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Rahul Gupta 1* ,Akash Kumar 1 , Saroj Dash 2 , Sujeet Chaudhary 1 and Pranaba Kishor Muduli 1 1 Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi-110016, India 2 Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296, Göteborg, Sweden *E-mail: [email protected] Abstract: We report on the ferromagnetic resonance (FMR) spectra of Permalloy (Py)/ Graphene (Gr) and pure Py thin films grown under identical conditions to study the phenomena of spin pumping into Graphene. An enhancement of Gilbert damping constant is observed for Py/Gr bilayer structure. Using detailed thickness dependence study we show an enhancement of spin mixing conductance of Ta/Py/Gr/SiO 2 /Si compared to Ta/Py/SiO 2 /Si, which can be attributed to spin pumping. This study is important for successful integration of graphene with commonly used ferromagnetic materials for spintronics applications. Introduction Ferromagnetic Resonance X-ray Reflectivity Raman Spectroscopy Experimental set-up Acknowledgement I would like to acknowledge Nano Research Facility (NRF) and Central Research Facility (CRF), IIT Delhi and Thin Film Laboratory, IIT Delhi for facilities. Conclusion References 0.5 1.0 1.5 2.0 2.5 3.0 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 Ta/Py(10nm)/SiO 2 /Si Fitted Reflectivity (R) 2 (deg.) Material Used Thickness (nm) Roughness (nm) SiO 2 600000 0.4 Py 10 1. 4 Ta 0.6 0.7 Ta 2 O 5 2.1 0.97 1500 2000 2500 3000 0 50 100 150 200 2D = 2694.78 cm -1 Gr/SiO 2 Fitted Intensity Raman Shift (cm -1 ) G = 1589.99 cm -1 0 40 80 0 40 80 1500 2000 2500 3000 0 40 80 p=2mTorr D G 2D =514nm Intensity (a.u.) p=6mTorr p=10mTorr Raman Shift (cm -1 ) J. Hirsch, Phys. Rev. Lett. 83, 1834 (1999). A. Brataas, Y. V. Nazarov, and G. E. W. Bauer, Phys. Rev. Lett. 84, 2481 (2000). Will Gannett et al., J. Appl. Phys. 117, 213907 (2015). Wei Han et al., Nature Nanotechnology 9, 794-807 (2014). A. K. Patra, S. Singh, B. Barin, Y. Lee, J.-H. Ahn, E. del Barco, E. R. Mucciolo, and B. Özyilmaz, Appl. Phys. Lett. 101, 162407 (2012). A large increase of Gilbert damping constant in Ta/Py/Gr/SiO 2 /Si as compared to Ta/Py/SiO 2 /Si thin film was observed which signifies spin pumping into graphene. The calculated value of spin mixing conductance (g ↑↓ ) of Ta/Py/Gr/SiO 2 /Si is found to be (2.621±0.001)⨯10 18 m -2 . We also find that the quality of Py films on graphene and without graphene to be very similar as determined from their material properties. This indicates that CVD grown graphene can act a good material for spin pumping studies. Here, we have prepared a series of samples; bare Py and Py/Gr by varying thickness of ferromagnetic films (Py) on the SiO 2 /Si and Gr/SiO 2 /Si substrates. We grow Py thin films by using DC-magnetron sputtering at 2 mTorr Argon working pressure. A 2 nm of Tantalum (Ta) capping layer is also deposited for protection of samples from oxidation. We have performed the FMR spectroscopy at room temperature for excitation frequency of 4-12 GHz using a broad-band FMR set-up as shown in above figure. The FMR set up is based on the Co-planar waveguide (CPW). The excitation of spin angular momentum can be measured by FMR spectroscopy. The commercial graphene samples used in this work were prepared by chemical vapour deposition [Graphenea, Spain]. The Raman Spectra of monolayer graphene is show that the G-peak (=1589.99 cm -1 ) and 2D-peak ( =2694.79 cm -1 )is observed as cited in the literature. The Raman spectra of Py deposit on the top of Graphene at different Argon working pressure is shown in figure. The G-peak and 2D-peak still observed whereas an additional peak, called Defect-peak (= 1351.54cm-1 ), is also observed. The intensity of D-peak increases as the Argon working pressure decreases. The X-ray reflectivity tells the thickness and roughness of thin film. Here, we have taken the XRR for the Ta/Py/SiO 2 /Si sample. The table given below tells thickness and roughness of thin films in the bilayer structure. Kittle Formula Line width FWHM 4 6 8 10 12 1.5 2.0 2.5 3.0 3.5 4.0 4.5 H (mT) frequency (GHz) Ta/Py(10nm)/Gr/SiO 2 /Si 100 110 120 130 140 Ta/Py(10nm)/Gr/SiO 2 /Si Ta/Py(10nm)/SiO 2 /Si FMR Sig. (a.u.) Field o H (mT) H r = (117.84 0.012) mT H r = (115.43 0.027) mT f = 10 GHz ( ) ( ) Ta/Py(10nm)/SiO 2 /Si 4 6 8 10 12 0 30 60 90 120 150 180 frequency (GHz) Ta/Py(10nm)/Gr/SiO 2 /Si M eff = (66.965 0.057) mT M eff = (69.589 0.038) mT H r (mT) Ta/Py(10nm)/SiO 2 /Si 0.05 0.10 0.15 0.20 0.25 0.30 0.35 40 45 50 55 60 65 70 75 Ta/Py(t)/Gr/SiO 2 /Si Ta/Py(t)/SiO 2 /Si M eff (mT) 1/t Py (nm -1 ) M s = 77.5 0.8 mT M s = 79.2 0.6 mT 0.05 0.10 0.15 0.20 0.25 0.30 0.35 8 12 16 20 24 Ta/Py(t)/Gr/SiO 2 /Si Ta/Py(t)/SiO 2 /Si Damping Cons. (10 -3 ) 1/t Py (nm -1 ) g = (2.621 0.001)10 18 m -2 g = (1.19 0.0002)10 18 m -2 2 4 6 8 10 12 14 16 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 H o (mT) t Py (nm) Ta/Py(t)/Gr/SiO 2 /Si Ta/Py(t)/SiO 2 /Si H eff -M×H D M Magnetization damping term Dissipation of angular momentum Transferred to conduction electrons Generation of spin current *Landau-Lifshitz-Gilbert equation The damping of magnetization term is introduced by Gilbert * When FM/NM interface is formed electrons losses it’s spin angular momentum via s-d interaction, known as spin pumping. Spin pumping generates spin current though FM/NM interface. Spin mixing conductance DC-magnetron Sputtering System Ferromagnetic Resonance = 2 [( + )( + + 4 )] ∆ = 4 + ∆ 0 ∆ = ↑↓ 4 1 Where, ↑↓ = Spin Mixing Conductance Using Line-width and Kittle formula given above, we have fitted the data and get the Gilbert damping constant & effective magnetization. The FMR absorption spectra for Py sample with graphene is fitted with the derivative of Lorentzian function to determine FMR linewidth (∆ ) and resonance field ( ). = − 0 × − [ × ( × )] = ℏ 4 ↑↓ [ × ] Effect of Graphene Incorporation on Spin Dynamics of Permalloy Thin Film
