Extrinsic SpinâOrbit Coupling-Induced Large Modulation of Gilbert

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Extrinsic Spin Orbit Coupling Induced Large Modulation of Gilbert Damping Coefficient in CoFeB Thin film on The Graphene Stack with Different Defect Density Sumona Sinha, Santanu Pan, Samiran Choudhury, Jaivardhan Sinha, and Anjan Barman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02790 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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The Journal of Physical Chemistry

Extrinsic Spin Orbit Coupling Induced Large Modulation of Gilbert Damping Coefficient in CoFeB Thin film on The Graphene Stack with Different Defect Density

Sumona Sinha, Santanu Pan, Samiran Choudhury, Jaivardhan Sinha and Anjan Barman*

Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700 106, India. ABSTRACT Control of Gilbert damping parameter is imperative for various spintronic and magnonic devices and various schemes have been attempted to achieve that. Here, we report a large tunability of Gilbert damping by varying the underlayer of CoFeB thin film from few-layer-graphene (FLG) to graphite layer. We measured the ultrafast magnetization dynamics of CoFeB, FLG/CoFeB and graphite/CoFeB by using time-resolved magneto-optical Kerr effect (TR-MOKE) magnetometry. While the magnetization precession frequency remained independent on the underlayer, a very large variation (~ 200%) in the value of the Gilbert damping coefficient α is observed from 1

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FLG/CoFeB (α ~0.035±0.005) to graphite/CoFeB (α ~0.008±0.001). This large variation of the damping coefficient is understood in terms of the extrinsic spin-orbit interaction of FLG and graphite films, which is very large in FLG due to the presence of large amount of surface defects in it. A faster demagnetization time and fast relaxation time (τ1) were noted for graphite/CoFeB bilayer system than that of FLG/CoFeB. In general, we infer that interfacial spin physics is primarily governed by the growth of CoFeB layer, from our bilayer systems. This finding suggests a new direction towards the control of precessional magnetization dynamics leading towards the applications in miniaturized high-speed magnetic devices.

INTRODUCTION Controlling the magnetic properties in low dimensional magnetic systems is significant for applications, such as magnetic data storage, memory, logic devices and sensors, as well as emerging technology concepts in spintronics and magnonics

1-3

. Employing spin current for

controlling magnetization in ferromagnetic (FM)/non-magnetic (NM) bilayer films is of substantial interest for low power consumption in spintronic devices. A number of approaches have been proposed and are being widely investigated to generate pure spin current, such as, electrical spin injection 4, spin Hall effect 5, spin Seebeck effect

6,7

and spin pumping

8, 9

. One

promising approach is to use spin-orbit interactions (SOI)s in heavy metal/FM bilayers which can produce strong current driven torques acting on the magnetic layer, via the spin Hall effect of the heavy metal or other interface spin-orbit effects such as the Rashba-Edelstein effect

10-15

.

In this context, it can be mentioned that the presence of Dzyaloshinskii-Moriya interaction 2


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(DMI) at the FM/heavy NM interfaces has been recently observed. Interfacial DMI interaction has found to occur as the inversion symmetry is broken at the interfaces with strong spin−orbit coupling

16, 17

. On the other hand, the spin pumping, the generation of spin current from

precessing magnetization in FM/NM bilayers carries special attention because this technique is non-local as well as no external stimulation is needed to flow the spin current. In the case of FM/NM bilayer thin films, the strong SOI of the heavy metal layer (NM) leads to increase in damping because of enhanced coupling of the electron spin with the lattice at the interface. This will facilitate the propagation and dissipation of transverse spin current generated by magnetization precession in the ferromagnetic layer. NM layers can therefore act as an absorber or sink for spin current pumped across the interface

18-20

. When considering the effect of spin

pumping on FM damping, it is useful to identify two limiting cases. If the FM is in contact with a material that strongly scatters or absorbs the pumped spin current, α increases substantially. In contrast, if the NM does not act as a spin sink, then pumped spins will accumulate in the NM and drive a diffusive spin current back toward the FM that cancels the spin pumping current in steady state. Generally, FM materials with low Gilbert damping are desired in spin-transfer torque magnetic random access memory and magnonic devices for low write-current and improved propagation of spin waves, respectively. In contrast, higher damping helps suppression of the magnetization precession during writing for data storage and memory devices 19. Nowadays a possible pathway towards controlling the spin transfer torques with 2D systems is given by the transition metal dichalcogenide (TMD) family of materials, which has attracted increased attention in the nanoelectronics community due to their layered structure, enabling 3



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readily the preparation of individual monolayers. However, much of the community’s interest in TMDs has been focused on transistor technology, while less work has been done with respect to their potential in applications in spintronics. Recently, few reports have been found on the study of the magnetization dynamics at NM/FM system by employing graphene as a NM layer

21-25

.

