Carrier Density and Thickness Dependent Proximity Effect in Doped Topological Insulator -- Metallic Ferromagnet Bilayers

TL;DR

This study investigates how ferromagnetic layers influence the surface states of doped topological insulators, revealing that magnetic proximity induces a gap at the Dirac point, dependent on carrier density and magnetization, with implications for spintronics.

Contribution

It demonstrates that magnetic proximity effects can open a tunable gap in topological insulator surface states, even across thick insulating layers, advancing understanding of magnetic interactions in these materials.

Findings

Magnetic proximity shortens the coherence length of surface states.

A magnetically induced gap persists through 170 nm of topological insulator.

The gap size correlates with the magnetization from the anomalous Hall effect.

Abstract

We use magneto-conductivity to study the magnetic proximity effect on surface states of doped topological insulators. Our bilayers consist 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 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 infra-red detectors etc.