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Proximity-Induced Magnetic Order in a Transferred Topological Insulator Thin Film on a Magnetic Insulator Xiaoyu Che, Koichi Murata, Lei Pan, Qing Lin He, Guoqiang Yu, Qiming Shao, Gen Yin, Peng Deng, Yabin Fan, Bo Ma, Xiao Liang, Bin Zhang, Xiaodong Han, Lei Bi, Qing-Hui Yang, Huaiwu Zhang, and Kang L. Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02647 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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Proximity-Induced Magnetic Order in a Transferred Topological Insulator Thin Film on a Magnetic Insulator Xiaoyu Che1, Koichi Murata1, Lei Pan1, Qing Lin He1, Guoqiang Yu1,2, Qiming Shao1, Gen Yin1, Peng Deng1, Yabin Fan1, Bo Ma3, Xiao Liang4, Bin Zhang5, Xiaodong Han5, Lei Bi4, Qing-Hui Yang3, Huaiwu Zhang3 and Kang L. Wang,1,6* 1

Department of Electrical and Computer Engineering, University of California, Los Angeles, California 90095, United States 2

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

3

State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China 4

National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu 610054, China

5

Beijing Key Lab of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China 6

Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States

*

To whom correspondence should be addressed: E-mail: [email protected]


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ABSTRACT

Breaking the time reversal symmetry (TRS) in a topological insulator (TI) by introducing a magnetic order gives rise to exotic quantum phenomena. One of the promising routes to inducing a magnetic order in a TI is utilizing magnetic proximity effect between a TI and a strong magnetic insulator (MI). In this article, we demonstrate a TI/MI heterostructure prepared through transferring a molecular beam epitaxy (MBE)-grown Bi2Se3 film onto a yttrium iron garnet (YIG) substrate via wet-transfer. The transferred Bi2Se3 exhibits excellent quality over a large scale. Moreover, through wet-transfer we are able to engineer the interface and perform a comparative study to probe the proximity coupling between Bi2Se3 and YIG under different interface conditions. A detailed investigation of both anomalous Hall effect and quantum corrections to conductivity in magneto-transport measurements reveals an induced magnetic order as well as TRS breaking in the transferred Bi2Se3 film on YIG. In contrast, a thin layer of AlOx at the interface obstructs the proximity coupling and preserves the TRS, indicating the critical role of the interface in mediating magnetic proximity effect.

KEYWORDS: topological insulator, magnetic insulator, wet-transfer, magnetic proximity effect, interface, quantum interference


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Three-dimensional topological insulators (TIs) featuring topologically protected surface states with massless Dirac fermions have attracted extensive studies in both condensed-matter physics and spintronics applications.1-3 The spin-momentum locked Dirac fermions in the topological surface states are immune to spin-independent backscattering as protected by the time-reversal symmetry (TRS).4-8 By breaking the TRS, exotic quantum phenomena emerge such as the topological magnetoelectric effect and the quantum anomalous Hall effect (QAHE).9-11 The QAHE with dissipationless chiral edge transport holds the promise for revolutionary electronic and spintronic devices, and an effective way to break the TRS is introducing magnetic dopant (e.g., Cr and V) into a TI, through which the QAHE has been observed.12-15 However, to date, the QAHE can only be observed at an extremely low temperature (milli-Kelvin range), which is probably due to crystalline disorders and local fluctuations of the Fermi level throughout the film induced by the random distribution of dopants.16-18 An alternative approach to introducing a magnetic order into a TI without doping is through magnetic proximity effect by interfacing a TI thin film with a magnet insulator (MI). In such a TI/MI heterostructure, magnetic exchange coupling at the interface can induce a magnetic order in the TI layer without the participation of dopants.19-23 One of the suitable MI candidates points to yttrium iron garnet (YIG, Y3Fe5O12), which is a ferrimagnetic insulator with a high Curie temperature (TC) of ~550 K. In molecular beam epitaxy (MBE)-grown TI/MI heterostructures, the signature of the proximity-induced magnetic order has been observed.21,23-27. So far, MBEgrowth remains the only approach to preparing high-quality TI/MI heterostructures, but epitaxial growth may introduce a chalcogenide-rich dead layer during the annealing of seed layer under Se/Te fluxes owing to a large lattice mismatch (lattice constant = 12.37 Å for YIG and = 4.14 Å for Bi2Se3),28,29 which not only jeopardizes the quality of the film but also hinders