Graphene is a promising candidate for spintronics due to the possibility of long spin lifetimes arising from low intrinsic spin-orbit coupling (SOC) and weak hyperfine coupling

26, 27

.

Multiple layers of nearly decoupled two-dimensional graphene stacks to form the graphite layer 28

. Electronically, single layer graphene is a zero-gap semiconductor, while graphite is a

semimetal with slightly overlapping electron and hole bands 29. As both graphene and graphite is comprised by carbon atoms, the value of intrinsic SOC is very small. Similar value of intrinsic SOC for few layer graphene and graphite is deduced which is order magnitude higher than that of single layer graphene

30

. In this situation, their extrinsic SOC strength can play a

crucial role to control the magnetization dynamics at graphene stack/FM system spin injection and transport has been achieved in both graphene and graphite

27, 31-33

. The

21-25, 28, 34-37

. The

experimental spin-relaxation length (∼ 2 µm) in single-layer graphene at room temperature 26, 38 is still one order of magnitude below theoretical predictions

38, 39

. As the spin relaxation is

believed to be caused mostly by extrinsic scatters in the substrate and on the graphene surface 38, 40

, reduction of the effect of these scatters should lead to an increase in the spin-relaxation

length 38. In a stack of graphene layers, the scattering potentials are screened by the outer layers with a screening length of about 5 layers, depending on the stacking order. This reduces the effect of external scatters, resulting in only weakly influenced inner layers 41, 42. In other words, the effects of external scatters decrease with the number of graphene layers in a stack. 4


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Moreover, it can be mentioned here that mechanical exfoliation of graphite onto oxidized Si wafers is the most successful and easy-going method to prepare graphene samples

43, 44

. In this

method, the minimum number of graphene layers is transferred to the substrate is few-layered graphene (FLG). The FLG has already emerged as very promising for spintronic devices 21, 43, 45, 46

. A few studies have been reported on the magnetization dynamics using graphene as NM

layer in FM/NM system using ferromagnetic resonance (FMR) techniques

22, 24, 33, 47-50

. Since

the lateral dimensions of mechanically exfoliated graphene by scotch tape method has been found to few tens to 100 µm 51, it is non-trivial to characterize the dynamic magnetic properties of such samples by using nonlocal electrical pumping based FMR technique. In addition, the damping coefficient measured by FMR technique can have several extrinsic contributions such as spatial inhomogeneities of the sample and instrumental linewidth, which may further complicate the problem. To overcome this problem, we introduced an all-optical measurement of magnetization dynamics in such systems. Moreover, controlling magnetization dynamics with ultrafast laser pulse is significant due to its potential to switch the magnetization at a faster timescale supporting development of high speed devices. The modal composition of the magnetization oscillations can also be observed in the time-domain and the damping for each mode can be separately assessed. Besides, using time-resolved magneto-optical Kerr effect (TRMOKE) over FMR technique does not require any additional fabrication of waveguide on the sample. Here, we have investigated how the magnetization dynamics, including the damping behaviour varies as the underlayer of CoFeB thin film is changed from FLG to graphite by using TR-MOKE microscopy. In local optical excitation and detection based TRMOKE technique, the damping was measured directly in the time-domain within the highly-localized 5



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probe area (~1 µm2) due to focused laser spot. It excluded any inhomogeneities or variation due to larger area averaging and provided better estimates of damping 52, 53.

