Magnetic topological insulators (Conference Presentation)

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.