Observation of High Spin-to-Charge Conversion by Sputtered Bismuth

Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/NanoLett

Observation of High Spin-to-Charge Conversion by Sputtered Bismuth Selenide Thin Films at Room Temperature Mahendra DC,†,§ Jun-Yang Chen,‡,§ Thomas Peterson,† Protuysh Sahu,† Bin Ma,‡ Naser Mousavi,‡ Ramesh Harjani,‡ and Jian-Ping Wang*,†,‡ †

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States

‡

Downloaded via BUFFALO STATE on July 21, 2019 at 12:33:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: We investigated spin-to-charge conversion in sputtered Bi43Se57/Co20Fe60B20 heterostructures with in-plane magnetization at room temperature. High spin-to-charge conversion voltage signals have been observed at room temperature. The transmission electron microscope images show that the sputtered bismuth selenide thin films are nanogranular in structure. The spin-pumping voltage decreases with an increase in the size of the grains. The inverse Edelstein effect length (λIEE) is estimated to be as large as 0.32 nm. The large λIEE is due to the spin-momentum locking and is further enhanced by quantum confinement in the nanosized grains of the sputtered bismuth selenide films. We also investigated the effect on spin-pumping voltage due to the insertion of layers of MgO and Ag. The MgO insertion layer has almost completely suppressed the spin-pumping voltage, whereas the Ag insertion layer has enhanced the λIEE by 43%. KEYWORDS: spin-to-charge conversion, topological insulator, quantum confinement effect, granular bismuth selenide, inverse Edelstein effect

T

conversion has been observed at room temperature by the GBS thin films. The spin-pumping voltage shows thickness dependence of the GBS layer. At room temperature, the figure of merit of spin-to-charge conversion, the inverse Edelstein effect length (λIEE),5,32 is estimated to be as large as 0.32 nm. We have also studied the influence of an insertion layer of MgO and Ag on the spin-pumping voltage. The MgO insertion layer blocks the spin transfer from the CoFeB to the GBS layer, whereas the Ag insertion layer enhances the spin-to-charge conversion voltage by nearly 40%. The enhancement of the spin-to-charge conversion by the insertion of a Ag layer is due to the additional spin-momentum locking originated from the Rashba−Edelstein effect.41,42 The thin films with the stack structure Si/SiO2/MgO (2 nm)/GBS (2, 4, 6, 8, 12, and 16 nm)/CoFeB (5 nm)/MgO (2 nm)/Ta (2 nm) were prepared by magnetron sputtering for the spin-pumping experiments. Unless otherwise stated, the labeling BS2-BS16 will be used for the samples with thickness of the GBS layer ranging from 2 to 16 nm, respectively. Roomtemperature magnetron sputtering was carried out in an ultrahigh vacuum (UHV) six-target Shamrock sputtering system with a base pressure of 5.0 × 10−8 Torr. A composite

opological insulators (TIs) have drawn a great deal of attention recently due to the efficient conversion of charge-to-spin and vice versa.1−7 The main mechanism behind the efficient interconversion between charge and spin is the spin-momentum locking in TIs due to spin−orbit coupling.1,8−11 Additionally, quantum confinement enhances the conversion of charge-to-spin in nanosized granular TIs.4 Furthermore, TIs have shown promise for potential applications by being able to switch magnetization at room temperature via spin−orbit torque (SOT).4,12−15 The efficient SOT switching of the magnetization can be utilized in spin− orbit torque magnetoresistive random access memory (SOTMRAM).16,17 Recently, a logic device known as magnetoelectric spin−orbit (MESO) has been proposed, which uses spin-to-charge conversion for the reading of data bits.18,19 The mechanisms that yield spin-to-charge conversion are the inverse spin Hall effect (ISHE) in heavy metals,20−22 the inverse Edelstein effect (IEE) in crystalline TIs3,23−31 grown by molecular beam epitaxy (MBE), and the inverse Rashba− Edelstein effect (IREE) in interfaces such as Ag/Bi,32−34 Fe/ Ge,35 Cu/Bi2O3,36 LAO/STO,37,38 and in two-dimensional materials.39,40 However, the spin-to-charge conversion by granular TIs in which quantum confinement contributes to the spin-momentum locking has yet to be realized.4 In this report, we present spin-pumping results from sputtered granular bismuth selenide (GBS)/CoFeB heterostructures at room temperature. Robust spin-to-charge © XXXX American Chemical Society

Received: December 16, 2018 Revised: June 5, 2019

A

DOI: 10.1021/acs.nanolett.8b05011 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters

Figure 1. (a) XPS spectra of Si/SiO2/GBS (8 nm) film. (b) High-resolution 4f bands of Bi and (c) 3d bands of Se, respectively. (d) Raman spectrum of 16 nm-thick GBS film.

