Development of Ferromagnetism in the Doped Topological Insulator Bi₂₋ₓ MnₓTe₃

The development of ferromagnetism in Mn-doped Bi2Te3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type Bi2-xMnxTe3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions Bi1.96 Mn0.04 Te3 and Bi1.91 Mn0.09 Te3, a second-order ferromagnetic transition is observed, with TC ~9-12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2 Te3 basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped Bi2Te3 crystals are p -type. Angle-resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine Bi2 Te3 are also present at 15 K in ferromagnetic Mn-doped Bi2-x MnxTe3 and that the dispersion relations of the surface states are changed in a subtle fashion