EXPERIMENTAL

a. Sample preparation i. Preparation of FLG and graphite films The FLG and graphite films are mechanically exfoliated from freshly cleaved highly oriented pyrolytic graphite (HOPG, grade ZYA, SPI supplies) and placed onto a thermally oxidized silicon [100] substrate with a 300-nm thick SiO2 (Si/SiO2). Before deposition, the Si/SiO2 substrate was cleaned by sonication in propanol and acetone for 5 min. each. ii. Deposition of CoFeB Thin Films The Co20Fe60B20 (CoFeB) thin films (nominal thickness ~2 nm) were grown by dc magnetron sputtering in high vacuum (base pressure ~2 × 10-7 Torr) on top of Si/SiO2, Si/SiO2/FLG and Si/SiO2/graphite. A capping layer of SiO2 (nominal thickness ~2 nm) was deposited onto the CoFeB layer by rf sputtering without breaking the vacuum of the chamber. CoFeB was grown using dc power of 28 Watt whereas the SiO2 was grown using rf power of 60 Watt at 13.56 MHz. The nominal thicknesses of the CoFeB and SiO2 films were calibrated by monitoring the evaporation rate with a quartz crystal microbalance. b. Sample characterization i. Optical Microscopy Imaging 6


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FLG and graphite films were viewed and identified by optical microscopy (Olympus DP12) from the difference in their optical contrasts. ii. AFM Imaging Surface morphology of Si/SiO2/FLG, Si/SiO2/graphite, Si/SiO2 as well as 2 nm thick CoFeB film deposited on these three substrates were studied by using a tapping mode atomic force microscope (AFM, Veeco) in air. The images were analysed by using WSxM software 54. Scans of different areas over several regions of the films were taken to check the consistency of the morphology of the samples. iii. Raman Spectroscopy The Raman scattering experiments were carried out using a micro-Raman set up comprising of a spectrometer (model Lab RAM HR, Jobin Yvon) and a Peltier-cooled charge-coupled-device (CCD) detector. An air-cooled argon ion (Ar+) laser with a wavelength of 488 nm was employed as the excitation light source, and a 100× objective with a numerical aperture (NA) of 0.9 was used to get the laser spot diameter of ∼0.7 µm. iv. TR-MOKE measurement The

ultrafast

magnetization

dynamics

was

measured

by

using

an

all-optical

time-resolved magneto optical Kerr effect (TR-MOKE) microscope based upon a collinear twocolour optical pump-probe technique 55. The second harmonic (λ = 400 nm, pulse width = 100 fs) of a mode locked Ti-Sapphire laser (Tsunami, Spectra physics) was used to pump the samples and the time-delayed fundamental laser beam (λ = 800 nm, pulse width = 80 fs) was 7



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used to probe the magnetization dynamics by measuring the magneto-optical Kerr rotation as a function of the time delay between the pump and the probe beams. Both the pump and the probe beams were made collinear and focused onto the central part of the sample surface through a microscope objective of numerical aperture (N.A.) of 0.65. The probe beam is tightly focused to a spot size of about 800 nm, while the pump beam is slightly defocused to a larger size of about 1 µm on the focal plane of the probe beam, respectively, which makes it easier to overlap the pump and probe beams on the sample surface. The probe beam is centred on the pump beam so that slight misalignment during the course of the experiment does not affect the pump-probe signals. A high precision piezoelectric scanning x-y-z stage is used for mounting the sample during the experiment to ensure precise positioning of the focused laser beams on the sample surface. A bias magnetic field was applied at a small angle (~10o) to the sample plane during the measurements; the in-plane component of which is defined as H during the measurements.

RESULTS AND DISCUSSIONS Figure 1 (a) shows the optical microscope image (at 50× magnification) of mechanically exfoliated Si/SiO2/FLG and Si/SiO2/graphite. The FLG and graphite can be identified from their optical contrasts. Further the thicknesses of FLG and graphite were measured by AFM technique (figure S1, Supporting Information). The thickness correspond to FLG is found to be about 5 nm (about 15 graphene layers) whereas the value is about 16 nm (about 48 graphene layers) for graphite, considering the thickness of a graphene monolayer to be 0.335 nm 21. The 8


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surface morphologies of Si/SiO2, Si/SiO2/FLG and Si/SiO2/graphite are displayed in figures 1(b)-(d). It can be observed that the bare SiO2 has a smooth surface. The Si/SiO2/FLG appears to have some wrinkles and crumples on the surface. It is known that a few layers of graphene can form wrinkles, ripples and crumples on its supporting substrate 43, 56-59. On the contrary, step edges on mechanically exfoliated graphite surface can be observed in the AFM images. The similar type of images for graphite surface has also been found in the literature 43, 56, 60. As FLG is more flexible than graphite, more defects appear on its surface during transfer as opposed to the graphite sample