previous reports.43,44 The reported values of binding energies of elemental Se 3d5/2 and 3d3/2 are 54.5 and 55.3 eV, respectively. Our GBS film shows Se red-shift by 0.34 eV. The atomic concentration of Bi and Se in our GBS film is obtained by estimating the area under Bi 4f and Se 4d levels, which is determined to be Bi: (42.81 ± 0.52)% and Se: (57.2 ± 0.70)%, respectively. This is very close to the stoichiometry of target Bi2Se3. The Raman spectrum of 16 nm thick GBS film is presented in Figure 1d. The peaks observed at 123 and 167 cm−1 correspond to E2g and A21g, respectively. These values agree well with previously calculated and experimental values of Bi2Se3 films.45−47 A11g mode is absent in the spectrum due to the use of filter. Thus, it is confirmed by Raman spectroscopy that Bi and Se form an alloy not individual clusters. The Raman measurement was performed using laser of wavelength 532 nm. The other peak present in the Raman spectrum at 520 cm−1 corresponds to Si substrate. The cross-sectional highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of BS8 sample is

target of Bi2Se3 was sputtered at 30 W dc power and at 3 mTorr Ar pressure, resulting in a deposition rate of ∼0.6 Å/s. The MgO layer was rf sputtered at a deposition rate of ∼0.07 Å/s, whereas the metallic layers Pt, Ta, Ag, and CoFeB were dc sputtered at 3 mTorr Ar pressure. We performed X-ray photoelectron spectroscopy (XPS) measurement on BS8 sample using Al Kα X-ray source. XPS spectra of GBS film is presented in Figure 1a. Figure 1b shows high-resolution Bi 4f spectra. At binding energies 156.95 and 162.22 eV, Bi 4f7/2 and 4f3/2 peaks are observed, respectively. The spin−orbit splitting of 5.27 eV of Bi 4f levels is comparable to the previous report.43 The reported values of binding energies of metallic Bi 4f7/2 and 4f3/2 are 156.6 and 161.9 eV, respectively. The shift compared to metallic Bi 4f with our GBS film is 0.35 eV (blueshift). In Figure 1c, high-resolution Se 3d spectra are presented. At binding energies 54.16 and 54.90 eV, Se 3d5/2 and 3d3/2 levels are seen, respectively. The spin−orbit splitting in the case of Se 3d is determined to be 0.74 eV. The binding energies and splitting of Se 4d levels are also comparable to the B


Letter

Nano Letters

Figure 2. Spin-pumping into GBS film and structural characterization. (a) Schematics of the experimental set up for the spin-to-charge conversion. (b) TEM image of the BS8 sample.

Figure 3. Conversion of spin-to-charge current by spin-pumping in to sputtered GBS thin films. (a) The spin-pumping voltage measured in BS2 sample at 9 GHz excitation frequency. (b) The spin-pumping voltage in BS2 sample as a function of the excitation frequency. (c) The spin-tocharge conversion current as a function of the GBS film thickness at 9 GHz excitation frequency and 2.0 V excitation amplitude. (d) The spinpumping voltage as a function of the excitation amplitude for different samples at 9 GHz excitation frequency.

thickness of ∼50 nm was deposited to insulate the CoFeB layer from the waveguide. The shorted coplanar wave-guides and contacts were patterned using lithography and Ti (10 nm)/Au (150 nm) was deposited using an e-beam evaporator. The spin-pumping measurements were performed on the symmetric waveguide with the signal line width of 75 μm, ground width of 225 μm, and separation between the ground and signal line of 37.5 μm. These spin-pumping devices are similar to our previous reports.3,48 The schematics of the spin-pumping into the GBS layer is shown in Figure 2a. The rf field induces a precessional magnetization of the CoFeB layer at a fixed frequency in the GHz range. At resonance, the CoFeB layer pumps spin into the

presented in Figure 2b (details in Supporting Information S3). The CoFeB and MgO layers are amorphous, as expected, whereas the GBS layer has a polycrystalline structure, and the atomic layers of Bi and Se are continuous within grains. The average grain size in the BS8 sample is approximately 18 nm wide and 8 nm high, which is consistent with our previous report.4 The grain size decreases with the decrease in the thickness of the GBS layer.4 Moreover, TEM image shows that the interface between GBS and CoFeB is sharp, which is necessary for efficient spin-pumping. The thin-film samples were patterned into rectangular strips with a width and length of 620 and 1500 μm, respectively, using photolithography and ion milling. Then SiO2 with a C


Letter

Nano Letters

Figure 4. Characterization of spin-injection efficiency and inverse Edelstein effect length. (a) Excitation frequency as a function of resonance field. (b) The line-width as a function of the excitation frequency. (c) Spin-mixing conductance (right axis) and damping constant (left axis) versus GBS film thickness. (d) Inverse Edelstein effect length as a function of the GBS film thickness.

VIEE shows parabolic behavior with the excitation amplitude consistent with previous reports.3 f as a function of the H0 is presented in Figure 4a for different BS samples. The data points are fitted to the Kittel formula, γ f = 2π H0(H0 + 4πMeff ) , where γ is the gyromagnetic ratio to extract Meff. The ΔH as a function of the excitation frequency is presented in Figure 4b. The damping constant (α) 4π is obtained by fitting the ΔH = Δ0 + 3 γ αf relation to the

GBS layer, and the spin-momentum locking present in the GBS layer creates a non-equilibrium charge accumulation.5,49 The open circuit voltage is probed by using a nanovoltmeter. Figure 3a shows the measured open-circuit voltage (V) as a function of the external magnetic field (Hext) at an excitation frequency (f) of 9 GHz and an excitation amplitude of 2.0 V (∼19.03 dBm). V can be attributed to the contribution from the IEE effect (VIEE), Seebeck effect (VSE), and anomalous Hall effect (VAHE) or anomalous magnetoresistance (VAMR). The VIEE and VSE can be separated from VAHE by fitting the experimental data to the symmetric and antisymmetric Lorentzian provided below in eq 1: V=

VSΔH2 2

2

ΔH + (Hext − H0)

+

ΔH versus f data, where Δ0 corresponds to the inhomogeneities present in the CoFeB layer. The α as a function of the different BS samples is presented in Figure 4c (left y-axis). The enhancement in the α of the BS samples as compared to the control sample: Si/SiO2/CoFeB (details in Supporting Information S2) value of α (0.003) corresponds to the spinto-charge conversion.51 The spin-current density (JS) injected from the CoFeB layer to the CoFeB/GBS interface is given by

