A research team led by Monash University has found that a structure containing an ultra-thin topological insulator between two 2D ferromagnetic insulators becomes a large-bandgap quantum anomalous Hall insulator.

Such heterostructures offer an opportunity towards viable ultra-low energy future electronics, or even topological photovoltaics.

**Topological Insulators: Filling in Sandwiches**

In the researchers’ new heterostructure, a ferromagnetic material forms the “bread” of the sandwich, while a topological insulator (i.e., a material exhibiting nontrivial topology) takes the place of the “filling”.

The combination of magnetism and non-trivial band topology gives rise to quantum anomalous Hall (QAH) insulators, as well as exotic quantum phases such as the QAH effect, where current flows without dissipation along quantized edge states.

Inducing magnetic ordering in topological insulators through the proximity of a magnetic material provides a promising route towards achieving the QAH effect at high temperatures (close to or higher than room temperature) for lossless transport applications.

A promising architecture consists of a sandwich structure consisting of two single layers of MnBi_{2}Feather_{4} (a 2D ferromagnetic insulator) ultra-thin Bi . either side of_{2}Feather_{3} in the middle (a topological insulator). This structure is predicted to yield a strong QAH insulator phase with a bandgap above the thermal energy available at room temperature (25 meV).

The new study led by Monash found MnBi. demonstrated the development of_{2}Feather_{4} / with_{2}Feather_{3} /mnbi_{2}Feather_{4} The heterostructure via molecular beam epitaxy, and investigated the electronic structure of the structure using angle resolved photoelectron spectroscopy.

Lead author Monash Ph.D. Candidate Kill Lee.

The magnetic origin of the gap was confirmed by observing the vanishing bandgap above the Curie temperature as well as the broken time-reversal symmetry and the Exchangeâ€“Rashba effect, in excellent agreement with density functional theory calculations.

“These findings provide insight into magnetic proximity effects in topological insulators, which will lead to lossless transport in topological insulators towards higher temperatures,” said Monash group leader and lead author Dr. Mark Edmonds says

**how it works**

2D MnBi_{2}Feather_{4} Ferromagnets induce magnetic ordering (that is, an exchange interaction with 2d Dirac electrons) in the ultra-thin topological insulator Bi._{2}Feather_{3} through magnetic proximity.

This creates a large magnetic gap, in which the heterostructure becomes a quantum anomalous Hall (QAH) insulator, such that the material becomes metallic (i.e. electrically conducting) along its one-dimensional edges, while electrically conducting in its interior. remains insulating. The near-zero resistance along the 1D edges of the QAH insulator make it such a promising path toward next-generation, low-energy electronics.

To date, several strategies have been used to realize the QAH effect, such as introducing thin amounts of magnetic dopants into ultrathin films of 3D topological insulators. However, introducing magnetic dopants into the crystal lattice can be challenging and result in magnetic disorder, which greatly suppresses the temperature at which the QAH effect can be observed and limits future applications.

Instead of incorporating 3D transition metals into the crystal lattice, a more advantageous strategy is to place two ferromagnetic materials on the upper and lower surfaces of a 3D topological insulator. This breaks the time-reversal symmetry in topological insulators with magnetic order, and thus opens a bandgap in the surface state of the topological insulator and gives rise to a QAH insulator.

**making the right kind of sandwich**

Nevertheless, inducing enough magnetic order to open a large gap via magnetic proximity effects is challenging due to the undesired effect of abrupt interface potential arising due to lattice mismatch between magnetic materials and topological insulators.

“To reduce the interface potential while inducing magnetic ordering through proximity, we needed to find a 2D ferromagnet that had chemical and structural properties similar to 3D topological insulators,” says Qile Li, a Ph.D. is also. student with the Australian Research Council Center for Excellence in Future Low-Energy Electronic Technologies (FLEET).

“In this way, instead of an abrupt interface potential, there is a magnetic expansion of the topological surface state in the magnetic layer. This strong interaction results in a significant exchange split in the topological surface state of the thin film and opens up a large gap,” says Lee. Huh.

Intrinsic magnetic topological insulator MnBi. single-septupal layer of_{2}Feather_{4} Particularly promising, as it is a ferromagnetic insulator with a Curie temperature of 20 K.

“More importantly, this setup is structurally similar to the well-known 3D topological insulator Bi._{2}Feather_{3}with a mesh mismatch of only 1%,” says Dr. Mark Edmonds, an associate investigator at FLEET.

The research team traveled to the Advanced Light Sources portion of the Lawrence Berkeley National Laboratory in Berkeley, US, where they developed ferromagnet/topological/ferromagnet heterostructures and investigated their electronic band structure in collaboration with beamline staff scientist Dr. Sung-Kwan Mo.

“Although we cannot directly observe the QAH effect using angle-resolved photoemission spectroscopy (ARPES), we can use this technique to probe the size of the bandgap opening, and then confirm that it is the original is essentially magnetic,” says Dr. Edmonds.

“Using angle-resolved photoemission we can also investigate hexagonal warping at surface conditions. It turns out that the warping strength in the Dirac fermion in our heterostructure is about twice that of Bi._{2}Feather_{3}says Dr. Edmondso

The research team was also able to confirm the electronic structure, the size of the gap, and the temperature at which this MnBi_{2}Feather_{4}/with_{2}Feather_{3}/mnbi_{2}Feather_{4} The heterostructure is likely to support the QHE effect by combining experimental ARPES observations with magnetic measurements to determine the Curie temperature (performed by Dr. David Corti, Fleet associate investigator at the University of Wollongong) and first-principles density functional theory calculations performed by the group. Dr. Shengyuan Yang (Singapore University of Technology and Design).

The growth of this heterostructure was initially detected at the Edmonds Electronic Structure Laboratory at Monash University. Later, heterostructure films were grown and characterized using ARPES measurements at the Advanced Light Source (Lawrence Berkeley National Laboratory) in California.

The study, “Large magnetic gap in a designer ferromagnet-topological insulator-ferromagnet heterostructure,” was published in *advanced Materials* in March 2022.

Electrons at the edge: the story of an intrinsic magnetic topological insulator

**more information:**

Qile Li et al, Large Magnetic Gap in a Designer Ferromagnet-Topological Insulator-Ferromagnet Heterostructure,

*advanced Materials*(2022). DOI: 10.1002/adma.202107520

FLEET. provided by

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