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the magnetic exchange coupling between the MI substrate and the TRS protected TI surface above the dead layer. Magnetic impurities are also prone to diffuse into the TI layer from the MI substrate during this procedure. Moreover, MBE-growth of thin films severely relies on the requirement of lattice matching with the substrates, and the flexibility of MI substrates for the epitaxial growth of TIs is very limited. Similar to TIs, two-dimensional van der Waals materials cannot be epitaxially grown on arbitrary substrates, but the wide adoption of transfer techniques has offered many possibilities in engineering device structures. Here, we demonstrate a TI/MI heterostructure prepared by transferring a MBE-grown Bi2Se3 film onto a YIG substrate via wettransfer. The resulting Bi2Se3/YIG heterostructure shows high crystallinity in a broad range without a dead layer at the interface. More importantly, through this approach the Bi2Se3/YIG interface can be manipulated, which is difficult to be implemented in direct MBE-growth without sacrificing sample quality. In this manner, it becomes feasible to probe the magnetic proximity effect under different interface conditions. In the transferred Bi2Se3 on YIG, a magnetic order originating from the magnetic proximity effect is observed, which manifests as anomalous Hall effect in the Hall measurement. The TRS breaking by the induced magnetic order also gives rise to a suppressed weak antilocalization (WAL) with a weak localization (WL) component. In contrast, the insertion of a thin AlOx layer between Bi2Se3 and YIG eliminates the interfacial proximity coupling, preserving the TRS in the Bi2Se3 layer.

RESULTS AND DISCUSSION Previous studies suggest that Bi2Se3 grown on sapphire (0001) substrates by MBE not only is among the best in terms of film quality but also can be transferred onto other substrates with


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minimal degradation.30-35 In this study, Bi2Se3 films were also grown on sapphire (0001) substrates for transfer. The thickness of the MBE-grown Bi2Se3 film was optimized to be 6 quintuple layers (QLs) in order to minimize the bulk conduction and at the same time avoid the hybridization between the top and bottom surface states. It has been demonstrated that the hybridization effect is not present in MBE-grown TI films with a thickness of 6 QLs or higher.31,36,37 The 2.3 µm-thick single-crystalline YIG was grown on a Gd3Ga5O12 (GGG) (111) substrate by liquid phase epitaxy (LPE) (see Supporting Information S1 for characterizations of the YIG substrate). The Bi2Se3 film was subsequently transferred to the prepared YIG substrate. As illustrated in Figure 1, the Bi2Se3/sapphire sample was firstly spin-coated with a thin polymethyl methacrylate (PMMA) layer once it was taken out of the MBE chamber. Then the whole sample was immersed in a potassium hydroxide (KOH) solution to initiate the exfoliation of the Bi2Se3 film from the sapphire substrate. When the Bi2Se3 film just started to detach from the sapphire substrate, the sample was removed from the KOH solution and left floating on freshly prepared DI water to complete the exfoliation. The partially detached Bi2Se3 film on DI water slowly got peeled off from the substrate, probably due to a tensile force exerted by the surface tension of water. After the detachment, the exfoliated Bi2Se3 film with PMMA coating was rinsed in DI water for 5 times. Finally, the Bi2Se3 film was fished out by the YIG substrate and dried up in a N2 ambient (see Supporting Information S2 for details). Figure 2a shows a picture of the transferred Bi2Se3 film on the YIG substrate. A visually smooth and continuous Bi2Se3 film is obtained, upon which the color gradient suggests the PMMA coating layer. The Bi2Se3 film remains largely intact after transfer, as evidenced by the optical microscope image in Figure 2b after the removal of PMMA. Interestingly, a corner of the film was unintentionally folded after transfer, allowing us to visualize the bottom surface of the


