Magnetic NiFe thin films composing MoS2 nanostructures for spintronic application

We demonstrate a nanostructure layer made of Ni80Fe20 (permalloy:Py) thin film conjugated MoS2 nano-flakes. Layers are made based on a single-step co-deposition of Py and MoS2 from a single solution where ionic Ni and Fe and MoS2 flakes co-exist. Synthesized thin films with MoS2 flakes show increasing coercivity and enhancement in magneto-optical Kerr effect. Ferromagnetic resonance linewidth as well as the damping parameter increaseed significantly compared to that of the Py layer due to the presence of MoS2. Raman spectroscopy and elemental mapping is used to show the quality of MoS2 within the Py thin film. Our synthesis method promises new opportunities for electrochemical production of functional spintronic-based devices.

Recent promising achievements in spintronics specially in magnetic thin films conjugated to two-dimensional (2D) materials has made this topic interesting for fundamental studies to explore their important role in the future spintronic-based memory and computing devices [1][2][3][4][5] . The core deriving fundamental phenomenon in such structures is the spin-orbit interaction (SOI) 6,7 . To benefit from SOI in spintronic devices, materials with high spin-orbit coupling (SOC), mostly heavy metals like Pt and Ta 8 are used in devices in contact to magnetic thin films. Also, due to the recent developments in the field of 2D materials, special focus is put into implementing 2D materials with their intriguing properties instead of those heavy metals 9,10 with high SOC. Many studies have demonstrated the successful usage of transition metal dichalcogenides (TMDCs) in contact to ferromagnetic thin films to enhance the SOI, induce surface anisotropy, etc. 11,12 . We have recently demonstrated that the magnetic anisotropy can be tuned by MoS 2 on the surface of Py thin films 13 and also predicted interfacial anisotropy can be changed in Co/black-phosphorene 14 . Here, we alternatively demonstrate the magnetic properties of Ni 80 Fe 20 change by embodiment of MoS 2 thin flakes. This shows the whole single ferromagnetic thin film to possess SOC-induced intrinsic magnetic properties.
Fabrication of thin films for spintronics devices based on physical techniques such as sputtering and thermal evaporation have shown the best performance 15,16 . Besides, electrodeposition method has established to be very promising in producing spin valves with very high number of layer repetitions (above 100 repeated layers 17 ) and also functional nanowires for spin caloritronic devices [18][19][20] . Although it should be mentioned that electrodeposition lacks the ability to provide ultra-thin films without voids or making multilayers of diverse types of materials in a single growth 21 . The implementation of 2D materials in contact to ferromagnetic thin films has been challenging 22,23 and such structures are made by transferring the as-made 2D layers on the ferromagnetic layers 24 . In addition to their multi-step fabrication method, the materials contacts are poor that hitherto limits their reproducibility and scalability 25 . Therefore, developing new fabrication method for making heterostructure of 2D materials/ferromagnetic layers is demanded to achieve higher yield and functionality.
In this work, we use electrodeposition method for fabrication of Py magnetic films and present the co-electrodeposition of MoS 2 thin flakes with ionic elements of the solution. The Raman spectroscopy indicates successful embodiment of thin MoS 2 flakes inside the grown magnetic film. The magnetic properties of the layer with MoS 2 flakes show prominent differences with bare ferromagnetic layer including higher magnetic coercivity and damping parameter which are directly related to the enhancement in SOC of the medium. Our results indicate that our fabrication method has resulted in a good proximity between the MoS 2 and the magnetic material for inducing SOC in the ferromagnet. Our method has the possibility of being used for growth of gradients or multilayers of the investigated material through control of the growth conditions like applied growth voltage/current.

Experimental section
Exfoliation of MoS 2 . Exfoliation of MoS 2 was done for a 1 g MoS 2 powder (Aldrich, 99%, < 2 μm) in 100 ml distilled water, equivalent to 10 g l −1 concentrations of MoS 2 (Fig. 1a). The MoS 2 powder was exfoliated for 4 h using a sonication probe unit equipped with a long step horn tip. In order to avoid excessive heating, the probe of sonic was set to operate 0.7 s and rest for 0.3 s and also an ice-water bath was used during the exfoliation. The resulting solution was centrifuged for 30 min at 200 rpm to remove non-exfoliated particles. For the electrodeposition of the Py conjugated MoS 2 sample (MoS 2 @Py), second solution was prepared as the following. First a 100 ml of the same solution with doubled molarities was prepared and then a 100 ml of the exfoliated MoS 2 solution was added to it. This way the final molarities of Ni and Fe ions is similar in both the solutions. Si substrates (single side polished surface) were cut by 1.5 × 1.5 cm slices. In order to clean surface of the Si from native oxide, they were dipped in 10% HF (hydrochloric acid) solution for 45 s, then washed by ethanol, acetone and distilled water respectively, and dried by air pump. Then the Si substrates were immediately transferred to the electrodeposition cell to prevent further surface oxidation. A two-electrode cell configuration with a DC current source was used for the electrodepositions. A 2 × 1 cm platinized Si was used as the anode and Si substrate as the cathode. The Py and MoS 2 @Py samples were electrodeposited by applying a direct voltage of 10 V at room temperature during 120 and 150 s, respectively.