56, 58

. The average roughness values of Si/SiO2, Si/SiO2/FLG and

Si/SiO2/graphite are found to be about 1.33 Ǻ, 3.26 Ǻ and 0.70 Ǻ, respectively. Furthermore, the surface topography of 2 nm thick sputter deposited CoFeB film onto Si/SiO2, Si/SiO2/FLG and Si/SiO2/graphite are shown in figures 1(e) - (g). It can be clearly observed from the figure that the CoFeB film on Si/SiO2 contains a grainy texture as was observed previously in the literature 61, 62. The CoFeB film on Si/SiO2/FLG shows highest roughness with irregular grains, while the CoFeB film on Si/SiO2/graphite shows very smooth surface with average rougness values for the three films are found to be about 0.27 Ǻ, 3.50 Ǻ and 0.16 Ǻ.

9



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Figure 1. (a) Optical microscope image (at 50× magnification) of mechanically exfoliated FLG film and graphite film on Si/SiO2; AFM images of (b) bare Si/SiO2, mechanically exfoliated (c) FLG, (d) graphite on Si/SiO2; 2 nm thick deposited CoFeB film onto (e) Si/SiO2, (f) Si/SiO2/FLG and (g) Si/SiO2/graphite. Moreover, the FLG and the graphite layer were also characterized by Raman spectroscopy. Raman spectra of FLG and graphite layer are displayed in figure S2, Supporting Information. Inset of the figure shows the Raman spectra of both samples in Raman shift regime: 400-800 cm1

. One can observe from the figure that the bulk (~1575 cm-1) and 2D peaks (~2710 cm-1) are

present for both FLG and graphite layer on Si/SiO2 substrate. However, defect peak appears only for FLG sample at about 1358 cm-1. The higher intensity ratio of 2D to bulk peak on FLG 10


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(I2D/IBulk=2.44) than graphite layer (I2D/IBulk= 1.14) infers that FLG is a few layer graphene stack and graphite layer is bulk one 63, 64. It appears that the few layer graphene may consist of about 712 graphene monolayers. In addition, the signal (~520 cm-1) of Si (substrate) 65 is visible only for FLG sample which further indicates that thickness of FLG film is much lower compared to that of the graphite film. Furthermore, the presence of defect peak in case of FLG sample suggests that FLG sample has more defects or in other words it is more inhomogeneous than the graphite layer. The study of ultrafast magnetization dynamics of these three samples have been carried out using a dual beam pump-probe technique based TR-MOKE experiment. Figure 2 shows the schematic diagram of the sample and the experimental geometry. The pump and the probe beams are spatially overlapped on the sample surface by the same microscope objective to locally excite and probe the magnetization dynamics from the same volume of the sample. In this measurement, the pump and probe beams are laterally moved and carefully placed on the three different regions of the sample surface containing Si/SiO2/CoFeB, Si/SiO2/FLG/CoFeB and Si/SiO2/graphite/CoFeB using the piezoelectric scanning stage with a feedback control.

11



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Figure 2. Schematic diagram of the samples and the experimental geometry for the TR-MOKE measurements. The raw experimental time-resolved Kerr rotation data can be divided into three distinct temporal regimes. First, an ultrafast demagnetization was observed within the initial hundreds of femtoseconds (fs), followed by a fast relaxation within few picoseconds (ps) and a slow relaxation within few hundreds of ps. The precession of magnetization is observed as a damped oscillatory signal superimposed on the slowly relaxing time-resolved Kerr rotation. The fast (τ1) and slow (τ2) relaxation times are extracted by fitting the post-demagnetization data with a biexponential decay function. Subsequently, a bi-exponential background was fitted to the decaying signal, and subtracted to isolate the precessional behaviour for analysing the data 55. First, we investigate the ultrafast demagnetization (τm) and subsequent fast relaxation (τ1) for all three samples. Figure 3 shows the time-resolved Kerr rotation data for the first few ps, revealing 12


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the ultrafast demagnetization and fast relaxation for all three samples at H = 1.38 kOe with pump fluence = 8.5 mJ/cm2.

