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Topological Insulator Interfaced with Ferromagnetic Insulators: ${\Rm{B}} PHYSICAL REVIEW MATERIALS 4, 064202 (2020) Topological insulator interfaced with ferromagnetic insulators: Bi2Te3 thin films on magnetite and iron garnets V. M. Pereira ,1 S. G. Altendorf,1 C. E. Liu,1 S. C. Liao,1 A. C. Komarek,1 M. Guo,2 H.-J. Lin,3 C. T. Chen,3 M. Hong,4 J. Kwo ,2 L. H. Tjeng ,1 and C. N. Wu 1,2,* 1Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany 2Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan 3National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan 4Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei 10617, Taiwan (Received 28 November 2019; revised manuscript received 17 March 2020; accepted 22 April 2020; published 3 June 2020) We report our study about the growth and characterization of Bi2Te3 thin films on top of Y3Fe5O12(111), Tm3Fe5O12(111), Fe3O4(111), and Fe3O4(100) single-crystal substrates. Using molecular-beam epitaxy, we were able to prepare the topological insulator/ferromagnetic insulator heterostructures with no or minimal chemical reaction at the interface. We observed the anomalous Hall effect on these heterostructures and also a suppression of the weak antilocalization in the magnetoresistance, indicating a topological surface-state gap opening induced by the magnetic proximity effect. However, we did not observe any obvious x-ray magnetic circular dichroism (XMCD) on the Te M45 edges. The results suggest that the ferromagnetism induced by the magnetic proximity effect via van der Waals bonding in Bi2Te3 is too weak to be detected by XMCD, but still can be observed by electrical transport measurements. This is in fact not inconsistent with reported density-functional calculations on the size of the gap opening. DOI: 10.1103/PhysRevMaterials.4.064202 I. INTRODUCTION (WAL) and induces weak localization (WL) [7]. The anoma- lous Hall effect (AHE) of a magnetized TI, on the other The quantum anomalous Hall effect (QAHE) is expected hand, has been observed in several TI/FI heterostructures, and observed when magnetic ordering is introduced in a such as in (Bi, Sb) Te on Tm Fe O for temperatures of topological insulator (TI) system [1–5]. This effect is due to 2 3 3 5 12 up to 400 K [8], (Bi Sb − ) Te on Y Fe O [9], Bi Te time-reversal symmetry breaking and can be experimentally x 1 x 2 3 3 5 12 2 3 on Cr Ge Te [10], and Bi Se on EuS [11]. X-ray mag- achieved by doping transition metals into the TI. There is 2 2 6 2 3 netic circular dichroism (XMCD) measurements have been also the possibility to use the magnetic proximity effect in performed to provide information on the magnetic properties TI/ferromagnetic insulator (FI) heterostructures to magnetize for each of the different elements in the TI. XMCD effects the topological surface state at the interface. Although the on the Cr-doped (Sb, Bi) Te [12] and Cr-doped Sb Te [13] QAHE has not yet been experimentally observed for such 2 3 2 3 systems have been observed, showing that the Sb and Te are TI/FI heterostructures, the magnetic proximity effect in TI/FIs magnetized. An XMCD experiment on a (Bi . Sb . ) Te has the advantage compared to doping with magnetic ions 0 25 0 75 2 3 film interfaced with Tm Fe O claims to have detected that that it will not introduce defects in the TI. Moreover, the 3 5 12 the Te is magnetized [14]. Curie temperatures of FIs are much higher than those of In order to study the magnetic proximity effect in TI/FI het- magnetically doped TIs so that one can hope to obtain the erostructures, we have grown Bi Te thin films by molecular- QAHE over a larger temperature window. To study the gap 2 3 beam epitaxy (MBE) on magnetic Y Fe O (111) substrates, opening of the topological surface state induced by the mag- 3 5 12 Tm Fe O (111) thin films, Fe O (111) substrates, and netic proximity effect, one can measure the magnetoresistance 3 5 12 3 4 Fe O (100) thin films as well as on nonmagnetic Al O (0001) and fit the Hikami-Larkin-Nagaoka (HLN) equation [6]tothe 3 4 2 3 substrates as a reference. X-ray photoelectron spectroscopy data. The gap opening suppresses the weak antilocalization (XPS) was utilized to examine the interface very carefully, and to ascertain that the Bi2Te3 films on the magnetite and iron garnets have no or minimal chemical reaction at the *[email protected] interface since this is a crucial aspect for the short-range magnetic proximity effect to be well defined. We conducted Published by the American Physical Society