The breaking of time-reversal symmetry by ferromagnetism is predicted to yield profound changes to the electronic surface states of a topological insulator. Here, we report on a concerted set of structural, magnetic, electrical, and spectroscopic measurements of Mn-Bi${}_2$Se${}_3$ thin films wherein photoemission and x-ray magnetic circular dichroism studies have recently shown surface ferromagnetism in the temperature range $15\text{K}\le T\le 100$ K, accompanied by a suppressed density of surface states at the Dirac point. Secondary-ion mass spectroscopy and scanning tunneling microscopy reveal an inhomogeneous distribution of Mn atoms, with a tendency to segregate towards the sample surface. Magnetometry and anisotropic magnetoresistance measurements are 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 an anomalous Hall effect suggests that this low-temperature ferromagnetism is unlikely to be a homogeneous bulk phase but likely originates in nanoscale near-surface regions of the bulk where magnetic atoms segregate during sample growth. Although the samples are not ideal, with both bulk and surface contributions to electron transport, we measure a magnetoconductance whose behavior is qualitatively consistent with predictions that the opening of a gap in the Dirac spectrum drives quantum corrections to the conductance in topological insulators from the symplectic to the orthogonal class.

• Received 23 June 2012

DOI:https://doi.org/10.1103/PhysRevB.86.205127