Kerr rotation (m deg.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 -0.2 -0.4

CoFeB FLG/CoFeB Graphite/CoFeB

-0.6

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time delay (ps) Figure 3. Time-resolved Kerr rotation data showing ultrafast demagnetization and fast relaxation signals from Si/SiO2/CoFeB(2 nm), Si/SiO2/FLG/CoFeB(2 nm) and Si/SiO2/graphite/CoFeB(2 nm) with pump fluence = 8.5 mJ/cm2 (Symbols correspond to experimental data while the solid curves are fits to Eq. 1) . The experimental data in Fig. 3 are fitted with the expression 66, 67

−θ k = {[

( A τ − Aτ ) τ ( A − A2 ) −t /τ 1 A1 − 2 1 1 m e − t / tM − e 1 e ]H (t ) + A3δ (t )} ⊗ G (t ) 0.5 (t / τ 0 + 1) τ1 − τ m τ1 − τ m

(1) where the time resolution is accounted for laser profile by a Gaussian function G(t), which is convoluted with the fit function containing two exponentials with time constants τm and τ1 representing the demagnetization and the fast relaxation time, respectively. H(t) and δ(t) 13



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represent the Heaviside step function and the Dirac delta function, respectively. A1, A2 and A3 are the constants. The extracted values of fast relaxation time (τ1) for CoFeB, FLG/CoFeB and graphite /CoFeB are 1.47±0.07 ps, 1.50±0.08 ps and 0.36±0.02 ps, respectively. Furthermore, a smaller value of demagnetization time (τm) for graphite/CoFeB bilayer system was determined than that of bare CoFeB and FLG/CoFeB systems. The values of demagnetization time (τm) for bare CoFeB, FLG/CoFeB and graphite/CoFeB are 220±6 fs, 270±7 fs and 120±4 fs, respectively. It can be mentioned that the origin of femtosecond laser induced ultrafast demagnetization is explained in terms of different mechanism for different system, till date. However, any unified model is not available to generally explain the ultrafast demagnetization 66-71. It has commonly been proposed that there exists some ultrafast dissipation channel for the spin angular momentum as the spin angular momentum is a conserved quantity.

From simulation, the

researchers previously noted that few states may appear near the Fermi level for FM/graphene bilayer sample with respect to the bare FM sample

25, 72-75

. Possibly, in our case the interfacial

interaction was relatively stronger for graphite/CoFeB sample than FLG/CoFeB due to the lower surface roughness of graphite underlayer (as shown in figure 1). As a result of enhanced interaction strength in between graphite and CoFeB surfaces, more number of allowed states appear near the Fermi level. This offers a faster dissipation channel for the spin angular momentum in case of graphite/CoFeB system which may lead a faster demagnetization. The fast relaxation within a few ps time scale is generally determined by the energy transfer rates between different degrees of freedom, viz. spin, electron, and lattice 55. On the similar ground of faster demagnetization process, the rate of energy transfer becomes faster in case of graphite/CoFeB as compared to FLG/CoFeB and bare CoFeB samples. 14


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Next the time-resolved magnetization precession data for three different samples under a fixed external bias magnetic field of H = 1.38 kOe are shown in figure 4(a). A fast Fourier transform (FFT) with a Welch window function was performed on the background subtracted timeresolved Kerr rotation data to obtain the power spectrum, which is displayed in figure 4(b). A single frequency magnetization precession is observed for all three samples indicating the uniform precessional dynamics, and is analysed by using macrospin solution of Landau-LifshitzGilbert (LLG) equation as follows 55,

r dmˆ dmˆ = −γ (mˆ × H eff ) + α (mˆ × ) dt dt where γ is the gyromagnetic ratio and is related to magneto-mechanical constant by γ =

(2) g µB , h

µ B is the Bohr-magneton and h is the reduced Plank’s constant. Heff is the total effective magnetic field consisting bias magnetic field, exchange field, dipolar field, anisotropy field, etc. and α is the Gilbert damping coefficient. The first term on right hand side of the above equation represents the precessional torque, while second torque term corresponds to the damping torque.