VA(Hext − H0) ΔH(ΔH2 + (Hext − H0)2 ) (1)

where ΔH is the line-width, which is also extracted by fitting, and Hext is the external dc magnetic field. The VIEE and VSE corresponds to the coefficient of symmetric component (VS) and the coefficient of antisymmetric component (VA) corresponds to (VAHE), respectively. Then the VIEE and VSE can be separated by using VIEE = (VS(+ H0) − VS(− H0))/2 and VSE = (VS(+ H0) + VS(− H0))/2, respectively. At a positive and negative resonance field (±H0), VS changes sign, which corresponds to the change in the spin direction. As expected, VA does not change sign at ± H0. Figure 3b shows V as a function of the excitation frequency at constant excitation amplitude for the BS2 sample. Increase in V as the excitation frequency decreases is consistent with the previous report.50 VIEE normalized by the device resistance (R) as a function of the GBS film thickness is presented in Figure 3c. VIEE/R decreases with increase in thickness of the GBS films. VIEE as a function of the excitation amplitude is presented in Figure 3d.

JS =

g↑↓γ 2hrf2 ℏ (MSγ + 8πα 2

(MSγ )2 + 4ω 2 ) 2e ℏ (MSγ )2 + 4ω 2

(2)

where MS, ω(2πf), hrf, g↑↓, ℏ, and eare saturation magnetization of CoFeB, excitation frequency, microwave rf field, spin-mixing conductivity, Planck’s constant, and electronic charge, respectively. The spin-mixing conductivity is given by g↑↓ =

4πMSt FM (α − αint) gμ B

(3)

where g, μB, tFM, and αint are Landé’s g-factor, Bohr magneton, CoFeB layer thickness, and intrinsic value of the CoFeB damping constant, respectively. The g↑↓ obtained by using eq 3 is presented in right axis of Figure 4c as a function of BS D


Letter

Nano Letters

Figure 5. Influence of interfacial layers on spin-to-charge current conversion. (a) The output dc voltage due to spin-to-charge current conversion in the BS4 sample. (b, c) Influence on the spin-pumping voltage due to the insertion layers MgO (1 nm) and Ag (2 nm), respectively. (d) The spinpumping voltage as a function of the excitation amplitude in BS-Ag sample. (e) The line-width as a function of the excitation frequency.

states of BS4 sample using relation29,32,49 λIEE = vfτm is determined to be 9.21 × 10−17 S, which is an order of magnitude shorter compared to the previous reports on TIs and Rashba interfaces29,37,53 but comparable to Cu/Bi54 and YIG/GBS55 interfaces. We also performed spin-to-charge conversion in a reference Pt sample (10 nm)/CoFeB (5 nm)/MgO (2 nm)/Ta (2 nm). At 9 GHz, JS, g↑↓, and spin Hall angle of Pt sample are determined to be 5.02 × 106 A/m2, 1.4 × 1019 m−2, and 0.1, respectively (details in Supporting Information S4). The spin Hall angle of Pt 0.1 is comparable to the previous reports.56−59 In addition to the Seebeck effect due to the microwave heating, thermoelectric signals induced by Nernst and anomalous Nernst effects can contaminate the spin-pumping signal31,60,61(details in Supporting Information S5). The spinpumping signal can be suppressed by the insertion of a barrier layer such as MgO between the FM and the spin-sink if the spin-to-charge conversion is due to a physical mechanism,35,62 otherwise if the voltage is coming from the thermal artifacts, it will remain almost the same. To identify whether the VS is

samples. The g↑↓ values estimated for the BS samples are comparable to the previous report.27 At the 9 GHz excitation frequency and the 2.0 V excitation amplitude the hrf is estimated to be 0.95 Oe. The JS of the samples BS2-BS16, obtained by using eq 2, is 4.24, 4.85, 3.57, 4.77, 4.06, and, 4.84 × 106 A/m2, respectively. The efficiency of the spin-to-charge conversion is given by 5 , 3 2 λIEE =

JC JS

=

VIEE , RwJS

w is width of the device. VIEE, R, and w for

the BS2 sample are 152 μV, 183 Ω, and 620 μm, respectively. The λIEE for the BS2 sample is estimated to be 0.32 nm. This value of λIEE is comparable or better than the previously reported values in TIs and interfaces.25,29,32,52 The λIEE as a function of the GBS is presented in Figure 4d. The λIEE shows the thickness dependence, which is opposite to the trend shown by crystalline Bi2Se3 (ref 27). The Fermi velocity (vf) of GBS (4 nm) is estimated to be 1.52 × 106 m/s by using the relation vf = ℏ (3π2n)1/3 where m* is 0.15 times the electronic m*

