Spin-orbit coupling (SOC) plays a vital role in spin-to-charge interconversion in heavy metal (HM)-ferromagnet (FM) bilayer structures [1]. Spin pumping is an efficient method to generate pure spin current by the precessional motion of magnetization, which transfers angular momentum to HM. The efficiency of spin pumping is quantified by an interfacial parameter called spin-mixing conductance (g↓↑). Electrical detection of spin current is possible via inverse spin Hall effect (ISHE) which converts spin current into charge current. The efficiency of spin-to-charge interconversion is quantified by spin Hall angle (θSH) [2]. The θSH of heavy metal highly depends on its crystalline phase and the effect of the seed layer on the phase of HMs is largely overlooked. Here, we report the effect of seed permalloy (Ni80Fe20, Py) layer thickness on the Tantalum (Ta) and Tungsten (W) crystalline phase and its θSH. We have observed a structural phase transition in Ta as a function of seed layer thickness (tPy) affecting the θSH of the Ta layer. Due to strain at the interface between crystalline Py and Ta, Ta exhibits a mixed-phase (α+β) on the seed permalloy layer when tPy > 12 nm. However, Ta nucleates as α-Ta on Py with tPy < 8 nm, Py does not exhibit prominent crystalline nature. The phase transition of Ta is primarily attributed to strain at the Py/Ta interface which is not observed in both Py(tPy)/W(10) and Co40Fe40B20 (tCFB)/Ta(18) bilayer structure. Ferromagnetic resonance (FMR) based spin pumping shows that effective damping is enhanced for all samples. The maximum g↓↑ of 10.1×1018 m-2 is observed for Si/Py(20)/Ta(18) and the minimum value of 7.9×1018 m-2 for Si/Py(8)/Ta(18) which corresponds to (α+β)-phase of Ta and α-phase of Ta, respectively. The estimated θSH for (α+β)-Ta is 0.15 ± 0.009, which is higher than α-Ta. The combined effect of symmetry breaking and low longitudinal resistance are the primary reason for the high g↓↑ and θSH. Our systematic study provides an insight into Ta phase transition via seed layer thickness and gives an alternative route for tuning [3]. 
**References*** [1] A. Brataas, Y. V. Nazarov, and G. E. W. Bauer, Finite-Element Theory of Transport in Ferromagnet–Normal Metal Systems, Phys. Rev. Lett. 84, 2481 (2000).
[2] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, Nonlocal Magnetization Dynamics in Ferromagnetic Heterostructures, Rev. Mod. Phys. 77, 1375 (2005).
[3] K. Sriram, A. Haldar, and C. Murapaka, Effect of Seed Layer Thickness on the Ta Crystalline Phase and Spin Hall Angle, Nanoscale 13, 19985 (2021). 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/89bfebd3bdb7b575d10ce82734453062.png)

Effect of seed layer thickness on Ta crystalline phase and spin Hall angle

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.

**References:**

1. Fabian, J., Sarma, S. Das & Igor Zutic, Jaroslav Fabian, S. D. S. Spintronics: Fundamentals and applications. Rev. Mod. Phys. 76, (2004). 2. Tserkovnyak, Y., Brataas, A. & Bauer, G. E. W. Enhanced Gilbert Damping in Thin Ferromagnetic Films. Phys. Rev. Lett. 88, 4 (2002). 3. Kumar, A., Bansal, R., Chaudhary, S. & Muduli, P. K. Large spin current generation by the spin Hall effect in mixed crystalline phase Ta thin films. Phys. Rev. B 98, 1–6 (2018). 4. Pham, T. K. H. et al. Interface morphology effect on the spin mixing conductance of Pt/Fe3O4 bilayers. Sci. Rep. 8, 1–8 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/5310e316ff29d92a2283e7123b2ddfa0.jpg)
Figure.1: (a) Deposition rate vs Power, (b) X-ray diffraction of Ta at varied deposition rate.

![](https://assets.underline.io/uploads/markdown_image/1/image/0bad8ec71162257913fa6146e434188e.jpg)
Figure.2: (a) Ferromagnetic response of Ta/Ni80Fe20 and reference Ni80Fe20 at 9 GHz, (b) spin-mixing conductance vs deposition rate of all bilayer structure

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

FOC: Spin Hall and Spin-Orbit Torque II

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Effect of seed layer thickness on Ta crystalline phase and spin Hall angle

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

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