Abstract
We prepared magnetic thin films Ni81Fe19 on single-crystal Si(001) substrates via single graphene layer through magnetron sputtering for Ni81Fe19 and chemical vapor deposition for graphene. Structural investigation showed that crystal quality of Ni81Fe19 thin films was significantly improved with insertion of graphene layer compared with that directly grown on Si(001) substrate. Furthermore, saturation magnetization of Ni81Fe19/graphene/Si(001) heterostructure increased to 477 emu/cm3 with annealing temperature °C, which is much higher than values of Ni81Fe19/Si(001) heterostructures with ranging from 200°C to 400°C.
1. Introduction
Efficient and robust injection of spin polarized electrons into semiconductor channel and manipulating the injected spin electrons have been persistent problems for semiconductor-based spintronic devices [1–3]. Hanle effect curves from ferromagnetic metal into semiconductor channel by three-terminal geometry have been reported [4–7]; moreover, the spin valve signals from ferromagnetic metal into semiconductor channel were investigated by four-terminal geometry [8–10]; various insulator layers such as MgO, Al2O3, and SiO2 were inserted between ferromagnetic metal and semiconductor to solve the conductance mismatch problem [11–14]. Although clear Hanle signals were obtained for three-terminal geometry in ferromagnetism/insulator/semiconductor system, the high contact resistance area products resulting from the insulator layer were a serious issue [15].
Graphene was a potential candidate for an insulator tunneling barrier because (1) it exhibited poor conductivity perpendicular to the plane although it is very conductive in plane [16] and (2) it is a highly uniform, defect-free, and thermally stable layer. Cobas et al. reported that clear magnetoresistance curves were obtained for graphene-based magnetic tunneling junctions in which graphene works as a tunneling barrier [17]. van’t Erve et al. reported spin injection from NiFe thin film into Si channel via graphene by three-terminal geometry [18]. However, the previous studies were restricted to the electrical properties of NiFe/graphene/Si heterostructures. The structural and magnetic properties of NiFe thin film grown on Si(001) substrate via graphene were rarely investigated.
Given this background, the purpose of the present study has been to fabricate Ni81Fe19/graphene heterostructures on Si(001) substrates and investigate their structural and magnetic properties. For this purpose, we prepared 50-nm thick Ni81Fe19 thin films on Si(001) substrate via single graphene layer by magnetron sputtering with various annealing temperature, . We also prepared 50-nm thick Ni81Fe19 thin films directly grown on Si(001) substrates for comparison. The intensity of 111 peak and saturation magnetization significantly increased for Ni81Fe19 thin films compared with that directly grown on Si(001) substrate with °C, indicating the improvement of crystal structure. These results demonstrated that the structural and magnetic properties of Ni81Fe19 thin film can be improved with insertion of graphene between ferromagnetic metal and semiconductor.
This paper is organized as follows. Section 2 describes materials and methods. Section 3 presents our experimental results regarding structural and magnetic properties of Ni81Fe19/graphene/Si(001) heterostructure and discussion. Section 4 summarizes our results and concludes.
2. Materials and Methods
Two kinds of layer structures were fabricated: firstly, (from the substrate side) graphene/Ni81Fe19 (50 nm)/ (1 nm) on a Si(001) single-crystal substrate. Grapehene was fabricated by chemical vapor deposition (CVD) on copper foil; then the copper foil was cut into the size of 20 × 20 mm2. Photoresist was coated on the surface of the graphene in order to assist the following wet-transfer process and add the copper foil into ferric trichloride to completely etch the copper. Then the photoresist-coated graphene was physically transferred on the Si(001) substrate which was cleaned by hydrofluoric acid solution and the photoresist was removed by acetone. Finally the sample was washed by deionized water. The prepared graphene/Si(001) substrate was installed in a high-vacuum chamber with base pressure of 5.0 × 10−4 Pa; the 50-nm thick Ni81Fe19 thin films were deposited by magnetron sputtering at room temperature (RT) and then subsequently annealing in situ at temperature °C. Secondly, we also fabricated 50-nm thick Ni81Fe19 films directly grown on Si(001) at RT, which were annealed with ranging from 200°C to 400°C.
We investigated the structural properties of the graphene through ALMEGA Dispersive Raman spectrometer (ALMEGA-TM) with a wavelength of 532 nm. The surface morphologies were observed using atomic force microscopy. The structural properties of Ni81Fe19 thin films were investigated by X-ray diffraction (XRD) - scan. The magnetic properties of Ni81Fe19 thin films were investigated through vibrating sample magnetometer (VersaLab, Quantum Design) at RT.
3. Results and Discussion
3.1. Structural Properties of Ni81Fe19/Graphene/Si(001) Heterostructures
Firstly, we describe the structural properties of Ni81Fe19/graphene/Si(001) heterostructure. Figure 1 shows the Raman spectra of graphene on single-crystal Si(001) substrate. The typical and peak were observed at shift of 1596 cm−1 and 2685 cm−1, respectively. The peak intensity ratio of is about 1.6; the higher peak intensity ratio for peak compared with that of peak indicated the graphene was single layer. Furthermore, the full width at half maximum (FWHM) of peak was about 21 cm−1, which is very close to the 25 cm−1 for the perfect single graphene layer. Although peak was observed at shift of 1352 cm−1 which is related to the defect in the graphene layer, the peak intensity ratio which is appreciably low, indicating the prepared graphene was almost defect-free [19].
Figure 2 shows the surface morphologies of Ni81Fe19 thin film on Si(001) substrate via single graphene layer by atomic force microscopy (AFM) measurements, the 50-nm thick Ni81Fe19 thin film had sufficiently flat surface morphologies with root mean square (rms) roughness of 0.47 nm, the Ni81Fe19 film was deposited at RT and subsequently annealed at 400°C on Si(001) substrate via graphene layer. This rms value was almost similar to that Ni81Fe19 thin film directly grown on MgO-buffered MgO substrate with value of 0.31 nm at annealing temperature of 400°C.
Figure 3 shows X-ray diffraction patterns of 50-nm thick Ni81Fe19 films grown on Si(001) substrate via single graphene layer with annealing temperature of 400°C. The X-ray diffraction patterns of 50-nm thick Ni81Fe19 films directly grown on Si(001) were also shown for comparison, in which the Ni81Fe19 films were as-deposited and postdeposition annealed at temperature ranging from 200°C to 300°C. No appreciable peak was observed for the as-deposited Ni81Fe19 films, indicating that it was amorphous film. With increasing the annealing temperature, the 111 peak of Ni81Fe19 films appeared for up to 200°C. In this sense, the crystal structure of the as-deposited Ni81Fe19 films was improved by the annealing. With continuous increasing the to 300°C, 002 peak of Ni81Fe19 films was obtained and the intensity of 111 peak slightly increased compared with that for °C; however, the 002 peak of single-crystalline silicon substrate disappeared with °C, which was possibly due to the interdiffusion between Ni81Fe19 films and Si at relatively high annealing temperature. In order to confirm the supposition, the annealing temperature was increased up to 400°C. No appreciable 111 and 002 peaks of Ni81Fe19 films and 002 peak of silicon substrate were observed. The graphene was inserted between Ni81Fe19 films and Si(001) substrate as shown in Figure 3; the intensity of 111 and 002 peaks was significantly increased with °C and the interdiffusion between Ni81Fe19 films and Si(001) substrate was prevented; the crystal structure of Ni81Fe19 films was significantly improved by inserting graphene layer. Furthermore, the XRD patterns show that the heterostructure is composed of face centered cubic (fcc) structure, which is consistent with the fact that fcc structure is dominant in high Ni content Ni81Fe19 film [20].
3.2. Magnetic Properties of 50-nm Thick Ni81Fe19 Thin Films Grown on Si(001) Substrate via Single Graphene Layer
Next, we describe the magnetic properties of Ni81Fe19 thin films grown on Si(001) via graphene layer and that directly grown on Si(001) substrate. Figure 4(a) shows the magnetic hysteresis (-) curves of Ni81Fe19 thin films at 300 K. The - curve with °C was that Ni81Fe19 thin film grew on Si(001) substrate via graphene layer; the other two curves were those Ni81Fe19 thin films directly grown on Si(001) substrates. The magnetic field () was applied in the plane of the film along direction of silicon substrate. The saturation magnetization () was 310 emu/cm3 for Ni81Fe19 thin films deposited at RT without annealing; the slightly increased to 350 emu/cm3 with °C. The obtained values were close to those Ni81Fe19 thin films grown on Ta and Ti substrate [21]. With increasing up to 400°C, no appreciable - curve was observed for Ni81Fe19/Si(001) heterostructure, which was due to the nonnegligible interdiffusion between Ni81Fe19 thin film and Si(001) substrate. However, with insertion of the graphene between Ni81Fe19 films and Si(001) substrate, the significantly increased up to 477 emu/cm3 for °C, indicating that the magnetic properties of Ni81Fe19 thin film significantly improved.
(a)
(b)
Figure 4(b) shows the coercive force () for Ni81Fe19 thin film as a function of . The values at 300 K decreased with increasing from Oe for the as-deposited film to Oe for °C. The decrease in with increasing was probably induced by a decrease in the pinning center density for the magnetic domain motion with increasing up to 400°C [22].
4. Conclusion
We prepared Ni81Fe19 thin films grown on Si(001) substrate via graphene layer. First, the Raman spectra showed that the graphene was single layer and almost defect-free. Second, the X-ray pattern of Ni81Fe19/graphene/Si(001) heterostructure confirmed that the crystal quality of Ni81Fe19 thin films significantly increased with insertion of graphene single layer. Third, sufficiently flat surface morphologies were obtained for Ni81Fe19 thin films grown on Si(001) via graphene layer. Fourth, relatively higher saturation magnetization values at 300 K were obtained for Ni81Fe19 thin films grown on Si(001) via graphene layer. These results confirmed that Ni81Fe19/graphene heterostructure is a potential candidate for spin injection source for spin injection into semiconductor channel.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This work was partly supported by The Fundamental Research Funds for the Central Universities (Grant no. 3102014JCQ01059), China.