“Magnetic Sandwich” Promises New Pathways in Lossless Electronics
Researchers have found a way to achieve a quantum anomalous Hall effect at elevated temperatures—potentially building new inroads to lossless electronics.
While the Hall effect is common terminology in an engineer's vernacular, the quantum anomalous Hall (QAH) effect may be a new concept. The QAH, a result of experiments on ferromagnetic materials, is a quantized Hall effect in the absence of an external magnetic field. In realizing the QAH effect, the coupling between intrinsic magnetization and spin-orbit coupling leads to the quantization of anomalous Hall effect conductivity.
The phase diagram of the Haldane model of the QAH effect. Image used courtesy of Nadeem et al.
The first model of the QAH effect was proposed by Duncan Haldane in 1988. Haldane’s model of the QAH effect includes a graphene lattice. The time-reversal symmetry is broken by a periodic magnetic field with complex matrix elements under a zero-net magnetic flux. Thanks to the Dirac-cone-shaped gapless band structure of graphene, the time-reversal symmetry makes a gap at the Dirac point to turn graphene into an insulator that has a unity Chern number.
Ever since Haldane proposed his QAH effect model, the main challenge faced by researchers centers on the lack of suitable materials to meet industry requirements. However, in recent years, researchers have devised methods to realize the QAH effect in viable systems. One such method involves inducing magnetic order in topological insulators through proximity to magnetic material. Now, researchers at Monash University used this approach to create a QAH effect that may be useful for lossless transport in both electronics and spintronics devices.
Topological Insulators: A Useful Tool for the QAH Effect?
Topological insulators feature conductive surface states that guard against impurities. They do this through topological invariants and host-spin, momentum-locked electron states. An example of a quantum hall insulator is the 3D topological insulator that has a non-trivial bond topology.
Time-reversal symmetry of a magnetic topological insulator is broken when induced by the ferromagnetic material. Image used courtesy of Chang et al.
The surfaces of a 3D topological insulator host-spin polarized, massless Dirac cones that are robust against time-reversal symmetry perturbations. When a long-range magnetic order is induced in a 3D topological insulator, time-reversal symmetry can be broken, which is demonstrated by a magnetic gap opening in the Dirac cones.
This results in a quantum anomalous Hall (QAH) insulator and axion insulator. The emergent QAH insulator is distinguished by dissipationless charge transport and perfect spin polarization. These insulators possess chiral edge states within the bandgap. A QAH effect is achieved when the chemical potential is tuned into the magnetic gap.
Tapping Into a Sandwich Structure
The novel research led by the researchers at Monash University exploited an ultrathin topological insulator sandwiched between two 2D ferromagnetic insulators in an attempt to achieve a QAH effect. The sandwich structure had a 4 quintuple-layer (QL) of bismuth telluride (Bi2Te3), which is a 3D topological insulator, in between two single septuple-layers (SL) of MnBi2Te4. Having two ferromagnetic materials on the top and bottom surfaces of a 3D topological insulator breaks time-reversal symmetry in the topological insulator with magnetic order.
The Dirac surface states of a 3D topological insulator. Image used courtesy of He et al.
Thus, the bandgap in the surface state of the topological insulator is opened. This results in a QAH insulator. The material becomes electrically conducting along its one-dimensional edges while its interior is electrically insulating. According to the researchers, the almost-zero resistance along the 1D edges of the QAH insulator is what makes it such a promising pathway toward next-generation, low-energy electronics.
Crystal structure of Bi2Te3 sandwiched between two MnBi2Te4. Image used courtesy of Advanced Materials
In general, the sandwich structure promises to yield a robust QAH insulator phase with a bandgap well above the thermal energy available at room temperature.
Using angle-resolved photoemission spectroscopy (ARPES), the researchers probed the size of the bandgap opening and discovered strong hexagonally-warped, massive Dirac fermions—and a bandgap of 75 ± 15 meV. It was also observed that the bandgap vanishes above the Curie temperature.
One of the researchers, Dr. Edmonds, however, noted that the QAH effect cannot be directly observed using angle-resolved photoemission spectroscopy (ARPES). The technique could only be employed to probe the size of the bandgap opening, and then confirm if it is magnetic in origin.