Development of ferromagnetism in the doped topological insulatorBi2−xMnxTe3

The development of ferromagnetism in Mn-doped ${\text{Bi}}_{2}{\text{Te}}_{3}$ 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 ${\text{Bi}}_{2\ensuremath{-}x}{\text{Mn}}_{x}{\text{Te}}_{3}$, 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 ${\text{Bi}}_{1.96}{\text{Mn}}_{0.04}{\text{Te}}_{3}$ and ${\text{Bi}}_{1.91}{\text{Mn}}_{0.09}{\text{Te}}_{3}$, a second-order ferromagnetic transition is observed, with ${T}_{C}\ensuremath{\sim}9--12\text{ }\text{K}$. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the ${\text{Bi}}_{2}{\text{Te}}_{3}$ basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped ${\text{Bi}}_{2}{\text{Te}}_{3}$ crystals are $p$-type. Angle-resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine ${\text{Bi}}_{2}{\text{Te}}_{3}$ are also present at 15 K in ferromagnetic Mn-doped ${\text{Bi}}_{2\ensuremath{-}x}{\text{Mn}}_{x}{\text{Te}}_{3}$ and that the dispersion relations of the surface states are changed in a subtle fashion.