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The breaking of time-reversal symmetry by ferromagnetism is predicted toyield profound changes to the electronic surface states of a topologicalinsulator. Here, we report on a concerted set of structural, magnetic,electrical and spectroscopic measurements of \MBS thin films whereinphotoemission and x-ray magnetic circular dichroism studies have recently shownsurface ferromagnetism in the temperature range 15 K $\leq T \leq 100$ K,accompanied by a suppressed density of surface states at the Dirac point.Secondary ion mass spectroscopy and scanning tunneling microscopy reveal aninhomogeneous distribution of Mn atoms, with a tendency to segregate towardsthe sample surface. Magnetometry and anisotropic magnetoresistance measurementsare insensitive to the high temperature ferromagnetism seen in surface studies,revealing instead a low temperature ferromagnetic phase at $T \lesssim 5$ K.The absence of both a magneto-optical Kerr effect and anomalous Hall effectsuggests that this low temperature ferromagnetism is unlikely to be ahomogeneous bulk phase but likely originates in nanoscale near-surface regionsof the bulk where magnetic atoms segregate during sample growth. Although thesamples are not ideal, with both bulk and surface contributions to electrontransport, we measure a magnetoconductance whose behavior is qualitativelyconsistent with predictions that the opening of a gap in the Dirac spectrumdrives quantum corrections to the conductance in topological insulators fromthe symplectic to the orthogonal class.

Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator