Enhancement of spin current to charge current conversion in Ferromagnet/Graphene interface
Mahammad Tahir Subhakanta Das Mukul Gupta Rohit Medwal Soumik Mukhopadhyay



Department of Physics, Indian Institute of Technology Kanpur, Kanpur- 208016, India

Department of Physics, Indian Institute of Technology Kanpur, Kanpur- 208016, India

UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore- 452001, India

Department of Physics, Indian Institute of Technology Kanpur, Kanpur- 208016, India

Department of Physics, Indian Institute of Technology Kanpur, Kanpur- 208016, India


The use of graphene in spintronic devices depend, on its ability to convert a spin current into an electric charge current. We have systematically investigated the spin pumping induced spin-to-charge current conversion at the Graphene/FM interface and the effect of interface modification through high spin orbit coupling (SOC) material (Pt) as an interlayer (IL) of varying thicknesses by using broadband FMR spectroscopy. The spin mixing conductance was enhanced from 1.66 × 10^18 m^-2 to 2.72 × 10^18 m^-2 where as the spin current density was enhanced from 0.135±0.003 to 0.242±0.004 MA/m^2 at the Graphene/FM interface due to the interface modification using high SOC material Pt as an interlayer. We observe the value of spin current to charge current conversion efficiency ≈ 0.292×10^-3 nm for the Graphene/FM interface. These findings support the idea that Graphene in contact with high SOC material (Pt) be a potential candidate for spintronic applications, specifically for spin-torque-based memory applications. 

Enhancement of spin current to charge current conversion in Ferromagnet/Graphene interface
    Soumik Mukhopadhyay
    April 29, 2024
==========================================================================================



§ INTRODUCTION

Generation, transportation and detection of pure spin current play a fundamental role in spintronic devices. The primary motivations behind ongoing spintronics research are to achieve, non-volatility <cit.>, improvement of information processing <cit.> and higher integration densities of spin current driven logic and memory devices  <cit.>. When a ferromagnetic (FM) thin film is coupled with a nonmagnetic (NM) thin film, interesting phenomena arise, which include perpendicular magnetic anisotropy (PMA) <cit.>, interfacial Dzyaloshinshkii-Moriya interaction (iDMI) <cit.>, chiral damping <cit.>, Rashba Edelstein effect (REE)  <cit.> and spin pumping  <cit.>. While most of these phenomena have been attributed due to high spin-orbit interactions, of the heavy metal (HM) layers being a natural choice for the NM layer. Two-dimensional (2D) materials have recently gained much interest in comparison with the conventional NMs because of their monolayer stability, high spin orbit coupling (SOC), and a lack of inversion symmetry in their crystal structures  <cit.>. These unique properties make them excellent candidates for interface magnetism engineering in spintronics.

Pure spin currents i.e. the transfer of spin angular momentum occurs from the FM layer to the NM layer across the interface, are generated by spin pumping via magnetization precession <cit.>. Quantitatively, the spin pumping is determined by the interfacial spin mixing conductance (g^↑↓) that takes into account the spin pumped, backflow spin currents and the enhanced Gilbert damping (α) <cit.>. The enhancement of Gilbert damping is more prominent in NM layers with high spin orbit coupling (SOC) because of the stronger interaction between electron spin and lattice. For spin current to charge current conversion there are two principal mechanisms: inverse Rashba-Edelstein effect (IREE) <cit.> and the inverse spin Hall effect (ISHE) <cit.>. The major difference between these two effects is that IREE is often referred to as an interface effect caused by the significant interfacial SOC and an electric field normal to the surface, induced by the breaking of the inversion symmetry while the spins and electrons are confined in the 2D state <cit.>. On the other hand, ISHE arises from the spin-orbit scattering within the bulk of the NM material, with the charge current being transverse to the spin current and spin polarization <cit.>. The basic mechanism can be identified by injecting a pure spin current J_s. For IREE, the surface charge current density J_c is given by:

    J_c = α_R(e/ħ)(ẑ×J_s)

where α_R is the Rashba parameter; J_s represents the non-equilibrium spin density caused by spin injection; ẑ represents the unit vector normal to the interface that aligns with the inversion symmetry breaking electric field.  <cit.>. The induced charge current J_c due to ISHE is described by:

    J_c = θ_SH(2e/ħ)(J_s×σ̂)

where J_c is perpendicular to both the spin current density J_s and the spin polarization vector σ. The spin Hall angle θ_SH quantifies the spin current to charge current conversion efficiency <cit.>.

Recently, for spin to charge current conversion, single layer graphene (SLG) <cit.> and semiconducting transition-metal dichalcogenide (TMD) monolayers (MLs) have attracted a great deal of interest. Owing to the low atomic number of carbon, pure graphene has a weak SOC, thus minimising the possibility of applications in spintronics <cit.>. In order to overcome this limitation, multiple approaches have been proposed mainly with the objective of enhancing SOC and local magnetic moments in graphene. These approaches includes (i) functionalization of graphene with tiny doses of adatoms or nanoparticles  <cit.> (ii) fabrication of nanostructures with ferromagnetic gate electrodes <cit.> and (iii) ferromagnetic insulator (FMI) or ferromagnetic metal (FM) directly attached to graphene in order to induce magnetic proximity effects <cit.>. Moreover, when graphene is synthesized directly on top of semiconducting TMDs, it demonstrates a significant increase in SOC  <cit.>. Recently, it was demonstrated that a Py electrode under FMR pumps spin current into SLG  <cit.>. When a heavy metal layer is deposited on graphene, the SOC strength at the interface changes, making SLG attached with a FM potential candidate for spintronic applications.

In the present work, we investigate the spin pumping and spin-to-charge current conversion in a heterostructure consisting of Quartz/SLG/Ni_80Fe_20 [Py(5 nm)] and the effect of interface modification through high SOC material (Pt) as an interlayer (IL) of varying thickness. We study the effect of interfacial SOC strength using IL thickness variation on the spin pumping efficiency in the SLG by using broadband FMR spectroscopy. The results are interesting for spintronics applications, specifically by provide an understanding of how the interface modification could further enhance the spin current to charge current conversion.







§ EXPERIMENTAL METHODS AND CHARACTERIZATION


We prepared the thin film heterostructures: Quartz/Py(5 nm), Quartz/SLG/Py(5 nm) and Quartz/SLG/IL/Py(5 nm) with Pt(1,2 nm) as an interlayer (IL). We used commercially available chemical-vapor deposited (CVD) graphene (manufactured by Graphenea) on a Quartz substrate. The thin films of Pt and Py were deposited using DC- magnetron sputtering with a base pressure below 3 × 10^-7 Torr. The deposition pressure and argon flow rate were 3 × 10^-3 Torr and 15 standard cubic centimeters per minute (SCCM), respectively. The target was pre-sputtered for two minutes to avoid any contamination. The SLG on Quartz substrate was characterized by Raman spectroscopy (Model: Princeton Instruments Acton Spectra Pro 2500i) equipped with a 532 nm Laser. Saturation magnetization of the thin films was measured by a SQUID-based magnetometer (MPMS 3, Quantum Design). In order to investigate the spin pumping, a lock-in-based broadband ferromagnetic resonance (FMR) set-up (NanOsc) was used. The samples of roughly 4 × 2 mm^2 were placed on a 200 μm wide coplaner waveguide (CPW) in a flip-chip manner and the FMR measurements were carried out in the broad frequency range (3–15 GHz) as a function of in-plane external DC magnetic field in a direction perpendicular to the radio frequency field (h_rf) at room temperature. For the spin current to charge current conversion measurements, electrical contacts were made using silver paste and gold wire beneath the inverted sample. The spin current to charge current conversion in in-plane and out of plane geometry were measured by a nanovoltmeter (Keithley 2182A) which is integrated to the FMR setup  <cit.>.
Out of plane angle-dependent spin current to charge current
conversion measurements were performed at the microwave
frequencies of 3 and 5 GHz.




The Raman spectra of CVD-grown SLG on a Quartz before and after the deposition of Py (5 nm) and Pt(2 nm)/Py(5 nm) bilayer thin films are shown in Fig. <ref>a. The Raman spectra is recorded at a laser wavelength of 532 nm having a spot size of 0.5 μm. The low laser power of 125  μW is used to avoid any heating effect and thermal damage. The two most commonly recognised SLG signatures are the G peak at ≈ 1583.9 cm^-1 and the 2D (or G^') peak at ≈ 2677.9 cm^-1. Here, the 2D peak is identified with second order peak of zone-boundary phonons and the G peak corresponds to a doubly degenerate zone-centre E_2g mode. We use the spectral weight ratio (I_2D/I_G) as a quantitative measure of sputtering damage. The I_2D/I_G ratio for SLG is 2.07 ± 0.02, which remains unchanged after the deposition of Py (5 nm) and Pt(1,2 nm)/Py (5 nm) bilayer thin films <cit.> . 
The typical in-plane (open black squares) and out-of-plane (open red circles) magnetic hysteresis loops for Quartz/Py(5 nm) and Quartz/SLG/Pt(2 nm)/Py(5 nm) stack measured at room temperature are shown in Fig. <ref>b and Fig. <ref>c, respectively. 
The saturation magnetization (4πM_s) value was determined from the magnetic hysteresis measurement for the Quartz/Py (5 nm) sample and was found to be ≈800 mT, while for the Quartz/SLG/Pt (2 nm)/Py (5 nm) bilayer, the value is ≈750 mT, which agrees reasonably well with the value estimated from the FMR measurements for the same sample.




§ MAGNETIZATION DYNAMICS AND SPIN PUMPING

The prepared thin film stacks are subjected to the flip-chip FMR method for magnetization dynamics studies. The measurement schematic with the orientation of the relevant quantities is given in our previous reports  <cit.>. The FMR measurements are performed in static fields well above the in-plane anisotropy fields so that magnetization M of the ferromagnetic layer can be considered parallel to the external magnetic field (H_ext). During magnetization precession, the ferromagnetic system absorbs the applied microwave power, thereby giving rise to the recorded FMR spectrum. The recorded FMR spectra of the Quartz/SLG/Py(5 nm) interface are shown in Fig. <ref>a, while the FMR spectra of the Quartz/Py(5 nm) and Quartz/SLG/Pt(1,2 nm)/Py(5 nm) interfaces are shown in Fig. <ref>a and Fig. <ref> (b,c) in the Appendix. The derivative Lorentzian function (Eq. <ref>) having symmetric and asymmetric coefficients is used to fit the recorded FMR spectra <cit.>.


    d I_FMR/d H = 4AΔH( H- H_res)/(4( H- H_res)^2+
    (ΔH)^2)^2-S(ΔH)^2-4(H-H_res)^2/(4( H- H_res)^2+(ΔH)^2)^2


Here  H, ΔH and H_res are the in plane applied DC magnetic field, FMR linewidth and resonance field of microwave absorption, respectively. The FMR signal's amplitudes S and A correspond to symmetric and antisymmetric coefficients, respectively <cit.>. We evaluate the ΔH and H_res from the fittings of the FMR spectra.
The plots of microwave absorption frequency f  versus  H_res are shown in Fig. <ref>b. The effective magnetization M_eff is calculated, using Kittel’s equation (Eq. <ref>) <cit.>.


    f = γ/2π[( H_res+ H_k) ( H_res+ H_k+ 4 πM_eff)]^1/2


Here γ = gμ_B/ħ = 1.88 × 10^2 GHz/T is the gyromagnetic ratio, μ_B is the Bohr magneton, g is the Lande's spectroscopic splitting factor, and ħ is the reduced Planck's constant, H_k is the in-plane anisotropy field of the FM layer. The variation of 4 πM_eff as a function of the interlayer Pt thickness is shown in Fig. <ref>c. We find that 4 πM_eff value is higher for Quartz/Py(5 nm) sample compared to Quartz/SLG/Pt(0,1,2 nm)/Py (5 nm) samples (Fig. <ref>c). The value of 4 πM_eff is given by  4 πM_eff = 4 πM_s- K_s/M_st_Py, where K_s is the surface/interface anisotropy constant, t_Py is the thickness of the Py layer, and M_s is the saturation magnetization. Since K_s is proportional to interfacial spin-orbit coupling, we believe the decrease of 4 πM_eff for Quartz/SLG/Pt(0,1,2 nm)/Py(5 nm) is caused by the increase in interfacial spin-orbit coupling due to d-d hybridization, similar to some previous reports for 2-D/FM systems <cit.>. 

The surface/interface imperfections and defects in the FM thin films can enhance the linewidth of the FMR spectra. The total FMR linewidth can be expressed as: 

    ΔH= ΔH_
    Gilbert+ΔH_inhomogeneous

The Gilbert contributions represent the intrinsic damping. It follows from the Landau Lifshitz-Gilbert equation that the FMR linewidth would depend linearly on the microwave frequency for intrinsic damping. The variation of ΔH versus f are shown in Fig. <ref>a. The solid line fit is given by the following equation <cit.>:

    ΔH=4πα_eff/γf+ΔH_0

Here ΔH_0 is the frequency-independent contribution known for inhomogeneous broadening, related to the magnetic inhomogeneity in the thin film. The linear behavior of ΔH versus f confirms the intrinsic origin of the damping parameter observed in our Quartz/SLG/Py(5 nm) and Quartz/Py(5 nm) samples, as shown in Fig. <ref>a. The FMR spectra of Quartz/Py(5 nm) and Quartz/SLG/Py(5 nm) recorded at 5 GHz are shown in the inset of Fig. <ref>a. The value of ΔH = 44.58 Oe for Quartz/Py(5 nm) enhances to 52.93 Oe at 5 GHz for the Quartz/SLG/Py(5 nm) interface. Spin pumping is responsible for the enhancement in the ΔH. In the present scenario, the value of α_eff measured from the slope of the linear frequency dependence of linewidth have the following contributions:

    α_eff = α_bulk+α_interface

Here, α _bulk is the damping contribution due to energy transfer to the lattice within the bulk of the ferromagnet <cit.>, whereas α _interface is mainly due to the spin pumping which is the result of the spin angular momentum loss due to spin current outflow from the FM layer into the NM layer or due to the spin-flip at the interface due to interfacial spin-orbit coupling <cit.>.
The value of α_eff is measured as a function of inetrlayer Pt thickness as shown in Fig. <ref>b. A large increase in α _eff is observed for the samples Quartz/SLG/Pt(0,1,2 nm)/Py(5 nm) compared to the reference sample Quartz/Py(5 nm) as shown in Fig. <ref>b. The enhancement in the Gilbert damping attributed to spin-pumping is estimated by Δα = α_SLG/Pt(t)/Py - α_Py, which is utilised to calculate the interfacial spin-mixing conductance (g^↑↓). The interfacial spin mixing conductance (g^↑↓) determines the amount of spin current injected or transmitted by the precessing magnetization vector of the FM layer across the interface and is given by the following expression: <cit.>

    g^↑↓ =4πM_st_Py/gμ_B(α_SLG/Pt(t)/Py-α_Py)

where 4 πM_s is the saturation magnetization. The extracted values of (g^↑↓) at the Quartz/SLG/Py(5 nm) interface is found to be increasing after the interface modification through high SOC material Pt(1,2 nm) as an interlayer. The values of (g^↑↓) for Quartz/SLG/Py(5 nm) interface is extracted to be 1.66 × 10^18 m^-2 which is further enhanced to 2.72 × 10^18 m^-2 for Quartz/SLG/Pt/Py(5 nm) interface as shown in Fig. <ref>c.
The enhancement of the Gilbert damping and spin mixing conductance observed in the Quartz/SLG/Pt(0,1,2 nm)/Py(5 nm) stacks could be attributed to: 1) the spin current injected in the SLG by the spin pumping mechanism at the SLG/Pt(0,1,2 nm)/Py(5 nm) interface, which creates a spin accumulation on the SLG. The dissipation of spin current at the SLG/Py and SLG/Pt/Py interfaces through spin-flip scattering acts as an additional channel for spin relaxation, leading to enhanced damping α_sp <cit.>. The diffusive flow of spins in SLG/Pt(0,1,2 nm)/Py(5 nm) can be described by spin current density J_s, evaluated using the following expression <cit.>:


    J_s≈(g^↑↓ħ/8π)(h_rfγ/α)^2 [4πM_sγ+√((4πM_sγ)^2+16(πf)^2)/(4πM_sγ)^2+16(πf)^2](2e/ħ)
 

Here h_rf is the RF magnetic field of 1 Oe (at 15 dBm rf power) in the strip line of the CPW. The calculated values of J_s are found to be dependent on interlayer (Pt) thickness and vary within the range of 0.135±0.003 to 0.242±0.004 MA/m^2 at 3 GHz. 


§ SPIN CURRENT TO CHARGE CURRENT CONVERSION


The study of spin current to charge current conversion is carried out with in-plane microwave excitation. The voltage V_dc is measured with a dc magnetic field (H_dc) sweep. The measuring wires are connected to a nanovoltmeter for a direct measurement of the V_dc. In Fig. <ref>a, the schematic of measurement geometry of the FM/NM stack shows how spin pumping is used for the generation and flow of spin current J_s across the FM/NM interface. A schematic of CPW shows the thin film stack in contact with the main signal transmission line (S), which is isolated from the adjacent ground line (G). In order to understand the effect of excitation frequency f on the V_dc, we have performed measurements for the Quartz/SLG/Pt(0,1,2 nm)/Py(5 nm) samples in the broad frequency range of 3 to 12 GHz. Fig. <ref>b shows the measured V_dc vs H_dc for different f values for Quartz/SLG/Pt(1 nm)/Py(5 nm) interface. The measured V_dc vs H_dc spectra are symmetric in shape and change sign with inversion of magnetic field direction which indicates that the voltage signal we measure may be primarily due to spin pumping induced spin to charge current conversion in the Pt(0,1,2 nm)/SLG interface. The sign reversal of V_dc with reversal of H_dc could be due to the fact that the V_dc ∝ J_s×σ ∝ J_s×M ∝ J_s× H ∝ sinθ_H where σ is the direction of spin polarization which gets reversed upon changing the direction of H_dc.  
We envisage two possibilities regarding the mechanisms responsible for spin-to-charge conversion. 1) The observed V_dc may have contributions of spin pumping induced ISHE voltage (V_ISHE) in the Pt(1,2 nm)/SLG interface and spin-rectification effects such as anisotropic magneto resistance (AMR) and anomalous Hall effect (AHE) of the Py layer. The V_ISHE is supposed to generate a symmetric Lorentzian shape, while the AMR/AHE is associated with an asymmetric Lorentzian shape  <cit.>. Therefore, in order to separate the V_sym and V_asym components of voltage we have used the following Lorentzian function to fit the experimental data:


    V(H) = V_sym(ΔH)^2/( H- H_res)^2+(ΔH)^2+V_asym2(ΔH)(H-H_res)/(H- H_res)^2+(ΔH)^2


Best fit to the experimental data is shown by a solid black line in Fig. <ref>c. Inset of Fig. <ref>b shows the V_sym and  V_asym components of voltage measured at 3 to 12 GHz frequencies for SLG/Pt(1 nm)/Py(5 nm) sample. It is clear that V_sym has significant non-zero values for all f, while V_asym is close to zero throughout the frequency range. This confirms that the observed voltage signal V_dc could be due to the ISHE and certainly not AMR/AHE. 

2) Spin to charge current conversion reported for 2D interfaces such as YIG/SLG  <cit.>, Ag/Bi  <cit.> and LAO/STO  <cit.> suggests an alternative mechanism in the presence of 2D SOC driven by broken inversion symmetry. In the case of SLG/Pt(1,2 nm)/Py(5 nm) interface, we propose that initially the spin current flows into the Pt layer and diffuses with the spin diffusion length λ_N. Subsequently, it is converted into a charge current density shown in Eq. <ref> by means of ISHE. This current density leads to a voltage as:


    V_SP(H)=ω R_N e θ_SH λ_N w P g_eff^↑↓/8π×tanh(t_N/2λ_N) (h/ΔH)^2 L(H-H_R) cosϕ

 
Here R_N, t_N, and w are, respectively, the resistance, thickness and width of NM layer, and P is a ellipticity factor and ω = 2π f <cit.>. The reported values of spin diffusion length (λ_N) and spin Hall angle (θ_SH) for Pt are 3.4 nm and 0.056 respectively <cit.>.

To interpret the origin of voltage in Py/SLG (Pt=0 nm) interface with the same spin pumping ISHE mechanism, we consider t_N/2λ_N <1, so that the expression for spin pumping voltage given by Eq. <ref> reduces to:

    V_SP(H)=f e R_N t_N θ_SH w P g_eff^↑↓/8×(h/ΔH)^2


The spin current is pumped from the FM layer into the Py/SLG interface and converted by the IREE into a charge current density J_c = (2e/ħ)λ_IREEJ_s. The measured voltage is related to this current density by V_SP = R_NwJ_c. Taking into account the values of the effective thickness of SLG t_N = 0.335 × 10^-9 m <cit.>, R_N = 1.2 kΩ, g^↑↓ = 1.66 × 10^18 m ^-2, w = 4 mm and the amplitude of peak voltage V = 0.5 μV at 3 GHz for the Py/SLG interface. We observe the value of IREE coefficient λ_IREE≈ 0.292×10^-3 nm for Py/SLG interface. The reported values of λ_IREE in SLG for YIG/SLG interface are of the order of ≈ 10^-3 nm. <cit.>
The spin pumping induced spin to charge current conversion voltage V_dc is enhanced for modified interfaces utilising the Pt IL, as compared to Py/SLG interface. The observed enhancement of the V_dc in Quartz/SLG/IL/Py (5 nm) interface at f = 3 GHz as shown in Fig. <ref>c (compared to Py/SLG) could be due to the increase of effective spin-orbit coupling due to the insertion of high SOC Pt interlayer.

To measure the voltage dependence on the out-of-plane (OOP) polar angle θ_H we rotate the sample with respect to the magnetic field. Fig. <ref>d shows the V_dc spectra observed at f = 3 GHz, with H_dc at θ_H = 0^∘ and 90^∘ for SLG/Pt(1 nm)/Py(5 nm) interface.
It can be observed that the sign of V_dc at θ_H = 0^∘ gets reversed when H_dc is applied in the reverse direction and falls below the noise level when θ_H = 90^∘ as shown in Fig. <ref>d. 
Fig. <ref>e shows the variation of V_sym and V_asym voltage components with varying OOP angle θ_H for SLG/Pt(1 nm)/Py(5 nm) interface at 3 GHz. As already discussed, the V_asym component of voltage arises due to the AMR and AHE while the V_sym is due to the V_ISHE as well as AMR and AHE <cit.>. We find that V_asym is negligible and remains unchanged with variation in θ_H , while V_sym is significantly larger and varies sinusoidally with θ_H as shown in Fig. <ref>e. This indicates that the voltage signal measured is primarily due to the spin pumping-induced spin current-to-charge current conversion. 

We calculate the value of J^Cal._s as shown in Table <ref> in order to verify the contribution of IL Pt in the spin current to charge current conversion in our SLG/Pt(0,1,2 nm)/Py interfaces. We take the λ_IREE value (observed for FM/SLG interface) fixed, which is of the order of 10^-3 nm. Then we calculate the value of J^Cal._s for our SLG/Py (Pt = 0 nm) interface, which is found to be 0.135±0.002 MA/m^2 and  0.404±0.005 MA/m^2 for the SLG/Pt(1 nm)/Py interface, which further enhanced to 0.456±0.003 MA/m^2  for the SLG/Pt (2 nm)/Py interface. We clearly observed the enhancement in the J^Cal._s with respect to the IL Pt thickness. Based on these findings, we speculate that the IREE may be responsible for the spin-to-charge conversion at the SLG/Py interface, while at the SLG/Pt (1,2 nm)/Py interface, both ISHE and IREE contributed to the spin-to-charge conversion.