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Bi2Se3 film which directly associates with the Bi2Se3/YIG interface quality. For clarity, three regions are marked on this sample as illustrated in Figure 2b: regions 1 and 2 represent the bottom and top surface of the Bi2Se3 film, respectively, while region 3 stands for the top surface of the YIG substrate. Atomic force microscopy (AFM) characterizations were carried out to further inspect the quality of the transferred Bi2Se3 film (Figures 2c-h). A terrace-like surface topography with a step height of 1 nm that corresponds to 1 QL of the Bi2Se3 is revealed for the Bi2Se3 top surface (region 2), confirming a high crystallinity of the Bi2Se3 film from latticematched MBE-growth on sapphire (0001). It also implies that the film was not corrugated, ruptured or chemically degraded after the wet-transfer. The Bi2Se3 bottom surface (region 1) presents a smooth surface morphology inherited from the ultra-flat top surface of the sapphire (0001) substrate, with a vanishingly small root-mean-square surface roughness = 0.14 nm. The YIG top surface (region 3) also shows a low roughness of = 0.16 nm. Such flatness of these two surfaces is essential to the formation of a sharp interface between Bi2Se3 and YIG, which is further verified by cross-sectional scanning transmission electron microscope (STEM) imaging. As displayed in Figure 2i, the quintuple-layer structure of Bi2Se3 with van der Waals gaps in between can be clearly noticed. The Bi2Se3/YIG interface is smooth and clean without any dead-layer. Energy dispersive X-ray spectroscopy (EDX) was simultaneously conducted, concluding no signature of intermixing between Bi2Se3 and YIG (see Supporting Information S3 for EDX mapping). To systematically study the interfacial proximity coupling under different interface conditions as well as to realize a better comparison, three samples were prepared in our experimental design, which are Bi2Se3/YIG (BS1), Bi2Se3/YIG with a thin AlOx insertion layer (BS2), and Bi2Se3/GaAs (BS3) (see Figure 3a). All three samples were prepared through the same transfer


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procedures. Particularly, in BS2 the AlOx insertion layer was implemented by depositing a 0.7 nm-thick Al on the YIG substrate using electron-beam evaporation, after which it naturally oxidized into AlOx with a thickness of about 1 nm. In principle, single-crystalline Bi2Se3 cannot be epitaxially grown on amorphous AlOx. But in this manner, we are able to engineer the trilayer structure despite the interfacial lattice mismatch. On the other hand, the Bi2Se3 films on all these samples are from the same Bi2Se3/sapphire sample (BS0), which offers a better control and circumvents the inevitable quality variation of Bi2Se3 on different substrates by growth. All samples were patterned into devices with Hall bar geometry through standard photolithography (see the inset of Figure 3b). Magneto-transport measurements were subsequently carried out to investigate the magnetic proximity effect. For all three samples, the Hall effect curves display negative slopes due to the n-type nature of Bi2Se3 as shown in the inset of Figure 3b. The extracted 2D carrier concentrations of the Bi2Se3 layer for BS0-BS3 are 4.1 × 10 cm , 8.7 × 10 cm , 8.9 × 10 cm , and 3.9 × 10 cm , while the mobilities are 119 cm , 332 cm , 533 cm , 148 cm , respectively. The carrier concentrations of the transferred Bi2Se3 films are found to be comparable or even lower than the as-grown Bi2Se3/sapphire sample while the mobilities are higher. These features verify that there is no degradation and negligible environmental doping effect (mostly n-type from oxygen) on the Bi2Se3 film during the wettransfer process. A plausible mechanism for the reduced carrier concentrations and the improved mobilites may be interfacial charge transfer or neutralization of surface charge defects, which has been reported in transferred Bi2Se3 films.34 Apart from the Hall measurement results, magnetoconductivity (MC) measurements also corroborates the robustness of the Bi2Se3 film and minimal sample degradation after transfer (see Supporting Information S4). After removing the