mass. The momentum relaxation time (τm) in the surface E


Letter

Nano Letters

with the hyperbolic tangent behavior. Furthermore, the λIEE estimated in sputtered GBS films is comparable to or larger than the reported values of it in other TIs (refs 3, 6, 24−27, and 29) and in interfaces such as Ag/Bi32 and Fe/Ge35 with mainly IEE and IREE being their origin of spin-to-charge conversion, respectively. With the overall comparable or larger value of λIEE, g↑↓ of GBS film thickness dependence agreeing with the IEE mediated spin-to-charge conversion rather than the heavy metals, and λIEE not showing hyperbolic tangent behavior, we can conclude experimentally that the observed high spin-to-charge conversion voltage present in the sputtered GBS samples is mainly due to the IEE. In addition, with substantial reduction of VIEE in the BS-MgO sample, any thermal related voltage is safely ignored. In our previous report, the high charge-to-spin conversion in sputtered GBS is observed, which is due to the spin-momentum locking.4 Furthermore, the sub-10 nm-sized grains present in the sputtered GBS films further enhances in the conversion of charge-to-spin due to the additional bands arising from quantum confinement.4 The figure of merits for both the charge-to-spin conversion and the spin-to-charge conversion show similar thickness-dependent behavior. The increase in thickness of the GBS films corresponds to the increase in the grain size and hence the decrease of the quantum confinement effect. The quantum confinement influences in spin-to-charge conversion as well, which can be seen in the λIEE as a function of GBS thickness in our case versus in YIG/Bi2Se3 showing opposite behavior.27 Not only in case of our GBS but also in other reports, grain dimensions have affected the spin-tocharge conversion as well as charge-to-spin conversion.41,67,68 We observed high spin-to-charge conversion in sputtered GBS thin films at room temperature due to the IEE, which is further enhanced by the quantum confinement. The spinpumping voltage shows grain size dependence. The MgO insertion layer suppressed the spin-to-charge voltage, whereas the Ag layer enhanced the spin-to-charge voltage. The successful sputtered growth of the GBS on silicon substrates makes for easier integration into complementary metal-oxide semiconductor (CMOS) devices, and high spin-to-charge conversion can be utilized for the reading scheme in the MESO device. Furthermore, the Ag insertion layer can enhance spin-pumping efficiency due to the additional spinmomentum locking originating from the Rashba−Edelstein effect.

contributed from the IEE or thermal effects, we prepared a sample with stack structure of Sub/MgO (2 nm)/GBS (4 nm)/MgO (1 nm)/CoFeB (5 nm)/MgO (2 nm)/Ta (2 nm), referred to as the BS-MgO sample. The V of the BS4 sample and the BS-MgO sample is presented in Figures 5a,b, respectively. It can be clearly seen that the VS decreases quite significantly in the BS-MgO sample compared to the BS4 sample. VIEE and VSE are 10.9 and 3.66 μV, respectively, in the BS-MgO sample. The α and g↑↓ of the BS-MgO sample are 0.0032 and 7.76 × 1017 m−2, respectively, which indicates that most of the spin-current pumped from the CoFeB layer is reflected back. The reduction of the VIEE by more than 6 times in the BS-MgO sample confirms a negligible presence of the thermal signals. Alves-Santos et al. observed a giant enhancement in the spin-pumping signal induced by Ag nanoparticles due to the IREE.41 Furthermore, it has been observed that the insertion of a Ag layer enhanced the efficiency of the conversion from charge-to-spin due to the additional spin-momentum locking, induced by the Rashba−Edelstein effect.42 We have prepared a sample with the stack structure Sub/MgO (2 nm)/GBS (4 nm)/Ag (2 nm)/CoFeB (5 nm)/MgO (2 nm)/Ta (2 nm), labeled as BS-Ag sample, to investigate if a Ag insertion layer can enhance spin-to-charge conversion. Indeed, as shown in Figure 5c, VIEE of the BS-Ag sample clearly indicates that the spin-to-charge conversion is enhanced due to the insertion of the Ag layer. VIEE is enhanced in the BS-Ag sample by approximately 40% compared to the BS4 sample. The VSE in the BS4 and the BS-Ag samples is estimated to be 3.22 and 0.6 μV, respectively, at 2.0 V excitation amplitude. Figure 5d shows ΔH as a function of the excitation frequency for the BSAg sample. The α and g↑↓ of the BS-Ag sample are determined to be 0.0051 and 1.02 × 1019 m−2, respectively. In comparison to the BS4 sample, both α and g↑↓ enhanced on the BS-Ag sample. The λIEE of BS-Ag is found to be 0.20 nm, which is approximately 43% enhancement as compared to the BS4 sample. A large value of λIEE 2.1 nm is reported at room temperature in α-Sn interfaced with Ag/Fe, however, the additional contribution of the Ag insertion layer was not estimated.29 A λIEE value of 0.075 nm was reported in (Bi0.22Sb0.78)2Te3 at room temperature.26 At 15 K, Shiomi et al. reported a λIEE value of 0.1 nm in Bi1.5Sb0.5Te1.7 but did not observe any spinto-charge conversion by Bi2Se3 (refs 24 and 25). Jamali et al. and Deorani et al. observed significant contribution of bulk effect in spin-to-charge conversion from Bi2Se3.3,63 However, Wang et al. and Fanchiang et al. observed that the spin-tocharge conversion in YIG/Bi2Se3 is mainly due to the Dirac surface states.27,64 In order to confirm the mechanism of spinto-charge conversion in our GBS, we analyze g↑↓ and λIEE as a function of thickness of the GBS films. The g↑↓ presented in Figure 4c does not show any specific pattern as a function of the film thickness. If the mechanism of spin-to-charge conversion is due to the bulk effect, g↑↓ should increase first and then almost saturates after a certain thickness as a function of GBS film thickness, as in the case of heavy metals.59,65,66 The g↑↓ as a function of film thickness dependence in our GBS agrees with the previous reports in YIG/Bi2Se3.27,64 Furthermore, g↑↓ as a function of GBS thickness in case of GBS/ CoFeB shows similar behavior as in YIG/GBS.55 We also performed the fitting of λIEE as a function of GBS film thickness using the ISHE formalism (Supporting Information Figure S5). The λIEE as a function of GBS film thickness does not agree