§ CONCLUSION

In summary, we have systematically investigated the dc voltage at the SLG/Py(5 nm) interface as a result of spin pumping induced spin current to charge current conversion by using coplanar waveguide based broadband ferromagnetic resonance setup. We have also observed the effect of the interlayer by inserting a high SOC material Pt (1,2 nm) at the SLG/Py interface. We demonstrate a strong correlation of measured spin mixing conductance and spin current to charge current conversion with the varying thickness of Pt interlayer. We contemplate that for the spin-current to charge-current conversion in our SLG/Py (5 nm) interface, the inverse Rashba-Edelstein effect is responsible, whereas in our SLG/Pt (1,2 nm)/Py (5 nm) interface, both the inverse spin Hall effect and the inverse Rashba Edelstein effect contributed to the conversion mechanism. These findings support the idea that graphene in contact with heavy metals may be a potential material for spintronic applications.





§ ACKNOWLEDGEMENTS

M.T. acknowledges the MHRD, Government of India for senior Research Fellowship. S.M. acknowledges the Department of Science and Technology (DST), Government of India for financial support. R.M. acknowledges Initiation grant, IIT Kanpur (IITK/PHY/2022027), and I-HUB Quantum Technology Foundation (I-HUB/PHY/2023288), IISER Pune  for financial support.



§  FMR MEASUREMENTS

Fig. <ref> shows the recorded FMR spectra of Quartz/Py(5 nm) stack and Quartz/SLG/Pt(1,2 nm)/Py(5 nm). 



§ MEASUREMENT OF SPIN CURRENT TO CHARGE CURRENT CONVERSION IN SLG/PT(2 NM)/PY(5 NM) INTERFACE


Fig. <ref>a shows the measured V_dc vs H_dc for different f values for Quartz/SLG/Pt(2 nm)/Py(5 nm) sample. As we already discussed in the main text that the sign of V_dc gets reversed when H_dc is applied in the reverse direction. The measured V_dc vs H_dc spectra are symmetric in shape and change sign with inversion of magnetic field direction which indicates that the voltage signal we measure  primarily due to spin pumping induced spin to charge current conversion in the SLG/Pt(2 nm)/Py(5 nm) interface. The spin pumping induced spin to charge current conversion voltage V_dc is enhanced for modified interfaces utilising the Pt IL, as compared to Py/SLG interface. The observed enhancement of the V_dc in Quartz/SLG/IL/Py (5 nm) interface at f = 5 GHz as shown in Fig. <ref>b (compared to Py/SLG) could be due to the increase of effective spin-orbit coupling due to the insertion of high SOC Pt interlayer.
 Best fit to the experimental data is shown by a solid black line in Fig. <ref>b. Inset of Fig. <ref>a shows the V_sym and  V_asym components of voltage measured at 3 to 12 GHz frequencies for SLG/Pt(2 nm)/Py(5 nm) sample. It is clear that V_sym has significant non-zero values for all f, while V_asym is close to zero throughout the frequency range. This confirms that the observed voltage signal V_dc could be due to the ISHE and certainly not AMR/AHE. 
To measure the voltage dependence on the OOP polar angle θ_H we rotate the sample with respect to the magnetic field. Fig. <ref>c shows the V_dc spectra observed at f = 3 GHz, with H_dc at θ_H = 0^∘ and 90^∘ for SLG/Pt(2 nm)/Py(5 nm) interface. It can be observed that the sign of V_dc at OOP angle θ_H = 0^∘ gets reversed when H_dc is applied in the reverse direction and falls below the noise level when θ_H = 90^∘ as shown in Fig. <ref>c. Fig. <ref>d shows the variation of V_sym and V_asym voltage components with varying OOP angle θ_H for SLG/Pt(2 nm)/Py(5 nm) sample. 
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