Electrodeposition. Electrodeposition
Characterization. UV-Visible (Perkin Elmer, Lambda25) and Raman spectroscopy (Teksan) were carried out at room temperature. Surface was probed via atomic force microscopy (AFM, nanosurf) measurement. Energy dispersive X-ray spectroscopy (EDX) and elemental mapping were measured by through field emission scanning electron microscope (FESEM, Hitachi). Magnetic hysteresis loops were measured by longitudinal magneto-optical Kerr effect (MOKE), with a 632 nm laser light (a home-made setup). Ferromagnetic resonance

Results
Schematic of the exfoliation condition is depicted in Fig. 1a where the force of water molecules results in exfoliation of the MoS 2 powder into thin flakes. To characterize the quality of the exfoliated MoS 2 in water, UV-Visible absorption measurement was used. Result of this measurements can be seen in Fig. 1b. The A and B peaks at 559 and 663 nm respectively are the characteristic of few layer MoS 2 dispersions. After solution preparation and electrodeposition of the layers (Fig. 1a below) Raman characterization is used to see if the MoS 2 flakes are imbedded in the body of the Py layer. Figure 1c presents the Raman spectrum for Py and MoS 2 @Py samples. Raman peaks at 379 and 403 cm −1 clearly show the presence of MoS 2 flakes in the electrodeposited layer 26 . The bare Py sample shows no peak in its Raman spectrum because it has a metallic nature. The surface topography of the Py and MoS 2 @Py samples has been observed with AFM and presented in Fig. 2a and b, respectively. The AFM images show that both samples have a similar surface structure with an increased mean surface roughness for the MoS 2 @Py sample to 45 nm from the 20 nm mean surface roughness of the Py sample. Also, FESEM characterization of the Py and MoS 2 @Py samples has been performed and results are presented in Fig. 2c and d, respectively. It can be seen that both the samples have a granular structure with an increased grain size for the MoS 2 @Py sample. Also cross sectional FESEM images of the samples are presented in supporting information which show thicknesses of ~ 50 and ~ 100 nm for Py and MoS 2 @Py samples (~ 10% error), respectively. The observed higher thickness of the MoS 2 @Py samples is related to the partial space occupation by MoS 2 and also the slightly higher electrodeposition time of this sample. Distribution of MoS 2 in Py layer has been evidenced by EDX measurement. Figure 2e-h represents the EDX mapping of Ni, Fe, Mo and S elements where the uniform color distribution shows the uniform embodiment of MoS 2 . Also, the atomic ratio of Ni:Fe is 4:1.
In continue, magnetic properties of the samples are investigated through the MOKE and FMR measurements. Longitudinal MOKE measurement results are presented in Fig. 3a showing that both the samples have an inplane magnetic anisotropy. Two prominent differences are appeared in the MOKE signal of the samples. One is the much higher coercivity (H c ) of the MoS 2 @Py sample which is depicted in Fig. 3b. The H c for the Py sample is ~ 10 Oe and has increased to ~ 30 Oe for the MoS 2 @Py sample which is equivalent to a 300% increase. In the case of films, the magnetic anisotropy of ferromagnetic layers has been demonstrated to change by proximity to TMDC layered materials due to the d-d hybridization at the interface 11,13,14 . In the case of MoS 2 @Py sample, all interfacial directions between Py and MoS 2 is possible which overall has resulted in the observed in-plane coercivity change. Generally, magnetic anisotropy is highly dependent on the SOC 27-29 and by addition of MoS 2 as a material with high SOC to the layer, changes in the magnetic anisotropy is expected. One should note that increase of the in-plane coercivity can result from the increase of thickness because of the emerging out of plane magnetic anisotropy component at higher thicknesses [30][31][32][33][34] . To see if the observed increase of H c in our samples is due to the relatively higher thickness of the MoS 2 @Py sample we performed MOKE measurements for different thicknesses of Py layer and only a slight change in the H c was observed (for the details see supporting information). Therefore, we conclude that the observed changes in the magnetic properties of Py is due to its proximity to MoS 2 . Also, the H c of the samples can be affected by the grain size 35 and the AFM and SEM images indicate a comparably bigger grain size for the MoS 2 @Py sample. But the totally different MOKE signal quality of the MoS 2 @Py sample, including the slope of the plot, indicates that presence of MoS 2 is playing a crucial role in this increased H c .