under the terms of the electrical transport measurements and observed the AHE as Creative Commons Attribution 4.0 International license. Further well as a suppression of the weak antilocalization, indicating a distribution of this work must maintain attribution to the author(s) topological surface-state gap opening induced by the magnetic and the published article’s title, journal citation, and DOI. Open proximity effect. Moreover, the XMCD was examined on the access publication funded by the Max Planck Society. Te M45 edges of our Bi2Te3 heterostructures. As reference, we 2475-9953/2020/4(6)/064202(8) 064202-1 Published by the American Physical Society V. M. PEREIRA et al. PHYSICAL REVIEW MATERIALS 4, 064202 (2020) also conducted XMCD measurements on a “Te on Fe” film X-ray absorption spectroscopy and x-ray magnetic circu- and a FexTe film, which showed clear and substantial XMCD lar dichroism (XAS/XMCD) measurements were conducted effects at the Te M45 edges. in situ at the 11A Dragon beam line of the Taiwanese Light Source at the National Synchrotron Radiation Research Cen- ter (NSRRC) (Taiwan) to probe the magnetic properties of the II. EXPERIMENT heterostructure selectively for each element. We connected an Magnetic insulators for the Bi2Te3 film growth were pre- MBE system to the XAS/XMCD chamber at the beamline end pared in various ways. Y3Fe5O12 (YIG) substrates were ul- station. 1–2-QL Bi2Te3 thin films were grown on YIG(111), trasonically cleaned by acetone and isopropanol, and were TmIG(111), and Fe3O4(111) at room temperature and then ◦ annealed in situ at 600 ◦C for 2 h in an oxygen pressure annealed at 240 C for 30 min to crystallize the films, and −6 of 1 × 10 mbar prior to the Bi2Te3 growth. Tm3Fe5O12 1-ML Te was grown on Fe3O4(100) at room temperature. The (TmIG) films (16 nm) were grown on Gd3Ga5O12 substrates samples were transferred in situ to the XAS/XMCD chamber. by off-axis sputtering [15,16]. The TmIG films were annealed In order to establish reference XMCD spectra of magnetized ◦ at 150 C to remove the moisture after being loaded into the Te, Te and Fe were codeposited into an FexTe film onto a MgO(100) substrate with a Te flux rate of 2.1 Å/min and an MBE chamber. Fe3O4(100) films were grown in situ by MBE Fe flux rate of 1 Å/min at a substrate temperature of 250 ◦C. on MgO(100) substrates [17–20]. The Fe3O4 single crystal was grown by the floating-zone method and cut and polished Moreover, a 1-ML Te film was deposited on an Fe film with a / ◦ into substrates with the surface normal along the (111) direc- Te flux rate of 4 Å min at a substrate temperature of 220 C. The spectra were recorded in the total electron yield mode at tion. The Fe3O4 (111) substrates were then loaded into the MBE chamber and annealed at 250 ◦C for 2 h in an oxygen around 80 and 300 K. All spectra have their photon energies −5 aligned using reference samples measured simultaneously (Cr pressure of 1 × 10 mbar. Nonmagnetic Al2O3(0001) sub- strates were also prepared using the same cleaning procedure L23 edges of Cr2O3 for Te M45 edges; Fe2O3 for Fe L23 edges). as for the YIG substrates. A magnetic field of about 0.28 T was applied for the XMCD measurements. Bi2Te3 films were grown on the magnetic oxides using MBE with a base pressure of about 2 × 10−10 mbar [21]. Bi and Te were evaporated from LUXEL Radak effusion cells III. RESULTS AND DISCUSSION at temperatures of about 458 and 228 ◦C, respectively. The flux rates were measured by a quartz crystal monitor at the Figure 1 shows the reflection high-energy electron diffrac- tion (RHEED) patterns of various magnetic oxides and the growth position. The Bi flux rate was set at 0.5 Å/min, while Bi Te films deposited on top of those. All the magnetic the Te flux rate was kept at 1.5 Å/min, giving a typical 2 3 substrates showed good and sharp RHEED patterns after the growth rate of Bi Te of about 0.15 quintuple layer per minute 2 3 in situ annealing, providing a very good starting condition for (∼0.15 QL/min). We first deposited 2 QLs of Bi2Te3 at ◦ the growth of the Bi2Te3 films. 2 QLs of Bi2Te3 were grown room temperature and then annealed the film at 240 Cfor ◦ at room temperature and annealed at 240 C for 30 min. The 30 min to crystallize the material [22]. For the second step, we RHEED patterns showed streaky lines (except 2-QLs Bi Te used 220 ◦C as the substrate temperature to grow the Bi Te 2 3 2 3 on TmIG), indicating a flat surface and good crystallinity to the desired thickness (6–10 QLs).
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