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linear Hall background, it becomes clear that BS1 displays a distinct nonlinear Hall curve with a saturation behavior at ~0.2 T, while in both BS2 and BS3 such behavior is not observed. The nonlinear Hall signal suggests anomalous Hall effect in the Bi2Se3 originating from an interfacial proximity-induced magnetic order, which can be further explained as the hybridization between the p-orbital of Bi2Se3 and the d-orbital of Fe in YIG at the interface. On the other hand, it is also possible that the nonlinear Hall signal stems from other origins, e.g., multiple transport channels due to the coexistence of the topological surface states and the bulk states with different carrier mobilities.38,39 However, in this scenario the multiple channels effect is vanishingly small due to the suppressed bulk channel in the Bi2Se3 film with only 6 QLs. And in carrier transport, it mostly occurs in a relative high magnetic field (typically larger than 1 T), in contrast to that of the nonlinear Hall signal in BS1 (within ± 0.4 T). Besides, the control samples BS2 and BS3 will display similar nonlinear Hall curves if the multiple channels effect is pronounced. Another possibility is the stray field created by the YIG substrate beneath instead of interfacial orbital hybridization, which can interfere with the Hall effect in Bi2Se3 following its own magnetic hysteresis. This possibility is ruled out by the Hall signal from BS2, where Bi2Se3 is adjacent to YIG but presents only an ordinary Hall effect. Consequently, by comparing with the control samples BS2 and BS3 we generally exclude other potential mechanisms, and the nonlinear behavior should predominantly originates from the anomalous Hall effect as a result of the proximity-induced magnetic order in Bi2Se3. Meanwhile, it is noticed that the 1 nm AlOx layer separating Bi2Se3 and YIG basically obstructs the interfacial proximity coupling as no anomalous Hall signal is observed in the Hall signal of BS2. This suggests the crucial role of a sharp Bi2Se3/YIG interface in inducing a magnetic order in Bi2Se3 through proximity effect.


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Figure 3c shows the temperature-dependent anomalous Hall effect curves in BS1 after subtracting the ordinary Hall background. The anomalous Hall resistance reaches its maximum of ~2.8 Ω at 1.9 K, slowly decreases as temperature increases, and eventually vanishes ~30 K. A natural question may arise that the TC of the Bi2Se3 layer seems to be much lower than that of the YIG substrate. In fact, both TC much lower than that of the magnetic substrate and TC that is above room temperature have been reported in the study of the magnetic proximity effect in TI.21,24,40 While the underlying mechanism is yet unclear, it is reasonable that the induced magnetic order is not as strong as the long-range exchange interaction in the interior of YIG. Moreover, the induced magnetic order also suffers from finite-size effect, imperfections at the interface, and sample-to-sample variation, which might be the reasons that account for the low TC in this system. This can be evidenced by the work by Jiang et al., where the TC of the proximity-induced magnetic order in TI/YIG can vary largely (20 K to 150 K) in different samples.26 The absence of an induced magnetic order in BS2 also provides important clues to the significance of interface in mediating magnetic proximity effect. Though the enhancement of the TC may be achieved by further improving sample and interface quality, it is beyond the scope of this work. The anomalous Hall signal of the Bi2Se3 layer is similar to the magnetic hysteresis of the YIG substrate along the out-of-plane direction measured by magneto-optical Kerr effect (see the inset of Figure 3c), again suggesting the proximity coupling between Bi2Se3 and YIG. No obvious coercivity is observed in the Bi2Se3 layer, due to the fact that YIG is a soft magnetic material with almost zero coercivity in its magnetic hysteresis.41-43 Similar experimental results were observed in another Bi2Se3/YIG sample prepared in the same manner in order to confirm reproducibility (see Supporting Information S5).


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To further understand the correlation between the induced magnetic order and the TRSprotected topological surface states, the MC of the three samples are investigated. We first start with the simplest scenario, BS3, where magnetic proximity effect is not present. As shown in Figures 4a, the MC of BS3 exhibits a prominent sharp cusp-like WAL behavior at 1.9 K, signifying a topological nontrivial surface transport carrying a Berry phase of ! in the quantum diffusive transport regime.44-46 In general, the WAL effect can be described by the HikamiLarkin-Nagaoka (HLN) formalism as ∆#(%) =

'( )

* ) ℏ

.)

.)