15



0.2

(a)

0.1

2 f = 13.20 GHz

0.0 -0.1 -0.2 0.3

1

Power (arb. Unit)

Kerr rotation (mdeg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

α = 0.017±0.003

(b)

0.0 -0.3

Page 16 of 33

α = 0.035±0.005

0.2

(c)

0 0.6 0.4 f = 13.07 GHz 0.2 0.0 1.0

0.0

f = 13.13 GHz

0.5

-0.2

0.0

α = 0.008±0.001

0.5 1.0 Time (ns)

1.5

0.0

0

5

10 15 20 25 30 f (GHz)

Figure 4. Precessional oscillations in time-resolved Kerr rotation data (left panels symbols correspond to experimental data while the solid curves are fits to Eq. 2) and the corresponding FFT power spectra (right panels) for (a) Si/SiO2/CoFeB(2 nm), (b) Si/SiO2/FLG/CoFeB(2 nm) and (c) Si/SiO2/graphite/CoFeB(2 nm) at H = 1.38 kOe. The optical images of the samples are shown in the insets of the right panels. We further extracted the Gilbert damping coefficient α directly from the time-domain decay of the precessional oscillation by fitting the oscillatory Kerr rotation data with a general sine wave equation superimposed with exponential decay function given by 55,

M (t ) = M (0)sin(ωt − ϕ )e

−

t

τ

16


(3)

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where M(t) and M(0) are the precessional magnetization component at finite and zero time delay, respectively, ω is the angular frequency, ϕ is the initial phase of oscillation and τ is the relaxation time, which has an inverse relationship to α as given by α = 1 / 2π f τ . The value of Gilbert damping coefficient α is found to be 0.017±0.003 for Si/SiO2/CoFeB(2 nm) but it increases drastically by a factor of two (0.035±0.005) in Si/SiO2/FLG/CoFeB(2 nm). Surprisingly, α reduces drastically to 0.008±0.001 in Si/SiO2/graphite/CoFeB(2 nm) i.e., as the underlayer is varied from FLG to graphite. This large change in α with the thickness of the graphene underlayer clearly represents an efficient modulation of damping in absence of any external stimulation. In contrast, we did not observe any discernible variation in the precessional frequency (f) within the experimental accuracy as a function of the graphene layer thickness. Constant value of thickness of the CoFeB layer ensures no effective change in the static ferromagnetic properties, which is further confirmed by the unaltered precessional frequency for different samples under a fixed value of the magnetic field. The reproducibility of the results was confirmed by measuring the time-resolved Kerr rotation data from different locations of the samples and another typical set of such data is shown in in figure S3, Supporting Information. We have further measured the time-dependent Kerr rotation data for few different values of bias magnetic field for all three samples as shown in figure 4. While the precession frequency is found to increase with the bias magnetic field as expected from the Kittel formula 55,

f =

γ 2π

H ext ( H ext + 4π M s )

17


(4)


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The damping is found to be independent on the frequency. The latter signifies that the damping is of intrinsic origin 76.

Figure 5. Precessional oscillations in time-resolved Kerr rotations (Symbols correspond to experimental data while the solid curves are fits to Equation 2) at different bias field values H for (a) Si/SiO2/CoFeB(2 nm), (b) Si/SiO2/FLG/CoFeB(2 nm) and (c) Si/SiO2/graphite /CoFeB(2 nm). Finally, we focus to understand the origin behind the large variation in the damping (although there is no change in static magnetic properties) with the variation in the underlayer i.e., the number of graphene layers. The damping value of the CoFeB thin film without any underlayer is similar to what has been reported in the literature

77

. On the other hand, a large increase or

decrease in the value of damping due to the presence of FLG or graphite underlayer is found to be of intrinsic origin due to the independence of damping on the precession frequency. Recent reports

22, 33, 48

showed an enhanced α value in various ferromagnetic/graphene bilayer systems 18


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measured primarily by ferromagnetic resonance technique. Different mechanisms including spin pumping and two-magnon scattering due to the presence of defects has been proposed to explain this enhanced α value 55. In our case the damping is found to be intrinsic in nature, which rules out extrinsic mechanism like two magnon scattering

55, 76

. Hence the observed increase in the

Gilbert damping in FLG/CoFeB sample could be related to spin pumping from the ferromagnetic CoFeB layer into the FLG. Surprisingly, the observed increase in damping is even larger than that observed in heavy metal systems (e.g., Pt or Pd), which possess high spin-orbit coupling and are known to be among the best existing spin sink materials

53, 77-80

. In the case of graphene, the

observation of such a large efficiency in absorbing spins is in contrary to the general expectations as its spin-orbit interaction (SOI) strength is estimated to be very small. The intrinsic SOI of single layer defect free and flat grapheme (SLG) is almost negligible (about 1-50 µeV) 31, 32, 40. The intrinsic SOI in FLG and graphite layer is about one order of magnitude larger

than in SLG, due to mixing between the π and σ bands by interlayer hopping 30. However, a 20fold enhancement of the SOI has been observed in case of SLG on some substrates because when placed on a substrate SLG loses its flatness and acquires ripples and crumples

