Effect of Ta crystallite size on spin-mixing conductance of Ta/Ni80Fe20 bilayer structure

Abstract

Spintronic devices explore spin degree of freedom in addition to charge of an electron. The success of spin-based electronics devices depends on the injection, manipulation and detection of spin currents1. Recently, ferromagnet (FM) and heavy metal (HM) bilayer structures have received great interest due to its spin-charge interconversion. A pure spin current can be generated at the interface between FM/NM via spin pumping. In general, ferromagnetic resonance (FMR) technique is employed in spin pumping to generate pure spin currents through magnetization precession in FM layer which will be injected in to NM layer through the interface. The efficiency of the spin injection is quantitatively measured as spin-mixing conductance(g↓↑) which estimates the amount of spin current injected into the NM layer. The spin-mixing conductance depends on several parameters such as interface roughness, crystalline phase and quality of the NM layer2. The influence of crystalline phase3 and roughness4 on spin-mixing conductance have been investigated but the studies on the effect of crystallite size and quality of NM layer on g↓↑ are limited.In this work, we have investigated the effect of crystallite size of Tantalum on g↓↑ in Ta (18 nm)/Ni80Fe20 (16 nm) bilayers. The bilayer structures are deposited using magnetron sputtering technique. The crystallite size is varied by tuning the growth rate during sputtering deposition by varying the sputtering power from 60 to 120 watt as shown in figure1(a). Grazing incidence X-Ray diffraction (GI-XRD) technique is used to study the phase of Ta. The Ta films that are deposited at different deposition rates in our study have exhibited BCC (α-Ta) crystal structure as shown in figure 1(b). Crystallite size was calculated by Scherrer formula and observed that with growth rate crystallite size is improved up to 120 watts. The thickness and surface morphology of the Ta films were measured using atomic force microscopy (AFM). The measured average rms roughness for all samples is found to be below 0.6 nm. The Ta/Ni80Fe20 bilayer structures are fabricated to investigate the spin-pumping mechanism at the interface.The FMR technique is used for spin pumping in Ta/Ni80Fe20 in the range of 4 -16 GHz while sweeping magnetic field from 0 – 3000 Oe. From the FMR spectrum shown in figure 2(a) we obtained linewidth and resonance field to determine the change in damping parameter which in turn reveals the spin-mixing conductance. We have quantitatively estimated g↓↑ of our bilayer structure and observed that it is in the order of 1018 m-2. Interestingly, increase in the crystallite size of Ta in Ta/Ni80Fe20 bilayer film enhances the spin-mixing conductance as shown in the figure 2(b). It shows a better spin pumping at Ta/ Ni80Fe20 interface with larger crystallite size. Enhancement of g↓↑ with Ta crystallite size is potentially due to the less scattering in Ta layer thus reducing the spin-back flow. **