(, - /) + 2 − 45 - /) 2) .0

.0

(1)

where , is the digamma function, 6 is the prefactor characterizing the number of independent channels contributing to the quantum interference, and 47 is the phase coherence length.47 Here a negative 6 suggests a symplectic case of non-magnetic scattering, where 6 = −0.5 indicates a single coherent channel and 6 = −1 implies two channels originating from both top and bottom surface states. Fittings to the MC at different temperatures using the HLN formula yield an 6 between -0.5 and -0.6. The fact that the resulting 6 > −1 despite the coexistence of two surfaces in the TI can be attributed to the coupling of the surface states and the gapped bulk states, which is typical for Bi2Se3 as the Fermi level resides in its conduction band.48 In contrast, in the case where the proximity coupling comes into play, it can be found that in BS1 the WAL feature is clearly diminished, and the MC cannot be simply described by the single-component HLN formula for the TRS-preserved scenario. This is due to the broken TRS and a small exchange gap opened in the surface states, which gives rise to a suppressed WAL dip and an additional WL component. To address the presence of both WAL and WL, a two-component HLN formula is employed as


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' ()

.)

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.)

∆#(%) = ∑; = 1 and > = 2 represent the WAL and the WL component in the formula, respectively.37,49-51 The inclusion of both WAL and WL terms in the quantum corrections to conductivity by adopting the two-component HLN formula offers an excellent fit to the MC of BS1. For BS2 with a thin AlOx insertion layer, its MC curve displays an intermediate behavior between that of BS1 and BS3. Fitting to the MC of BS2 using the HLN formula does not give an appreciable value of the WL component, however, it reveals a decrease of the phase coherence length and will be further discussed in the next section. The WAL and WL contributions can be estimated by extracting the prefactors as well as the phase coherence lengths from the fittings by using Eqs. (1) and (2), allowing a quantitative analysis of the correlation between the proximity effect and the quantum interference corrections in the Bi2Se3 layer. As Figure 4b shows, the WAL prefactor value of BS2 (-0.5 ~ -0.6) closely resembles that of BS3, while in both samples the WL component is absent. This is consistent with the Hall measurement results, implying that the insertion of the AlOx layer in BS2 blocks the proximity coupling and preserves the TRS in the Bi2Se3 layer due to the absence of induced magnetic order (Figure 4c). On the other hand, it also clarifies that the WAL and WL components are not affected by potential charge transfer occurring at the interface. In BS1, the absolute value of the WAL prefactor (|6 | = 0.3 ~ 0.4) is ~0.2 smaller than that of BS2 and BS3, in accordance with the aforementioned suppressed WAL in BS1. For the WL prefactor in BS1, from the fitting, it is extracted to be 6 ≈ 0.06 at 1.9 K. Therefore, the gap size of the surface state can be calculated with the obtained 6 and 6 , since both 6 and 6 are explicit functions of ∆/CD , where ∆ is the gap size and CD is the Fermi level. The surface state gap size is estimated


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to be ∆≈ 70 meV at 1.9 K (see Supporting Information S6 for details). It would be of interest to compare the surface state gap size in the proximity effect scenario to that in the magnetic doping scenario, which can provide some important clues to the magnetization of the transferred Bi2Se3 since it is difficult to obtain a specific value of its magnetic moment. The gap size of BS1 is found to be comparable to that of Bi2Se3 film with 2% Cr dopant (∆ ≈ 75 meV), as well as Bi2Se3 film with 1-2.5% Mn dopant (∆ ≈ 50-100 meV).49,50 In addition to the gap size, it could be noticed that the negative WAL feature still dominates the MC of BS1 despite the presence of the WL component (Figure 4a), which makes it counterintuitive because WL typically manifests as a positive MC. In fact, if we compare the WAL component |6 | and the WL component |6 | in BS1, |6 | has a much smaller value than |6 |. This is due to a high CD from the Dirac point (~0.3 eV) in Bi2Se3 and thereby a small ∆/CD , which results in the WAL-dominated feature in the MC of BS1 (see Supporting Information S6). A transition to a positive WL behavior is expected if the Fermi level can be tuned closer to the surface state Dirac point.37,51,52 Besides, both theoretical calculations and experimental investigations by polarized neutron reflectometry have suggested that the effective proximity length in the TI layer is about 1-2 QLs, and the gap is opened in the Dirac cone of the bottom surface.20,23,53 Therefore, the TRS protected top surface also explains the dominance of the WAL feature. Figure 4d presents the temperature dependence of the extracted phase coherence lengths 47 of the three samples. Theoretically, 47 can be described by a power law function as G H , where the exponent value I = 1/2 in 2D systems and I = 3/4 in 3D systems.54 The extracted phase JK JK JK coherence lengths in the WAL component of all three samples 47 , 47 , and 47 well match