The other observation from the MOKE signal is the much higher signal intensity of the MoS 2 @Py sample relative to Py sample which is increased by 275%. Two mechanism can play role in the observed MOKE signal increase for the MoS 2 @Py sample: (1) increase of the saturation magnetization (M s ) and (2) increase of light interaction with matter 36,37 . Here the increase of the MOKE signal cannot be due to the increase of M S , as the M S decreases for the MoS 2 @Py sample (See FMR section). Therefore, increase of the MOKE signal can be related to the increased interaction between light and the MoS 2 @Py sample. For the case of multilayered ferromagnet/ TMDC heterostructures it has been reported that proper thickness, refraction index and incident angle, can form a cavity 38,39 . Here we achieved enhancement of the MOKE signal via composing MoS 2 flakes with the magnetic layer. Enhancement of light-mater interaction also has been achieved via electrophoretic deposition of MoS 2 nanostructures 40 . Simulation of the MOKE signal for the composed MoS 2 @Py sample are encouraged to obtain the optimized composing structure. Also, we do not ignore the possibility of the larger grain size in the MoS 2 @Py being responsible partially for the observed MOKE signal intensity increase. For determining the exact contribution of each parameter further experiments should be designed.
We have also measured the FMR characteristics of the samples to see how MoS 2 can affect the magnetization dynamics of the Py layer. The spin dynamic response of the samples investigated at different constant microwave frequencies ranging from 2 to 20 GHz and by sweeping H from 0 to 4500 Oe. The observed FMR spectra were fitted with the derivative of Lorentzian function to determine the resonance field (H r ) and FWHM (∆H) at each frequency. Figure 3c presents the measured FMR signal of the samples (dots) and their fit (solid line) at f = 14 GHz. The frequency dependence of H r for the Py and MoS 2 @Py samples can be seen in Fig. 3d (dots). and fitting the data we can calculate the damping parameter of the samples. Here ΔH 0 is the inhomogeneous broadening, and α is the Gilbert damping parameter. The fitting of the data based on this equation can be seen in Fig. 3e. The value of ΔH 0 for Py and MoS2@Py samples are 114 and 231 Oe respectively. All the obtained parameters from the FMR data are presented in Table 1.
In the obtained results, we see a doubling of the damping coefficient in the MoS 2 @Py sample compared to the Py sample, which has increased from 0.018 to 0.023. There are many reports that show the coating of a nonmagnetic layer on a ferromagnetic layer can lead to the enhancement of the damping parameter [43][44][45] . Several mechanisms have been proposed for the enhancement of damping parameter in such bilayers. SOC and interfacial d-d hybridization cause the enhancement of the intrinsic damping, while extrinsic enhancement of the damping can arise from two-magnon scattering processes, due to roughness and defects at the interface region [46][47][48] . In the case of our sample, embodiment of MoS 2 in the Py layer results in the increased interfaces  www.nature.com/scientificreports/ between Py and MoS 2 and therefore both intrinsic and extrinsic contribution can contribute in the observed increase of the damping parameter. Moreover, the coupling between a FM layer and an adjacent NM layer can enhance the effective damping of the magnetization precession via spin-pumping effect 44,49 . For example, many groups have recently demonstrated the generation of spin-orbit torque in devices made with the Py/WTe 2 50 , CoFeB/MoS 2 or WSe 2 51 , and Py/MoS 2 52 and the reciprocal process (voltage generation from spin-pumping) in MoS 2 /Al/Co heterostructures 53 . In the case of our MoS 2 @Py sample we observe the phenomena via enhanced damping parameter in a relatively thick magnetic layer thanks to the embodiment of the MoS 2 flakes inside the layer which gives a high contact surface area between MoS 2 and Py.

Conclusions
MoS 2 was successfully embodied within the structure of the Py magnetic thin film by electrodeposition method. The layer with MoS 2 flakes shows a higher magnetic coercivity and Gilbert damping parameter, indicating the proper bonding between the MoS 2 and the magnetic material. In addition, the cavity of light in the MoS 2 @Py sample resulted in a three-fold increase of the MOKE signal which opens a pathway for the research on the optimization of MOKE sensors and also fundamental studies in the field. Due to the capability of applying our method for a large set of ferromagnetic/TMDC materials, there is a great potential for further development of functional spintronic and magnonic devices.