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b05011. Electrical transport measurement, intrinsic damping constant, and saturation magnetization measurement of Co20Fe60B20, TEM characterization of spin-pumping sample, spin-pumping on reference sample Pt, thermal contribution in spin-pumping signal related discussion, and estimation of the inverse spin Hall effect present in the spin-pumping signal (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F


Letter

Nano Letters ORCID

Torque without Exclusive Surface Conduction. Sci. Adv. 2018, 4, eaap8294. (14) Han, J.; Richardella, A.; Siddiqui, S. A.; Finley, J.; Samarth, N.; Liu, L. Room-Temperature Spin-Orbit Torque Switching Induced by a Topological Insulator. Phys. Rev. Lett. 2017, 119, 077702. (15) Wang, Y.; Zhu, D.; Wu, Y. Y.; Yang, Y.; Yu, J.; Ramaswamy, R.; Mishra, R.; Shi, S.; Elyasi, M.; Teo, K.-L.; et al. Room Temperature Magnetization Switching in Topological Insulator-Ferromagnet Heterostructures by Spin-Orbit Torques. Nat. Commun. 2017, 8, 1364. (16) Cubukcu, M.; Boulle, O.; Drouard, M.; Garello, K.; Onur Avci, C.; Mihai Miron, I.; Langer, J.; Ocker, B.; Gambardella, P.; Gaudin, G. Spin-Orbit Torque Magnetization Switching of a Three-Terminal Perpendicular Magnetic Tunnel Junction. Appl. Phys. Lett. 2014, 104, 042406. (17) Liu, L.; et al. Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum. Science 2012, 336, 555−558. (18) Manipatruni, S.; Nikonov, D. E.; Young, I. A. Beyond CMOS Computing with Spin and Polarization. Nat. Phys. 2018, 14, 338−343. (19) Manipatruni, S.; Nikonov, D.; Lin, C.; Gosavi, T. A.; Liu, H.; Prasad, B.; Huang, Y.; Bonturim, E.; Ramesh, R.; Young, I. A. Scalable Energy-Efficient Magnetoelectric Spin-Orbit Logic. Nature 2019, 565, 35. (20) Mosendz, O.; Pearson, J. E.; Fradin, F. Y.; Bauer, G. E. W.; Bader, S. D.; Hoffmann, A. Quantifying Spin Hall Angles from Spin Pumping: Experiments and Theory. Phys. Rev. Lett. 2010, 104, 046601. (21) Ando, K.; Saitoh, E. Inverse Spin-Hall Effect in Palladium at Room Temperature. J. Appl. Phys. 2010, 108, 113925. (22) Ando, K.; Takahashi, S.; Ieda, J.; Kajiwara, Y.; Nakayama, H.; Yoshino, T.; Harii, K.; Fujikawa, Y.; Matsuo, M.; Maekawa, S.; et al. Inverse Spin-Hall Effect Induced by Spin Pumping in Metallic System. J. Appl. Phys. 2011, 109, 103913. (23) Edelstein, V. M. Spin Polarization of Conduction Electrons Induced by Electric Current in Two-Dimensional Asymmetric Electron Systems. Solid State Commun. 1990, 73, 233−235. (24) Shiomi, Y.; Nomura, K.; Kajiwara, Y.; Eto, K.; Novak, M.; Segawa, K.; Ando, Y.; Saitoh, E. Spin-Electricity Conversion Induced by Spin Injection into Topological Insulators. Phys. Rev. Lett. 2014, 113, 196601. (25) Yamamoto, K. T.; Shiomi, Y.; Segawa, K.; Ando, Y.; Saitoh, E. Universal Scaling for the Spin-Electricity Conversion on Surface States of Topological Insulators. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 024404. (26) Mendes, J. B. S.; Santos, O. A.; Holanda, J.; Loreto, R. P.; De Araujo, C. I. L.; Chang, C.-Z.; Moodera, J. S.; Azevedo, A.; Rezende, S. M. Dirac-Surface-State-Dominated Spin to Charge Current Conversion in the Topological Insulator (Bi0.22Sb0.78)2Te3 Films at Room Temperature. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, No. 180415. (27) Wang, H.; Kally, J.; Lee, J. S.; Liu, T.; Chang, H.; Hickey, D. R.; Mkhoyan, K. A.; Wu, M.; Richardella, A.; Samarth, N. Surface-StateDominated Spin-Charge Current Conversion in Topological-Insulator−ferromagnetic-Insulator Heterostructures. Phys. Rev. Lett. 2016, 117, 076601. (28) Song, Q.; Mi, J.; Zhao, D.; Su, T.; Yuan, W.; Xing, W.; Chen, Y.; Wang, T.; Wu, T.; Chen, X. H.; et al. Spin Injection and Inverse Edelstein Effect in the Surface States of Topological Kondo Insulator SmB6. Nat. Commun. 2016, 7, 13485. (29) Rojas-Sánchez, J.-C.; Oyarzún, S.; Fu, Y.; Marty, A.; Vergnaud, C.; Gambarelli, S.; Vila, L.; Jamet, M.; Ohtsubo, Y.; Taleb-Ibrahimi, A.; et al. Spin to Charge Conversion at Room Temperature by Spin Pumping into a New Type of Topological Insulator: α-Sn Films. Phys. Rev. Lett. 2016, 116, 096602. (30) Baker, A. A.; Figueroa, A. I.; Collins-Mcintyre, L. J.; Van Der Laan, G.; Hesjedal, T. Spin Pumping in Ferromagnet-Topological Insulator-Ferromagnet Heterostructures. Sci. Rep. 2015, 5, 7907. (31) Jiang, Z.; Chang, C.-Z.; Masir, M. R.; Tang, C.; Xu, Y.; Moodera, J. S.; Macdonald, A. H.; Shi, J. Enhanced Spin Seebeck

Mahendra DC: 0000-0003-1249-336X Author Contributions §

These authors contributed equally.