such power law with I = 0.48, since the carrier transport is mostly 2D-like in the Bi2Se3 layer JK with a thickness of only 6 QLs. In terms of the phase coherence length values, 47 in the


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JK in BS3 without scenario of proximity coupling is about 1.5 times smaller compared to 47

proximity coupling. In the picture of quantum diffusive transport, the destructive interference of electrons in a self-intersecting loop due to spin-orbit coupling can be destroyed by either JK magnetic scattering or an external magnetic field, which results in a reduced 47 in BS1. JK Interestingly, on the other hand, 47 of BS2 with AlOx between Bi2Se3 and YIG displays a JK reduction similar to 47 . It implies that the destructive interference of electrons can also be

affected by the stray field originating from the YIG substrate. The resistance-temperature (RT) plot of the three samples is shown in Figure 5a, where a pronounced resistance upturn in BS1 comparing to BS2 and BS3 in the regime of 1.9 K-10 K can be noticed. After converting it to a conductivity plot, all three samples demonstrate a logarithmic conductivity increase as temperature increases below 10 K (Figure 5b). The experimental values of the conductivity curve slopes L =

*M

NO

( ) N.P(Q)

between 1.9 K and 10 K are obtained from the plot

as L JK = 0.22, L JK = 0.138, and L JK = 0.135 for BS1, BS2, and BS3, respectively. While the curve slopes of BS2 and BS3 closely match each other, BS1 exhibits an appreciably larger slope that manifests as the resistance upturn in the RT curve. The conductivity curve slopes L can be quantitatively analyzed as follows. In general, quantum corrections to conductivity from both WAL and WL can be calculated using the formula below: STU(SU)

∆#RR

=

': H: ( ) *) ℏ

Q

45(Q ) V

(3)

where GU is the characteristic temperature at which quantum correction vanishes, > = 1 represents the WAL component and > = 2 stands for the WL component.55-57 Here, the close match of the curve slopes in the samples without proximity coupling (BS2 and BS3) indicates


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similar values of 6 and agrees well with the aforementioned extracted WAL component. However, while the WAL in Bi2Se3 should lead to a negative slope of the conductivity curve at low temperature due to a negative sign of 6 , the observed slopes in BS2 and BS3 are positive. This logarithmic conductivity increase has been extensively reported and should be attributed to WWX an extra electron-electron interaction (EEI) term ∆#RR in the quantum diffusive transport

regime.44,58,59 For BS1, on the other hand, the additional WL component with a positive sign of 6 due to the TRS breaking promotes the further increase of the conductivity and thereby a WWX JK STU WWX larger slope in BS1. In this manner, ∆#RR can be estimated by ∆#RR = ∆#RR + ∆#RR , where JK STU the conductivity change of BS3 ∆#RR is obtained from the experimental curve and ∆#RR can

be calculated using Eq. (3). This subsequently yields an electron screening factor Y ≈ 0.042, WWX implying the presence of typical strong electron screening in Bi2Se3.57 Alternatively, ∆#RR also JK JK JK can be calculated from ∆#RR , which yields a similar result because ∆#RR is close to ∆#RR and

the WAL component in BS2 is identical to that in BS3. Finally, the conductivity change in BS1 JK STU SU WWX can be calculated as ∆#RR = ∆#RR + ∆#RR + ∆#RR by employing Eq. (3) as well as the

extracted 6; and I; from MC, yielding an estimated slope value of L JK = 0.225 (See Supporting Information S7 for calculation details). The agreement between the calculated slope value and the experimental result (L JK = 0.22) demonstrates an excellent consistency between the MC and RT results in the quantum interference picture, which are correlated to the proximity-induced magnetic order in the Bi2Se3 film.