27, 30-33

, which

increases the extrinsic SOI. It has also been observed that this surface defect reduces with the number of SLG when transferred on a substrate 56, 58, 59. Our AFM images (figure 1) also support this observation and clearly show wrinkles and crumples formation (absent in Si/SiO2/graphite and Si/SiO2) on the surface of Si/SiO2/FLG. Surface roughness reduces systematically from Si/SiO2/FLG to Si/SiO2/graphite through Si/SiO2. This is imprinted on the CoFeB layer grown on these three surfaces and consequently the AFM images show highest roughness for Si/SiO2/FLG/CoFeB and lowest in Si/SiO2/graphite/CoFeB. This reveals the role of surface 19



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defects in enhancing extrinsic SOI in case of Si/SiO2/FLG/CoFeB as opposed to Si/SiO2/ graphite/CoFeB. Consequently, the Gilbert damping parameter α enhances drastically in Si/SiO2/FLG/CoFeB as opposed to Si/SiO2/graphite/CoFeB where the surface roughness is minimum. Our investigation reveals that the surface defects of underlayer play an important role for controlling the Gilbert damping in ferromagnetic/graphene bilayers with different thicknesses of graphene. A more rigorous and systematic variation of the number of single layer graphene in such bilayer system would be required to control of the damping parameter for applications in various spintronics and magnonic devices. It is interesting to note here that irrespective of the same origin, i.e., spin-orbit interaction strength for ultrafast demagnetization and Gilbert damping,

the

experimental

observations

indicate

anomalous

behaviour

of

ultrafast

demagnetization in FLG (graphite)/CoFeB bilayer system. We believe that these anomalous observations will trigger further investigations to unravel the mechanism involved ultrafast demagnetization in graphene/ferromagnet based systems.

CONCLUSION In conclusion, we measured the time-resolved ultrafast magnetization dynamics of CoFeB thin film without and with few-layer-graphene and Graphite underlayers for investigating the effects of graphene layer on the precessional frequency and damping of CoFeB layer. While the precession frequency is found to be independent on the underlayer, the damping showed a huge variation by a factor of more than four as the underlayer is varied from FLG (α ~0.035±0.005) to graphite (α ~0.008±0.001). We explained this huge variation of damping coefficient in terms of extrinsic spin orbit interaction of the underlayer. The FLG layer shows very large amounts of 20


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surface defects on the Si/SiO2 substrate, which reduces drastically in the graphite surface, which showed the smoothest surface even better than the Si/SiO2 substrate which got imprinted on the 2 nm thick CoFeB layer, which also showed highest roughness on FLG and lowest roughness on graphite. This variation of damping is much larger than the conventional method of variation of damping by using spin pumping and related phenomena using heavy metal underlayer. In addition, we noted a faster demagnetization time and fast relaxation time for graphite/CoFeB bilayer system than that of FLG/CoFeB. We conclude from our studied bilayer systems that interfacial spin physics is mostly governed by the growth of CoFeB layer on such 2D layered materials. Our findings suggest a new direction towards the control of damping and its application in high speed spintronic and magnonic devices.

ASSOCIATED CONTENT

*S Supporting Information 1) Estimates of thickness of FLG and graphite layer from AFM images. 2) Raman spectra of FLG and graphite layer. 3) An additional set of precessional oscillations in time-resolved Kerr rotations at bias field H=1.38 KOe for three studied cases to address the reproducibility issue.

AUTHOR INFORMATION

*Corresponding Author: [email protected] 21



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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

We gratefully acknowledge Mr. Tara Shankar Bhattacharya and Prof. Achintya Singha from Bose Institute, Kolkata for helping us in the Raman measurements. We also acknowledge the financial support from the Department of Science and Technology, Government of India (DST, Grant no. SR/NM/NS-09/2011(G)) and S. N. Bose National Centre for Basic Sciences (SNBNCBS), India (Grant no. SNB/AB/12-13/96). SP thanks DST for INSPIRE fellowship, while SC thanks SNBNCBS for senior research fellowship.

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