Funding

This work was supported in part by ASCENT, one of six centers in JUMP, a Semiconductor Research Corporation (SRC) program sponsored by DARPA. Portions of this work were conducted in the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nano Coordinated Infrastructure Network (NNCI) under award number ECCS-1542202. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

We would like to thank Jason C. Meyers for TEM. We would also like to thank J. C. Rojas Sánchez for fruitful discussion.

(1) Hasan, M. Z.; Kane, C. L. Colloquium: Topological Insulators. Rev. Mod. Phys. 2010, 82, 3045−3067. (2) Mellnik, A. R.; Lee, J. S.; Richardella, A.; Grab, J. L.; Mintun, P. J.; Fischer, M. H.; Vaezi, A.; Manchon, A.; Kim, E. A.; Samarth, N.; et al. Spin-Transfer Torque Generated by a Topological Insulator. Nature 2014, 511 (7510), 449−451. (3) Jamali, M.; Lee, J. S.; Jeong, J. S.; Mahfouzi, F.; Lv, Y.; Zhao, Z.; Nikolic, B. K.; Mkhoyan, K. A.; Samarth, N.; Wang, J. P. Giant Spin Pumping and Inverse Spin Hall Effect in the Presence of Surface and Bulk Spin-Orbit Coupling of Topological Insulator Bi2Se3. Nano Lett. 2015, 15 (10), 7126−7132. (4) DC, M.; Grassi, R.; Chen, J.-Y.; Jamali, M.; Hickey, D. R.; Zhang, D.; Zhao, Z.; Li, H.; Quarterman, P.; Lv, Y.; et al. Room-Temperature High Spin-Orbit Torque Due to Quantum Confinement in Sputtered BixSe(1‑x) Films. Nat. Mater. 2018, 17, 800−807. (5) Zhang, S.; Fert, A. Conversion between Spin and Charge Currents with Topological Insulators. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 184423. (6) Kondou, K.; Yoshimi, R.; Tsukazaki, A.; Fukuma, Y.; Matsuno, J.; Takahashi, K. S.; Kawasaki, M.; Tokura, Y.; Otani, Y. Fermi-LevelDependent Charge-to-Spin Current Conversion by Dirac Surface States of Topological Insulators. Nat. Phys. 2016, 12, 1027−1032. (7) Wang, Y.; Deorani, P.; Banerjee, K.; Koirala, N.; Brahlek, M.; Oh, S.; Yang, H. Topological Surface States Originated Spin-Orbit Torques in Bi2Se3. Phys. Rev. Lett. 2015, 114 (25), 257202. (8) Zhang, H.; Liu, C.-X.; Qi, X.-L.; Dai, X.; Fang, Z.; Zhang, S.-C. Topological Insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a Single Dirac Cone on the Surface. Nat. Phys. 2009, 5, 438−442. (9) Ando, Y.; Hamasaki, T.; Kurokawa, T.; Ichiba, K.; Yang, F.; Novak, M.; Sasaki, S.; Segawa, K.; Ando, Y.; Shiraishi, M. Electrical Detection of the Spin Polarization Due to Charge Flow in the Surface State of the Topological Insulator Bi1.5Sb0.5Te1.7Se1.3. Nano Lett. 2014, 14, 6226−6230. (10) Li, C. H.; Van ’t Erve, O. M. J.; Robinson, J. T.; Liu, Y.; Li, L.; Jonker, B. T. Electrical Detection of Charge-Current-Induced Spin Polarization Due to Spin-Momentum Locking in Bi2Se3. Nat. Nanotechnol. 2014, 9, 218−224. (11) Dankert, A.; Geurs, J.; Venkata Kamalakar, M.; Charpentier, S.; Dash, S. P. Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators. Nano Lett. 2015, 15, 7976−7981. (12) Khang, N. H. D.; Ueda, Y.; Hai, P. N. A Conductive Topological Insulator with Large Spin Hall Effect for Ultralow Power Spin-Orbit Torque Switching. Nat. Mater. 2018, 17, 808. (13) Li, Y.; Ma, Q.; Huang, S. X.; Chien, C. L. Thin Films of Topological Kondo Insulator Candidate SmB6 : Strong Spin-Orbit G