CONCLUSIONS


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In summary, we demonstrated a Bi2Se3/YIG heterostructure prepared by transferring an MBEgrown Bi2Se3 film onto a YIG substrate using the wet-transfer technique. The resulting Bi2Se3/YIG heterostructure exhibits high-quality and a sharp interface. Furthermore, through wet-transfer we are able to modify the Bi2Se3/YIG interface in the study of magnetic proximity effect and perform a systematic comparison. Anomalous Hall effect observed in the Bi2Se3/YIG sample suggests an induced magnetic order in Bi2Se3 through the proximity coupling, and a detailed analysis of the magneto-transport results reveals a WL component in the Bi2Se3/YIG due to the breaking of TRS and gap opening in the Bi2Se3 surface states. In contrast, the interfacial proximity coupling is blocked by a thin AlOx insertion layer between Bi2Se3 and YIG, and the TRS in the topological surface states is preserved in this scenario.


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Figure 1. Schematic illustration of the procedures in transferring a MBE-grown Bi2Se3 film onto a YIG substrate via wet-transfer. Detachment of the Bi2Se3 film is firstly initiated in a KOH solution, after which the sample is transferred to DI water where the Bi2Se3 film get fully peeled off. The floating Bi2Se3 film is finally fished out by the YIG substrate, followed by overnight drying and the removal of PMMA.


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Figure 2. Characterizations of the Bi2Se3/YIG sample after transfer. (a) An image of the Bi2Se3/YIG sample before removing PMMA. (b) An optical microscope image of the transferred Bi2Se3 film on YIG, where a smooth and crack-free film surface can be observed. Three regions are marked in the image as: (1) bottom surface of the transferred TI film; (2) top surface of the transferred TI film; (3) top surface of the YIG substrate. (c)-(e) AFM characterization results of the three regions, with the line profiles illustrated in (f)-(h). The AFM images reveal an atomically flat surface in regions (1) and (3), while region (2) shows a pristine terrace-like surface morphology. (i) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the Bi2Se3/YIG sample, revealing a high crystallinity of the Bi2Se3 film and a clean interface after transfer.


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Figure 3. A comparative study of the Hall effect in the Bi2Se3 samples with different sample structures. (a) Three distinct sample structures prepared by the same wet-transfer procedures, named as BS1 (Bi2Se3/YIG), BS2 (Bi2Se3/AlOx/YIG) and BS3 (Bi2Se3/GaAs), respectively. The ~1 nm AlOx layer in BS2 was prepared by depositing 0.7 nm Al on YIG using electron-beam evaporation, after which it naturally oxidized into AlOx. (b) Hall measurement results of the three samples at 1.9 K after subtracting the linear background. BS1 displays a nonlinear Hall signal with a saturating behavior at ~0.2 T compared to BS2 and BS3. Inset 1: Device geometry for magneto-transport measurements with a schematic of the instrument setup. Inset 2: Hall measurement results of the three samples at 1.9 K before subtracting the linear background (c) Anomalous Hall resistance of BS1 at different temperatures. The anomalous Hall resistance decreases as temperature increases and eventually vanishes at ~30 K. Inset 1: A schematic


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illustrating the interfacial magnetic proximity effect in BS1. Inset 2: Normalized out-of-plane magnetic hysteresis loop of YIG measured by MOKE at 300 K.

