Proximity effect between a topological insulator and a magnetic insulator with large perpendicular anisotropy

Abstract

We report that thin films of a prototype topological insulator, Bi 2 Se 3 , can be epitaxially grown onto the (0001) surface of BaFe 12 O 19 (BaM), a magnetic insulator with high Curie temperature and large perpendicular anisotropy. In the Bi 2 Se 3 thin films grown on non-magnetic substrates, classic weak antilocalization (WAL) is manifested as cusp-shaped positive magnetoresistance (MR) in perpendicular magnetic fields and parabolashaped positive MR in parallel fields, whereas in Bi 2 Se 3 /BaM heterostructures the low field MR is parabolashaped, which is positive in perpendicular fields and negative in parallel fields. The magnetic field and temperature dependence of the MR is explained as a consequence of the suppression of WAL due to strong magnetic interactions at the Bi 2 Se 3 /BaM interface.The surface of a three-dimensional topological insulator (TI) hosts a fascinating Dirac electron system with momentum locked to real electron spins, 1,2 in contrast to the valley-related pseudospins in graphene.3 The helical spin structure has been exploited theoretically as the basis for realizing topological magnetoelectric effects and spintronic applications. [4][5][6][7][8][9][10][11][12] In many proposals, a key ingredient is to open an energy gap near the Dirac point via the proximity effect between a TI and a magnetic insulator (MI). In case of magnetization of the MI parallel to the interface, obtaining a sizable gap would require significant Fermi surface warping.13,14 In contrast, an MI with out-of-plane magnetic order can break time reversal symmetry, thereby opening a large energy gap on any TI surface as long as the interfacial exchange interaction is sufficiently strong. Unfortunately, the easy magnetization axis in most known MIs, such as ferro- 23-27 It is highly insulating and has a Curie temperature of 723 K.27 In this work, the flat (0001) surfaces of nearly hexagon-shaped single crystalline thin plates (Fig. 1a), were used as the substrates for epitaxial growth of Bi 2 Se 3 thin films. Fig. 1b shows magnetization curves of a typical BaM sample with magnetic field H applied perpendicular and parallel to the (0001) plane at T =2 K. The magnetization M reaches saturation at µ 0 H=0.5 T and 1.75 T for perpendicular and parallel field orientations, respectively. For both orientations, M has a nearly linear dependence on H below the saturation. These features are in agreement with those previously reported for high quality single crystals.23,25 The large perpendicular anisotropy, the simple M -H relationship, and the high Curie temperature make BaM a valuable platform for investigation of the interfacial interactions between TIs and magnetic materials. Furthermore, the large remnant magnetization in some specially engineered BaM thin films 28 could be very useful for pursuing topological magnetoelectric effects without external magnetic fields. Fig. 1c is a high resolution cross-section transmission electron microscopy (TEM) image of a Bi 2 Se 3 /BaM heterostructure. It shows that the 1 nm thick Se-Bi-Se...

Large Spin‐Valley Polarization in Monolayer MoTe2 on Top of EuO(111)

“…An alternative approach of inducing spin polarization is given by the magnetic proximity effect, which has been studied intensively, both theoretically and experimentally, in topological insulators and magnetic insulator heterostructures . It has been proposed that proximity to a magnetic material will induce an energy gap in the surface states of a topological insulator and thus result in a quantum anomalous Hall phase.…”

The dimensional crossover of quantum transport properties in few-layered Bi2Se3 thin films

“…15,21,22 If the WAL is broken or in competition with weak localization (WL), then a positive MC or the transition from a negative to a positive MC will arise. The positive MC in TI lms may originate from the magnetic doping of SS or the proximity effect between TI and magnetic materials, 23,24 from the gap opened up in TI ultrathin lms, [25][26][27] from the 2D bulk sub-bands in ultrathin lms, 28,29 from 2DEG tied to a surface band bending effect or from the disorder induced by nonmagnetic doping. 30,31 In the SS of the TI, the spin-momentum locking leads to the destructive interference, and thus the electrons of the SS exhibit WAL, which leads to higher conductivity at lower temperatures.…”

Weak localization effect in topological insulator micro flakes grown on insulating ferrimagnet BaFe12O19

“…Moreover, the topological insulator-ferromagnetic insulator heterostructure is expected to suppress external magnetic impurities and magnetic clusters; therefore, it may be a better experimental candidate to induce the gap for the topological surface states. A number of heterostructures have been studied 29 30 31 32 33 34 with different ferromagntic insulator substrates, such as EuS 30 31 , yttrium iron garnet 29 33 , GdN 32 and BaFe 12 O 19 (BaM) 34 . In the experiments, only a suppressed weak antilocalization effect with a negative magnetoconductivity was achieved.…”

“…However, it is not sufficient to conclude that the magnetic proximity has indeed opened the gap since the finite-size effect can also open gaps in thin films 35 and leads to the weak localization effect 22 . Very recently, a low-field positive magnetoconductivity was observed in a Bi 2 Se 3 -BaM heterostructure in parallel magnetic fields 34 , but the perpendicular magnetoconductivity remains negative. Domain walls may be the possible origins of the positive parallel-field magnetoconductivity in very weak parallel fields.…”