Letter

Nano Letters

ple Layers of Bi2Te3, Bi2 Se3, and Sb2Te3 Materials. J. Appl. Phys. 2012, 111, 054305. (48) Jamali, M.; Klemm, A.; Wang, J.-P. Precessional Magnetization Induced Spin Current from CoFeB into Ta. Appl. Phys. Lett. 2013, 103, 252409. (49) Shen, K.; Vignale, G.; Raimondi, R. Microscopic Theory of the Inverse Edelstein Effect. Phys. Rev. Lett. 2014, 112, 096601. (50) Harii, K.; An, T.; Kajiwara, Y.; Ando, K.; Nakayama, H.; Yoshino, T.; Saitoh, E. Frequency Dependence of Spin Pumping in Pt/Y3Fe5O12 Film. J. Appl. Phys. 2011, 109, 116105. (51) Tserkovnyak, Y.; Brataas, A.; Bauer, G. E. W. Enhanced Gilbert Damping in Thin Ferromagnetic Films. Phys. Rev. Lett. 2002, 88, 117601. (52) Shiomi, Y.; Yamamoto, K. T.; Nakanishi, R.; Nakamura, T.; Ichinokura, S.; Akiyama, R.; Hasegawa, S.; Saitoh, E. Efficient Edelstein Effects in One-Atom-Layer Tl-Pb Compound. Appl. Phys. Lett. 2018, 113, 052401. (53) Kondou, K.; Tsai, H.; Isshiki, H.; Otani, Y. Efficient Spin Current Generation and Suppression of Magnetic Damping Due to Fast Spin Ejection from Nonmagnetic Metal/Indium-Tin-Oxide Interfaces. APL Mater. 2018, 6, 101105. (54) Isasa, M.; Martínez-Velarte, M. C.; Villamor, E.; Magén, C.; Morellón, L.; De Teresa, J. M.; Ibarra, M. R.; Vignale, G.; Chulkov, E. V.; Krasovskii, E. E.; et al. Origin of Inverse Rashba-Edelstein Effect Detected at the Cu/Bi Interface Using Lateral Spin Valves. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 14420. (55) DC, M.; Liu, T.; Chen, J.; Peterson, T.; Sahu, P.; Li, H.; et al. Room-Temperature Spin-to-Charge Conversion in Sputtered Bismuth Selenide Thin Films via Spin Pumping from Yttrium Iron Garnet. Appl. Phys. Lett. 2019, 114, 102401. (56) Rojas-Sánchez, J.-C.; Reyren, N.; Laczkowski, P.; Savero, W.; Attané, J.-P.; Deranlot, C.; Jamet, M.; George, J.-M.; Vila, L.; Jaffrès, H. Spin Pumping and Inverse Spin Hall Effect in Platinum: The Essential Role of Spin-Memory Loss at Metallic Interfaces. Phys. Rev. Lett. 2014, 112, 106602. (57) Hahn, C.; De Loubens, G.; Klein, O.; Viret, M.; Naletov, V. V.; Youssef, J. Ben. Comparative Measurements of Inverse Spin Hall Effects and Magnetoresistance in YIG/Pt and YIG/Ta. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 174417. (58) Wang, H. L.; Du, C. H.; Pu, Y.; Adur, R.; Hammel, P. C.; Yang, F. Y. Scaling of Spin Hall Angle in 3d, 4d, and 5d Metals from Y3Fe5O12 /Metal Spin Pumping. Phys. Rev. Lett. 2014, 112, 197201. (59) Tao, X.; Liu, Q.; Miao, B.; Yu, R.; Feng, Z.; Sun, L.; You, B.; Du, J.; Chen, K.; Zhang, S.; et al. Self-Consistent Determination of Spin Hall Angle and Spin Diffusion Length in Pt and Pd: The Role of the Interface. Spin Loss 2018, 4, No. eaat1670. (60) Hull, R.; Osgood, R. M., Jr.; Parisi, J.; Warlimont, H. Springer Series in Materials Science. Introduction to Thermoelectricity; Springer: Heidelberg, 2016. (61) Yue, D.; Lin, W.; Li, J.; Jin, X.; Chien, C. L. Spin-to-Charge Conversion in Bi Films and Bi/Ag Bilayers. Phys. Rev. Lett. 2018, 121, 037201. (62) Mosendz, O.; Pearson, J. E.; Fradin, F. Y.; Bader, S. D.; Hoffmann, A. Suppression of Spin-Pumping by a MgO TunnelBarrier. Appl. Phys. Lett. 2010, 96, 022502. (63) Deorani, P.; Son, J.; Banerjee, K.; Koirala, N.; Brahlek, M.; Oh, S.; Yang, H. Observation of Inverse Spin Hall Effect in Bismuth Selenide. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90 (9), 094403. (64) Fanchiang, Y. T.; Chen, K. H. M.; Tseng, C. C.; Chen, C. C.; Cheng, C. K.; Yang, S. R.; Wu, C. N.; Lee, S. F.; Hong, M.; Kwo, J. Strongly Exchange-Coupled and Surface-State- Modulated Magnetization Dynamics in Bi2Se3/ Yttrium Iron Garnet Heterostructures. Nat. Commun. 2018, 9, 223. (65) Tserkovnyak, Y.; Brataas, A.; Bauer, G. E. W.; Halperin, B. I. Rev. Mod. Phys. 2005, 77, 1375. (66) Huo, Y.; Zeng, F. L.; Zhou, C.; Wu, Y. Z. Spin Pumping and the Inverse Spin Hall Effect in Single Crystalline Fe/Pt Heterostructure. AIP Adv. 2017, 7, 056024.