Figure 4. A suppressed WAL feature and an emergent WL component in BS1 due to the surface state gap opened by the interfacial proximity coupling. (a) MC of BS1, BS2, and BS3 at 1.9 K. A cusp-like WAL feature is observed in BS1, which is less pronounced in BS2 and clearly diminished in BS3. (b) Quantum correction prefactor 6 in a temperature-dependent manner. Prefactor 6 for the WAL component is denoted as squares in the cyan region, while 6 representing the WL component is marked as circles in the yellow region. (c) A sketch of the


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band structures of the Bi2Se3 in BS1, BS2 and BS3. “BS” and “SS” represent bulk states and surface states, respectively. The TRS in BS2 and BS3 is preserved, however, in BS1 it is broken by the magnetic order and a gap is opened in the topological surface states. (d) Temperature JK JK dependence of the phase coherence lengths 47 . The obtained 47 and 47 in BS1 and BS2 are JK smaller than 47 in BS3, but the phase coherence lengths of all three samples follow the power

law function G H with I ≈ 0.48.

Figure 5. RT plot of the three Bi2Se3 samples, and a logarithmic scaling of their conductivities as a function of temperature. (a) RT curves of the three samples from 300 K to 1.9 K. A pronounced resistance upturn is noticed in BS1 when the temperature further decreases below 10 K. (b) A logarithmic increase of the conductivities as the temperature increases. Straight lines denote the slopes of the conductivity curves below 10 K. The conductivity curve slope of BS1 is larger compared to BS2 and BS3 and can be attributed to the WL component in BS1 originating from the proximity-induced magnetic order.


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METHODS MBE Growth of Bi2Se3 Films. 6 QLs of Bi2Se3 thin films were grown on epi-ready sapphire (0001) substrates using an ultrahigh vacuum Perking Elmer 430 molecular beam epitaxy (MBE). Before the growth, the sapphire substrates were pre-annealed at 690 ℃ . High-purity Bi (99.9999%) was evaporated from a conventional effusion cell and Se (99.99%) flux was created by a cracking cell. The Bi and Se cells were kept at 470 ℃ and 270 ℃, respectively during the growth, while the sapphire substrates were maintained at 220 ℃. The epitaxial growth was monitored by an in-situ reflection high-energy electron diffraction (RHEED). RHEED patterns were optimized to very sharp, smooth, streaky patterns, and the intensity oscillations were recorded to calibrate film thickness. Device Fabrication. Detailed steps of transferring the 6-QL Bi2Se3 thin film onto the YIG substrate have been provided in Supporting Information S2. The Bi2Se3/AlOx/YIG sample and the Bi2Se3/GaAs sample were prepared through the same wet-transfer procedures. The transferred Bi2Se3 films were from the same as-grown Bi2Se3/sapphire sample to ensure a consistent quality. All three samples were patterned into a micrometer-scale Hall bar geometry using conventional optical photolithography and a subsequent CHF3 dry etching of 15 s. Contact pads of Ti/Au (10 nm/100 nm) were deposited by e-beam evaporation. Characterizations. HAADF-STEM. The HAADF-STEM experiments were carried out using FEI Titan Cs-corrected ChemiSTEM operating at 200 KV with a Super-X detector. MagnetoTransport Measurements. Standard low-frequency four-probe magnetoresistance measurements and Hall measurements were conducted in a Quantum Design Physical Properties Measurement


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System (PPMS). Multiple SR830 lock-in amplifiers and a Keithley 6221 AC/DC current source were connected to the PPMS to achieve high-sensitivity. A constant ac current of 1 µA at 15.8 Hz was fed in the transport measurements. The temperature range of the PPMS is from 1.9 to 300 K, and the magnetic field can be up to 9 T.

ASSOCIATED CONTENT Conflict of Interest The authors declare no competing financial interest. Supporting Information Characterizations of the YIG substrate, transferring the Bi2Se3 film onto the YIG substrate, EDX mapping of the transferred Bi2Se3/YIG, quality check for the Bi2Se3 film after transfer, reproducibility of the magnetic proximity effect in the transferred Bi2Se3/YIG, estimation of the surface state gap size, decomposition of the WAL, WL and EEI components in the conductivity corrections. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: (K.L.W.) [email protected] ACKNOWLEDGMENT


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We acknowledge the support from the Army Research Office accomplished under Grant Number W911NF-16-1-0472 and W911NF-15-1-10561. We are also grateful to the support by the Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under award #S000686, and the National Science Foundation (DMR-1411085). LB thanks the support from National Natural Science Foundation of China (Nos. 61475031 and 51522204).

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