Carrier density and thickness-dependent proximity effect in doped-topological-insulator-metallic-ferromagnet bilayers

We use magnetoconductivity to study the magnetic proximity effect on surface states of doped topological insulators. Our bilayers consist of a layer of Fe7Se8, which is a metallic ferrimagnet, and a layer of Bi0.8Sb1.2Te3, 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 170 nm. Furthermore, the size of the gap is found to be proportional to the magnetization that we extract from the anomalous Hall effect. Our results give information on the ties between carrier density, induced magnetization, and magnetically induced gap in topological-insulator-ferromagnet bilayers. This information is important both for a theoretical understanding of magnetic interactions in topological insulators and for the practical fabrication of such bilayers, which are the basis of various suggested technologies, such as spintronic devices, far-infrared detectors, etc.