Carrier density And Thickness Dependent Proximity Effect In Doped Topological insulator - Metallic Ferromagnet Bilayers

We use magneto-conductivity to study magnetic proximity effect on surface states of doped topological insulators. Our bilayers consists of a layer of Fe$_7$Se$_8$, which is a metallic ferrimagnet and a layer of Bi$_{0.8}$Sb$_{1.2}$Te$_{3}$ which is a highly hole-doped topological insulator. Using transport measurements and a modified Hikami-Larkin-Nagaoka model, we show that the ferromagnet shortens significantly the effective coherence length of the surface states, suggesting that a gap is opened at the Dirac point. We show that the magnetically induced gap persists on surface states which are separated from the magnet by a topological insulator layer as thick as 170nm. Furthermore, the size of the gap is found to be proportional to the magnetization that we extract from the anomalous Hall effect. Our results gives information on the ties between carrier density, induced magnetization and magnetically induced gap in topological insulator/ferromagnet bilayers. This information is important both to theoretical understanding of magnetic interactions in topological insulators and to the practical fabrication of such bilayers, which are the basis of various suggested technologies which depends on these interactions, such as spintronic devices, far infra-red detectors etc.