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.
Irradiation with high-energy electrons (HEE) at cryogenic temperatures is a subtle tool for shaping matter. Unlike irradiation with heavy particles, e.g. protons, neutrons, or ions, HEE irradiation produces very low local damage generating exclusively point lattice defects. In the interaction process, the primary high-energy electron transfers a minute quantity of energy to a lattice ion, just enough for displacing it from its lattice site. The concentration of induced vacancies depends on the irradiation dose and in this way can be carefully adjusted. Since the lattice defects can act as donor or acceptor states in semiconductors, electron irradiation enables accurately-controlled compensation of electrically-active impurities introduced in a semiconductor crystal during growth. In this article, we present a study of the evolution of electronic properties of β-gallium oxide with step-by-step compensation of initial n-type doping through controlled introduction of point defects (gallium vacancies) produced by a 2.5-MeV electron beam. Our analysis relies on a set of electron paramagnetic resonance, luminescence, and transport data obtained at different temperatures.
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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