Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructures

Abstract

We demonstrate the van der Waals epitaxy of the topological insulator compound Bi 2 Te 3 on the ferromagnetic insulator Cr 2 Ge 2 Te 6 .The layers are oriented with (001 Introducing a magnetic exchange gap into the Dirac spectrum of a topological insulator (TI) surface generates circumferential, half-integer quantum Hall states. 1-4Recently, independent experimental groups 5-7 have detected the quantum anomalous Hall effect (QAHE) in thin films of magnetically doped topological insulators, in which the upper and lower surfaces are ferromagnetically gapped. However, the host TI materials are disordered due to the extreme concentration of magnetic dopants, which may contribute to the milli-Kelvin temperature scale at which the QAHE is observed and the reported sensitivity of the effect to fabrication details.5 In contrast, introducing ferromagnetism into the TI surface by proximity to a ferromagnetic insulator (FI) leaves the TI ordered, and can break time reversal symmetry without directly alloying the TI with magnetic elements. To date, epitaxy of TI-FI systems has been investigated using the FI materials EuS, GdN, and Y 3 Fe 5 O 12 .8-11 As compared to the layered TI crystal structure, these FI compounds possess 3D structures (e.g. the NaCl-type structure in the case of EuS) producing radicals at a TI-FI interface. To create a pristine interface, a FI compound is sought with a stable, lattice-matched surface. The strong anisotropy of the Bi 2 Te 3 crystal structure causes films to grow in the [001] direction, further constraining the problem by requiring a match to that lattice plane.

Interface-induced phenomena in magnetism

“…Heterostructures of topological insulators with ferromagnetic insulators, such as Bi 2 Se 3 /EuS, (Bi,Sb) 2 Te 3 /GdN, Bi 2 Se 3 /YIG or Bi 2 Se 3 /Cr 2 Ge 2 Te 4 , break time-reversal symmetry while still preserving topological symmetry [Wei et al ., 2013; Yang et al ., 2013; Kandala et al ., 2013; Richardella et al ., 2015; Lang et al ., 2014; Alegria et al ., 2014]. Ideally, transport in such bilayers would be entirely carried by the Dirac surface, modified by exchange coupling with the ferromagnetic layer.…”

Electrically tunable exchange splitting in bilayer graphene on monolayer Cr2X2Te6 with X = Ge, Si, and Sn

“…Several ferromagnetic insulators (FMI), like EuO and Yttrium-Iron-Garnet, have been considered in the context of graphene spintronics [22,[31][32][33][34]. However transition-metal trichalcogenides Cr 2 Si 2 Te 6 , Cr 2 Ge 2 Te 6 (CGT), and transition-metal trihalides CrI 3 , and their monolayers are now extensively discussed in literature as promising materials for low-dimensional spintronics because of their FMI ground state properties [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]. These materials have attracted attention as substrates for topological insulators (TI) [49,50] and graphene [27], because ferromagnetic coupling is present.…”

Reverse Stark effect, anomalous optical transitions, and control of spin in topological insulator quantum dots

“…From the existing experimental works 6,19-23 , we have that the non-uniform mass confinement can be implemented via a closed magnetic heterostructure of ferromagnetic insulators such as EuS, GdN, YIG or Y 3 Fe 5 O 12 , and Cr 2 Ge 2 Te 6 , with different magnetizations of each side deposited on the surface of a 3D topological insulator. Depending on the local exchange coupling of the topological insulator and ferromagnetic insulator as well as the interface quality between them, the magnitude of the induced mass potential can be several to hundred meV 19,21,23 . Interestingly, the sign of the induced mass is tunable by changing the direction of the magnetization in the ferromagnetic insulator caper layer, which can be achieved by means of the anisotropic feature of the materials 6,22 .…”