting of (tTa) revealed an anomalous negative interfacial spin mixing conductance, and spin diffusion length,. The increase in αeff observed above tTa = 6 nm is attributed to the weakening of SOT at higher tTa. The study highlights the potential of employing β-Ta based nanostructures in developing low power spintronic devices having tunable as well as low value of α.
Highlights
In this communication, we present the experimental evidence of anti-damping SOT in a β-Ta/Py/SiO2/Si(100) bilayers without any DC-current flowing through Ta
Based on the analyses of the line boarding in the ferromagnetic resonance (FMR) spectra recorded on the bilayers having different Ta layer thickness deposited in-situ over the Py layer of constant thickness tPy = 18 nm, it is proposed that the observed anti-damping effect has its origin associated with a Rashba like interfacial SOT arising due to the spin accumulation at the β-Ta/ Py interface[27,28,29,30,31]
TTa ~ 6 nm, its strength decreases monotonically and becomes more or less independent of t Ta layer thickness (tTa) above 8 nm. These experimental results which are manifestations of the FMR induced spin-pumping mechanism in β-Ta/Py bilayers are explained in terms of negative interfacial effective spin mixing conductance (g↑↓) which depends on tTa
Summary
We present the experimental evidence of anti-damping SOT in a β-Ta/Py/SiO2/Si(100) bilayers without any DC-current flowing through Ta. Within the error of fitting, the inhomogeneous line broadening ∆H0 can be seen to be nearly constant at ~0.2 mT for different thicknesses of Ta. It may be noted from Fig. 6(b) that irrespective of the values of tTa, the observed values of αeff for these β-Ta(tTa)/Py bilayers are smaller than that of the bare Py sample which possessed α = 0.0093 ± 0.0003.
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