Journal of Magnetism and Magnetic Materials 432 (2017) 10–13
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Magnetic transport property of NiFe/WSe2/NiFe spin valve structure Kangkang Zhao a,b, Yanhui Xing a,⇑, Jun Han a, Jiafeng Feng c, Wenhua Shi b, Baoshun Zhang b, Zhongming Zeng b,⇑ a
Key Lab of Opto-electronics Technology, Ministry of Education, College of Electronic Information and Control Engineering, Beijing University of Technology, Beijing 100124, PR China Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), Suzhou 215123, PR China c Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences (CAS), Beijing 100190, PR China b
a r t i c l e
i n f o
Article history: Received 22 December 2016 Received in revised form 20 January 2017 Accepted 22 January 2017 Available online 26 January 2017 Keywords: WSe2 Spin valve Magnetoresistance Spintronics
a b s t r a c t Two-dimensional (2D) materials have been proposed as promising candidate for spintronic applications due to their atomic crystal structure and physical properties. Here, we introduce exfoliated few-layer tungsten diselenide (WSe2) as spacer in a Py/WSe2/Py vertical spin valve. In this junction, the WSe2 spacer exhibits metallic behavior. We observed negative magnetoresistance (MR) with a ratio of 1.1% at 4 K and 0.21% at 300 K. A general phenomenological analysis of the negative MR property is discussed. Our result is anticipated to be beneficial for future spintronic applications. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDs), have attracted great attention as building blocks for future electronic technologies due to their specific layered structures and novel physical properties [1–6]. Recently, 2D materials have also been demonstrated to have potential for applications in the field of spintronics. Graphene exhibits exceptional spin-transport characteristics such as long spin diffusion lengths owing to its low spin–orbit interaction [7,8]. Monolayer TMDs have both strong spin–orbit coupling and inversion symmetry breaking [9–12], which has stimulated ideas for novel spin–orbit torque devices . Thanks to their atomically thin, smooth, and chemically inert nature, 2D materials have offered new perspectives on interlayer in spin-valve or magnetic tunnel junctions (MTJs) [14,15], in which the most commonly used interlayer (or barrier) materials are Cu, MgO or Al2O3. However, these materials are difficult to control the thickness and uniformity at the monolayer scale. Many efforts have focused on exploiting 2D materials such as h-BN or MoS2 as an interlayer [16–18], but the observed magnetoresistance effects have been low. Therefore, there still is strong demand to search new materials for spintronic applications. WSe2 crystal, as an emblematic representative of the TMDs, has been demonstrated to be an appealing candidate material owing to ⇑ Corresponding authors. E-mail addresses: [email protected]
(Y. Xing), [email protected]
(Z. Zeng). http://dx.doi.org/10.1016/j.jmmm.2017.01.066 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
its exotic physical properties. For instances, WSe2 has a tunable bandgap which varies from 1.2 eV for bulk material to 1.65 eV for a single layer [4,19]. Moreover, WSe2 crystal also shows a weak interlayer coupling and wave function overlap because of weakly bound through van der Waals interactions of adjacent layers. In addition, in contrast to other transition metal sulfides, WSe2 has been predicted to have a giant spin splitting (475 ± 5 meV) in the valence band at the K/K0 point and a giant valence band splitting . Because of these unique properties, few-layer WSe2 could be an ideal material to study spin dependent properties. So far, there is no report on spin-dependent properties based on WSe2-based spin-valve structures. In this paper, we report the fabrication and characterization of WSe2-based spin-valve devices. Two Permalloy (Py, Ni0.8Fe0.2) electrodes are decoupled by thin film WSe2 spacer in spin-valve. The magnetoresistance curve indicates high and low resistance states dependent on the two NiFe electrode magnetization directions. The spin valve signals were detected and found to have negative magnetoresistance (MR) ratios of 0.21% at 300 K to 1.1% at 4 K. A general phenomenological analysis of the negative MR property is discussed. We also studied the magnetoresistance as function of bias current.
2. Experimental details The thin-layer WSe2 film was obtained by mechanically exfoliation from bulk WSe2 crystals with help of adhesive tape, then transferred onto a pre-patterned NiFe (30 nm) bar (20 lm length)
K. Zhao et al. / Journal of Magnetism and Magnetic Materials 432 (2017) 10–13
6 4 2
Intensity (a. u.)
Raman Shift (cm-1)
Fig. 1. (a) Schematic view of WSe2 spin valve consisting of two NiFe electrodes and WSe2 interlayer; (b) The measurement configuration of spin valve device. Magnetic field (H) is applied in plane and oblique to the ferromagnetic electrodes axis; (c) Upper Left: Optical micrograph of the WSe2 spin valve device; Upper Right: the AFM of the WSe2 spin valve; Lower: height profile of the AFM image; (d) Raman spectrum (532 nm wavelength) of 5.4 nm WSe2 film after transferred to the SiO2/Si substrate.
made by photo-lithography and deposition on a SiO2 (300 nm)/Si substrate. The device fabrication process was performed in three steps. Firstly, the WSe2 film was located and photographed by optical microscopy. Secondly, the magnetic top electrode was patterned using e-beam lithography. A standard lift-off procedure was used to obtain the magnetic electrodes after evaporating NiFe with a thickness of 70 nm. Thirdly, non-magnetic 100 nm-thickness Au electrodes were patterned by using e-beam lithography and lift-off process. A schematic of the WSe2 spin valve with NiFe electrodes and a WSe2 spacer is shown in Fig. 1(a). For magnetic transport measurement, the in-plane magnetic field was applied at 45° to the easy axis of the ferromagnetic electrodes. The resistances were performed with a 4-Probe measurement where the current goes through the junction area vertically, as shown in Fig. 1(b). One terminal of each electrode was connected to a constant current source, while the other was connected to voltmeter. This method of measurement can effectively reduce extra resistances, including contact resistance, and allow more accurate detection of real device resistance. The image in the upper left corner of Fig. 1(c) displays an optical micrograph of the complete spin valve device, while the image in the upper right corner of Fig. 1(c) shows the atomic force microscope (AFM) of the WSe2-based spin valve device, and the image in the lower shows the height profile of the WSe2-spacer, which is indicated by a white line in the AFM image. The height profile reveals a 5.4 nm thickness for the WSe2 interlayer. Fig. 1(d) shows the Raman spectrum of the measured WSe2 film, which shows typical signals of in-plane E12g and outof-plane A1g of WSe2. The frequencies of the A1g and E12g modes are so close that two modes overlap into a single peak around 250 cm1. This result proves the presence of WSe2 film in our structure, which is in accord with previous reports [20,21]. 3. Results and discussion We investigated the basic transport properties of the NiFe/ WSe2/NiFe spin-valve device. Fig. 2 (a) displays the voltage– current (I–V) curves for several temperatures and the linear behaviors indicate good Ohmic contact characteristics of the WSe2 and NiFe electrodes. The resistance is found to decrease with
reducing temperature as shown in Fig. 2(b), which exhibits a metallic behavior despite the semiconducting nature of WSe2. This phenomenon is similar to what has reported for the MoS2 spin valve device . This behavior can be explained by hybridization of magnetic atoms (Ni, Fe) and Se atoms. Because the distance between NiFe surface atoms and nearest Se atom is very small, strong bonding between WSe2 and NiFe allows a strong wavefunction overlap between W and Py states, similar to what was reported for MoS2 spin valve . The hybridization between the interface Se atoms and the Fe/Ni atoms is a short range interaction which takes place in the direct contacting area. Note that both the valence and conduction band of WSe2 mainly originate from W–d orbitals, and therefore surface Se atoms act as ligands and have a great importance for metallic behavior at the interface. Thus in a nanoflakes of WSe2, the most part covered by the metal shows metallic behavior. The magnetoresistance data for a representative NiFe/WSe2/NiFe spin valve at 300 K and 4 K are shown in Fig. 2(c). When a magnetic field is applied in-plane, the magnetizations of the NiFe electrodes reverse at fields corresponding to their respective coercivities with the top NiFe switching at a larger field than the bottom NiFe. Thus their magnetizations can be aligned either parallel or antiparallel, and two distinct resistance states are observed, i.e. a low resistance (RP) for parallel alignment and a high resistance (RAP) for is antiparallel alignment. The MR ratio (MR = (RAP RP)/RP) of the NiFe/WSe2/NiFe device is determined to be 0.21% at 300 K and 1.1% at 4 K. According to discussion in Refs. [16,22], the MR ratio of the device with a conducting interlayer (without considering the interface resistance) can be approximated as MR = 2P1P2/(1 P1P2), where P is the spin polarization of the NiFe electrode. Assuming that two NiFe electrodes have the same composition, then the polarization (P1 P2 = P) of the two NiFe electrode is estimated to be 7%, which is a little larger than what can be found in the literature , but smaller than that (P 0.3) in the NiFe/Al2O3 interface. The possible reason for the lower value obtained for the NiFe/WSe2/NiFe spin valve may be due to air exposure of our NiFe surface prior to application of the WSe2 layer. Future well-controlled fabrication process in situ without air exposure of the interfaces may improve the interface quality, maximizing the MR effect.
K. Zhao et al. / Journal of Magnetism and Magnetic Materials 432 (2017) 10–13
Fig. 2. (a) I–V curve of one typical NiFe/WSe2/NiFe spin valve at various temperatures; (b) Device resistance as a function of temperature; (c) Magnetic resistances as a function of magnetic field at 300 K and 4 K; (d) Phenomenological representation of negative magnetoresistance signal for NiFe/WSe2/NiFe spin valve.
Fig. 3. (a) The MR curves of one typical NiFe/WSe2/NiFe spin valve at various temperatures; (b) MR ratios as a function of temperature.
It is interesting to note that the NiFe/WSe2/NiFe spin valves exhibit negative MR behavior, which is different from the previous works . Negative MR was reported for a vertical single-layer graphene spin valve , which was explained by the absence of majority spin states of nickel and cobalt near the K point of graphene. Fig. 2(d) shows phenomenological representation of negative MR signal for NiFe/WSe2/NiFe spin valve. Only minority spins have a continuous transport channel through WSe2 to the ferromagnetic electrode, while majority spins have no direct conduction path, which may be the reason for negative MR of NiFe/ WSe2/NiFe spin valve. The detail physical mechanism behind this negative MR remains unknown and will require further research. Fig. 3(a) illustrates the MR curves of the NiFe/WSe2/NiFe spin valve at various temperatures. It can be seen that the conversion point of magnetoresistance become broader at low temperature, which is ascribe to an increased conversion field of FM electrode with weaker thermal agitation . The MR is found to increases with reducing the temperature as reported in the TMD-based spinvalves [16,17] and a magnetic tunnel junction (MTJ) . The decrease in MR amplitude at higher temperature may be attributed to the inelastic scattering with phonons, magnetic impurity scattering, surface states and thermal smearing of electron energy
distribution in the FM metals . We analyzed the temperature dependence of the MR ratio by fitting the data to the Bloch’s law, where the spin polarization is described by
PðTÞ ¼ P0 ð1 aT 3=2 Þ
By substituting P1 and P2 of Eq. (1) with P (T), the material dependent constant a can be estimated to be 8.1 105 K3/2. This value is comparable to that of 3–5 105 K3/2 reported in the literature . Finally, we study the bias current dependence of the spin-valve effect. Fig. 4(a) shows the MR curves at various bias currents. It is clear that the spin valve effect is present at each bias current. Although the MR signals trend of with respect to current somewhat distorted, it is evident that the magnitude of MR signal of the NiFe/WSe2/NiFe spin valve increases as the current decreases. The trend of change in magnetoresistance as a function of bias current, as shown in Fig. 4(b), is similar to what has been reported previously for WS2-based spin valve . We attribute this decrease in MR value at larger bias current to the spin excitations localized at the interfaces between the FM electrodes and the WSe2 interlayer  as well as the localized trap states in the WSe2 interlayer .
K. Zhao et al. / Journal of Magnetism and Magnetic Materials 432 (2017) 10–13
Fig. 4. (a) The MR curves as a function of magnetic field with different bias currents; (b) The MR ratios versus bias currents.
4. Conclusion In summary, we have fabricated a spin valve using WSe2 fewlayer as a spacer between NiFe electrodes. The devices exhibit spin-valve effect with a MR ratio of 1.1% at 4.2 K and 0.21% at room temperature. The magnitude of relative magnetoresistance decreases with increasing temperature and bias current. Both I–V curves and temperature dependence of the resistance suggest that the WSe2 layer in the NiFe/WSe2/NiFe spin valve behaves as a metallic conductor instead of an insulator. The results provide a possible approach to use the emerging TMD materials for future spintronic applications such as magnetic memory and magnetic sensors. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 61204011, 61574011, 11474311), the Natural Science Foundation of Beijing, China (Grant No. 4142005) and Natural Science Foundation of Jiangsu Province (Grant No. BK20130363). References  M. Osada, T. Sasaki, Two-dimensional dielectric nanosheets: novel nanoelectronics from nanocrystal building blocks, Adv. Mater. 24 (2012) 210–228.  K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669.  B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150.  Y. Zhang, M.M. Ugeda, C.H. Jin, S.F. Shi, A.J. Bradley, A. Martin-Recio, H. Ryu, J. Kim, S.J. Tang, Y. Kim, B. Zhou, C. Hwang, Y.L. Chen, F. Wang, M.F. Crommie, Z. Hussain, Z.X. Shen, S.K. Mo, Electronic structure, surface doping, and optical response in epitaxial WSe2 thin films, Nano Lett. 16 (2016) 2485–2491.  G.Z. Magda, J. Peto, G. Dobrik, C. Hwang, L.P. Biro, L. Tapaszto, Exfoliation of large-area transition metal chalcogenide single layers, Sci. Rep. 5 (2015) 14714.  Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Electronics and optoelectronics of two-dimensional transition metal dichalcogenides, Nat. Nanotechnol. 7 (2012) 699–712.  N. Tombros, C. Jozsa, M. Popinciuc, H.T. Jonkman, B.J. van Wees, Electronic spin transport and spin precession in single graphene layers at room temperature, Nature 448 (2007) U571–U574.  W. Han, W.H. Wang, K. Pi, K.M. McCreary, W. Bao, Y. Li, F. Miao, C.N. Lau, R.K. Kawakami, Electron-hole asymmetry of spin injection and transport in singlelayer graphene, Phys. Rev. Lett. 102 (2009) 137205.
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