Magnetic topological insulators are a new class of materials that combine magnetism with topology, which leads to exotic quantum phenomena such as the quantum anomalous Hall effect and the axion insulator phase. Of the magnetic topological insulators, those with MnBi2Te4 magnetic septuple layers self-assembled in a non-magnetic topological Bi2Te3 host material are of particular interest and have recently been extensively studied. Here, we present an overview of our recent advances in understanding the influence of several factors such as the ordering of Mn impurities, omnipresent magnetic disorder, and the position of the Fermi level on ferromagnetism and magnetotransport in such systems. In particular, the consequences of these effects for observation or lack of the quantized anomalous Hall effect are discussed. Both theoretical and experimental research on these issues is crucial for gaining controllable access to the quantum anomalous Hall effect and other spintronic phenomena, which have potential applications in low-power consumption electronic devices, data storage, and quantum computing.
A platform where magnetism meets topology opens up wide possibilities for implementation of new ideas, such as the quantum anomalous Hall effect, the magnetoelectric effect, the axion insulator state. Among magnetic topological insulators, the MnBi2Te4/(Bi2Te3)n family has recently attracted a special interest. Here, the classic Bi2Te3 topological insulator is combined with a compatible magnetic MnBi2Te4. Interestingly, the MnBi2Te4/(Bi2Te3)n superlattices with different spacing (n) between the MnBi2Te4 septuplet layers self-organize both structurally and magnetically during standard (e.g. Bridgman) crystal growth. Mn in MnBi2Te4 is oriented planarly, forming a 2D ferromagnet with the out-of-plane easy axis. The interplay between magnetism of this highly-ordered layer and the topology of Bi2Te3 is manifested in the high-temperature quantum anomalous Hall effect (Nature Physics (2020), DOI: 10.1038/s41567-020-0998-2) due to breaking down the time-reversal symmetry. In this communication I will discuss the influence of the MnBi2Te4 septuplet layer on the electronic structure of topological surface states, which is very broad due to possible different surface terminations of MnBi2Te4/(Bi2Te3)n superlattice. This problem is currently controversial due to many, often contradictory literature reports, and the assignment of the bands is still under discussion. Next, I will present the complex magnetism of MnBi2Te4/(Bi2Te3)n, which consists of both intra-layer interactions between Mn spins in MnBi2Te4 and inter-layer interactions between individual septuplet layers. Understanding magnetism and its effect on surface states is critical to applying the material to new phenomena. We would like to acknowledge NCN (Poland) grant no 2016/21/B/ST3/02565.
Topological insulators (TI) belong to category of phases which go beyond the theory of spontaneous symmetry breaking, well describing classical phases. TI are materials of strong spin-orbit interaction that leads to the inversed band structure. Thus, they belong to different topological class than surrounding “normal” world. Consequently, these materials behave as insulators in their volume while their surface hosts metallic states, that appear as a result of the need to meet boundary conditions. The metallic states have the unusual spin structure described by the Dirac-type Hamiltonian, with the electron spin locked to its momentum. They are protected by the time reversal symmetry, thus are resistant to non-magnetic disturbances. Introducing magnetic impurities breaks the time reversal symmetry, opening the energy gap at the Dirac point and eventually modifying spin texture. In research of magnetically doped TI there are still many challenges and open questions. Here, I will present results of our recent studies of three-dimensional TI from the Bi2-xSbxTe3-ySey family, doped with Mn ions. I will discuss possible locations of Mn impurity in the crystal host lattice, the influence of doping on the crystal structure and magnetic properties. Ferromagnetism was successfully obtained in Bi2Te3 and BiSbTe3 doped with 1.5-2 at. % of Mn, with the Curie temperature of the order of ~ 15 K. The role of free carriers in ferromagnetic interactions is not clear. Ferromagnetism is observed at diluted Mn concentrations suggesting a need for a medium mediating the long-range ferromagnetic order, but the Tc does not scale with the concentration of free carriers.
We would like to acknowledge National Science Center, Poland, grant no 2016/21/B/ST3/02565.
Recent theoretical predictions confirmed by experimental observations provided evidence that there exist materials which behave as insulators in the bulk but possess gapless, spin-momentum-locked, linearly dispersed states on the surface. They are called topological insulators (TI). The conducting surface states of TIs are immune to localization as long as the disorder potential does not violate time reversal symmetry. One way to break the time reversal symmetry is to introduce magnetic dopants into the TIs that can induce ferromagnetism and open the surface energy gap. Opening a gap at the topological surface may result in exotic quantum phenomena including magnetoelectric effect and quantized anomalous Hall effect.
In this work, we studied magnetic and electrical properties of the bismuth telluride doped with 2 % of Mn atoms. Ferromagnetic resonance (FMR) measurements show two resonance lines with different spin relaxation times, which we assigned to Mn2+ ions located at different lattice sites. Hall resistance measurements reveal that below 15 K the curve becomes hysteretic that is typical for ferromagnetic conductors. Hall as well as FMR demonstrate that the Curie temperature of the studied sample is between 10 and 15 K. Furthermore, the electric transport measurements reveal n-type conductivity indicating that Mn atoms may occupy interstitial position in van der Waals gaps. Magnetoresistance data show weak localization effect which may be one of the signature of the gap opening on the topological surface, other possible explanations related to the crystal structure will be also discussed.
We would like to acknowledge National Science Center, Poland, grant no 2016/21/B/ST3/02565.
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