Development of ferromagnetism in the doped topological insulator Bi _2-x Mn_xTe₃

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 Bi _2-xMn_xTe₃, 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 Bi _1.96 Mn_0.04 Te₃ and Bi_1.91 Mn _0.09 Te₃, a second-order ferromagnetic transition is observed, with T_C ∼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 Bi_2-x Mn_xTe₃ and that the dispersion relations of the surface states are changed in a subtle fashion.