Emerging weak localization effects on a topological insulator–insulating ferromagnet (Bi2Se3-EuS) interface

Abstract

Thin films of topological insulator Bi2Se3 were deposited directly on insulating ferromagnetic EuS. Unusual negative magnetoresistance was observed near the zero field below the Curie temperature (TC), resembling the weak localization effect; whereas the usual positive magnetoresistance was recovered above TC. Such negative magnetoresistance was only observed for Bi2Se3 layers thinner than t ∼ 4nm, when its top and bottom surfaces are coupled. These results provide evidence for a proximity effect between a topological insulator and an insulating ferromagnet, laying the foundation for future realization of the half-integer quantized anomalous Hall effect in three-dimensional topological insulators.A topological insulator (TI) has a full energy gap in the bulk, and contains gapless surface states that cannot be destroyed by any non-magnetic impurities. Because of time reversal symmetry, the surface states cannot be back-scattered by non-magnetic impurities. 1,2 When a thin magnetic layer is applied on the surface, a full insulating gap is opened, and an electric charge close to the surface is predicted to induce an image magnetic monopole. 3,4 Probably the most extensively studied threedimensional TI (3D-TI) has been bismuth-selenide (Bi 2 Se 3 ), 5-7 exhibiting crystal structure that consists of atomic quintuple layers (QLs), with three QLs forming a unit cell. As made, uncompensated samples typically have a Fermi level above the Dirac point and intersecting the bulk conduction band. 8,9 In particular, low temperature transport measurements on ungated and uncompensated TI films show positive magnetoresistance (MR) at low magnetic fields and in a wide range of film thicknesses. [10][11][12] This was explained in terms of weak antilocalization (WAL) that results from spin-momentum locking on the surface state Dirac cone. 10,13 While the inability to account for the bulk bands (presumably because of their low mobility) has challenged this simple assignment, the discovery of weak localization (WL) effects at higher fields 12,14 and the ability to accurately separate quantum oscillation effects 15 in high-quality films may provide a first step towards a more comprehensive understanding of transport in these systems.By adopting topologically non-trivial Hamiltonians to describe these materials, various recent theories predict WL or negative MR near the zero field in a TI as a result of gap-opening at its surface state Dirac point. The negative MR may arise from the surface state when the Fermi level is sufficiently close to the top of the gap. 16 Alternatively, it can also be produced by bulk conduction and can only be observed when the surface conduction is sufficiently suppressed. 17,18 Such a phenomenon was reported in cases of gated ultra-thin Bi 2 Te 3 films 19 and magnetically doped Bi 2 Se 3 films. 20 To further elucidate the uniqueness of transport in the surface state of TI materials, and as an initial step towards realizing half-integer quantized anomalous Hall effect (QAHE) and other applications, we studied ...

Growth and characterization of hybrid insulating ferromagnet-topological insulator heterostructure devices

“…However, electrical transport experiments require an insulating ferromagnet to ensure current paths that flow solely through the TI. Recent advances in this context have exploited the synthesis of EuS/Bi 2 Se 3 heterostructures [13,14]. In this Letter, we demonstrate an alternative scheme towards such "magnetic gating" proposals by creating hybrid electrical transport devices wherein we interface the insulating FM GdN with a TI (Bi 2 Se 3 ).…”

Breaking time reversal symmetry in topological insulators

“…27 With similar objectives, Yang et al 28 and Kandala et al 29 at H C coincides with the occurrence of resistance minima (or higher electrical conduction). This unconventional behavior was attributed to the isotropic distribution of domains and hence the observed isotropic planar MR. Interestingly, the Dirac SSs along the domain wall were theoretically predicted to have zero mass carrying chiral conduction modes similar to the quantum Hall edge channels, 1,2 in agreement with the experimental observations of the MR dips (increased conduction) at H C .…”

“…27 Generally, all the FIs have an in-plane easy magnetization axis, although canted magnetization or small perpendicular magnetic hysteresis has been reported. [27][28][29] However, the perpendicular magnetic anisotropy (PMA) can be engineered by choosing a proper interface layer, which is attributed to the hybridization of the electronic states. 34 Strong PMA in transition metal/rare-earth multilayers has been reported.…”

“…In this case, the allimportant advantage is that the TRS breaking happens mostly at the interface between the TI and the FI, rather than affecting the majority bulk states, while also not introducing defects ( Figure 1b). However, in general, thin films of FIs show in-plane easy axis of magnetization, [27][28][29] which hinders the observation of the QAHE or the zero-field half-integer QHE. 7 We provide an overview of the behavior of FI/TI heterostructures and that of magnetically doped Se/Te-based TI compounds as well as bring out their differences.…”

Breaking time-reversal symmetry at the topological insulator surface by metal-organic coordination networks

“…The latter can be realized by adding magnetic functionality to TI, i.e., either by surface [14][15][16] or bulk [17][18][19][20][21][22] doping with magnetic atoms or bringing the TI in contact with (an) a (anti)ferromagnetic [(A)FM] material [23][24][25][26][27][28][29]. In the presence of FM order with an out-of-plane magnetization the TI energy band structure becomes fully gapped due to the splitting of the Dirac point (DP).…”