Effect Signal Due to Spin-Momentum Locked Topological Surface States. Nat. Commun. 2016, 7, 11458. (32) Rojas Sánchez, J. C.; Vila, L.; Desfonds, G.; Gambarelli, S.; Attané, J. P.; De Teresa, J. M.; Magén, C.; Fert, A. Spin-to-Charge Conversion Using Rashba Coupling at the Interface between NonMagnetic Materials. Nat. Commun. 2013, 4, 2944. (33) Matsushima, M.; Ando, Y.; Dushenko, S.; Ohshima, R.; Kumamoto, R.; Shinjo, T.; Shiraishi, M. Quantitative Investigation of the Inverse Rashba-Edelstein Effect in Bi/Ag and Ag/Bi on YIG. Appl. Phys. Lett. 2017, 110, 072404. (34) Zhang, W.; Jungfleisch, M. B.; Jiang, W.; Pearson, J. E.; Hoffmann, A. Spin Pumping and Inverse Rashba-Edelstein Effect in NiFe/Ag/Bi and NiFe/Ag/Sb. J. Appl. Phys. 2015, 117, 17C727. (35) Oyarzún, S.; Nandy, A. K.; Rortais, F.; Rojas-Sánchez, J.-C.; Dau, M.-T.; Noël, P.; Laczkowski, P.; Pouget, S.; Okuno, H.; Vila, L.; et al. Evidence for Spin-to-Charge Conversion by Rashba Coupling in Metallic States at the Fe/Ge(111) Interface. Nat. Commun. 2016, 7, 13857. (36) Tsai, H.; Karube, S.; Kondou, K.; Yamaguchi, N.; Ishii, F.; Otani, Y. Clear Variation of Spin Splitting by Changing Electron Distribution at Non-Magnetic Metal/Bi 2 O 3 Interfaces OPEN. Sci. Rep. 2018, 8, 5564. (37) Lesne, E.; Fu, Y.; Oyarzun, S.; Rojas-Sánchez, J. C.; Vaz, D. C.; Naganuma, H.; Sicoli, G.; Attané, J.; Jamet, M.; Jacquet, E.; et al. Highly Efficient and Tunable Spin-to-Charge Conversion through Rashba Coupling at Oxide Interfaces. Nat. Mater. 2016, 15, 1261− 1267. (38) Song, Q.; Zhang, H.; Su, T.; Yuan, W.; Chen, Y.; Xing, W.; Shi, J.; Sun, J.; Han, W. Observation of Inverse Edelstein Effect in RashbaSplit 2DEG between SrTiO3 and LaAlO3 at Room Temperature. Sci. Adv. 2017, 3, 1602312. (39) Mendes, J. B. S.; Santos, O. A.; Meireles, L. M.; Lacerda, R. G.; Vilela-Leão, L. H.; Machado, F. L. A.; Rodríguez-Suárez, R. L.; Azevedo, A.; Rezende, S. M. Spin-Current to Charge-Current Conversion and Magnetoresistance in a Hybrid Structure of Graphene and Yttrium Iron Garnet. Phys. Rev. Lett. 2015, 115, 226601. (40) Dushenko, S.; Ago, H.; Kawahara, K.; Tsuda, T.; Kuwabata, S.; Takenobu, T.; Shinjo, T.; Ando, Y.; Shiraishi, M. Gate-Tunable SpinCharge Conversion and the Role of Spin-Orbit Interaction in Graphene. Phys. Rev. Lett. 2016, 116, 166102. (41) Alves-Santos, O.; Silva, E. F.; Gamino, M.; Cunha, R. O.; Mendes, J. B. S.; Rodríguez-Suárez, R. L.; Rezende, S. M.; Azevedo, A. Giant Spin-Charge Conversion Driven by Nanoscopic Particles of Ag in Pt. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, No. 060408. (42) Shi, S.; Wang, A.; Wang, Y.; Ramaswamy, R.; Shen, L.; Moon, J.; Zhu, D.; Yu, J.; Oh, S.; Feng, Y.; et al. Efficient Charge-Spin Conversion and Magnetization Switching through the Rashba Effect at Topological-Insulator/Ag Interfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 97, No. 041115. (43) Nascimento, V. B.; De Carvalho, V. E.; Paniago, R.; Soares, E. A.; Ladeira, L. O.; Pfannes, H. D. XPS and EELS Study of the Bismuth Selenide. J. Electron Spectrosc. Relat. Phenom. 1999, 104, 99− 107. (44) Yang, X.; Wang, X.; Zhang, Z. Synthesis and Optical Properties of Single-Crystalline Bismuth Selenide Nanorods via a Convenient Route. J. Cryst. Growth 2005, 276, 566−570. (45) Richter, W.; Becker, C. Raman and Far-Infrared Investigation of Phonons in the Rhombohedral V2−VI3 Compounds Bi2Te3, Bi2Se3, Sb2Te3 and Bi2(TexSex)3 (0< X< 1),(Bi1‑ySby)2Te3. Phys. Status Solidi B 1977, 84, 619−628. (46) Zhang, J.; Peng, Z.; Soni, A.; Zhao, Y.; Xiong, Y.; Peng, B.; Wang, J.; Dresselhaus, M. S.; Xiong, Q. Raman Spectroscopy of FewQuintuple Layer Topological Insulator Bi2Se3 Nanoplatelets. Nano Lett. 2011, 11, 2407−2414. (47) Shahil, K. M. F.; Hossain, M. Z.; Goyal, V.; Balandin, A. A. Micro-Raman Spectroscopy of Mechanically Exfoliated Few-QuintuH


Letter

Nano Letters (67) Zucchetti, C.; Dau, M.-T.; Bottegoni, F.; Vergnaud, C.; Guillet, T.; Marty, A.; Beigné, C.; Gambarelli, S.; Picone, A.; Calloni, A.; et al. Tuning Spin-Charge Interconversion with Quantum Confinement in Ultrathin Bismuth Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 98, 184418. (68) Demasius, K.-U.; Phung, T.; Zhang, W.; Hughes, B. P.; Yang, S.-H.; Kellock, A.; Han, W.; Pushp, A.; Parkin, S. S. P. Enhanced SpinOrbit Torques by Oxygen Incorporation in Tungsten Films. Nat. Commun. 2016, 7, 10644.

I


Observation of High Spin-to-Charge Conversion by Sputtered Bismuth

Recommend Documents