Reasearch: Metamagnetism of weakly coupled antiferromag­netic topological insulators

3D topological insulators ideally have an insulating bulk and 2D gapless topological surface states (TSS). In magnetic materials, time-reversal symmetry is broken by the internal exchange field, so that an energy gap opens for the TSS, resulting in novel topological states. For a homogenous magnetization, it realizes a Chern insulator or the quantum anomalous Hall state (QAH), with dissipationless spin-polarized transport at edges only, or a Weyl semimetal phase. For more complex magnetic structures, additional symmetries can generate other topological states. For instance, antiferromagnets are candidates to realize the axion insulator phase. In general, the magnetization becomes an easily tunable parameter with some potential to modify topological electronic phases by applying small external magnetic fields.

Recently, van der Waals multilayers of 2D ferromagnets have raised specific interest, with the possibility of tailoring multilayers of exchange-coupled 2D ferromagnets having a nontrivial band structure. A unique example is the so-called MBT family, [MnBi₂Te₄][Bi₂Te₃]_n with the integer n ≥ 0, that ideally realizes stoichiometric magnetic topological insulators. The magnetic base unit, a single MnBi₂Te₄ septuple layer, is a 2D ferromagnet with a perpendicular anisotropy K_U that stabilizes an out-of-plane ferromagnetic order and generates the QAH state. Stacks of septuple layers form the MnBi₂Te₄ compound, with an antiferromagnetic interlayer coupling leading to 3D antiferromagnetic order. Other compounds have n units of the nonmagnetic Bi₂Te₃ spacer in between 2D ferromagnetic layers and therefore a reduced interlayer exchange coupling.

We studied the magnetic properties of the van der Waals magnetic topological insulators MnBi₂Te₄ and MnBi₄Te₇ by magnetotransport measurements. We evidence that the relative strength of the interlayer exchange coupling to the uniaxial anisotropy K_U controls a transition from an A-type antiferromagnetic order to a ferromagnetic-like metamagnetic state. A bilayer Stoner-Wohlfarth model allows us to describe this evolution, as well as the typical angular dependence of specific signatures, such as the spin-flop transition of the uniaxial antiferromagnet and the switching field of the metamagnet. These results are a major advance in the search for new classes of topological materials, in particular to search for the QAH phase above 1K, which requires to control the micromagnetic configuration of thin films. Thanks to its versatile magnetic and electronic properties, the MBT family is a unique material platform for the realization of tunable topological quantum phenomena.

A. Tan, V. Labracherie, N. Kunchur, A.U.B. Wolter, J. Cornejo, J. Dufouleur, B. Büchner, A. Isaeva, R. Giraud,
Metamagnetism of Weakly Coupled Antiferrimegnetic Topological Insulators,
Phys. Rev. Lett. 124, 197201 (2020)