Two-Stage Proximity-Induced Gap-Opening in Topological Insulator - Insulating Ferromagnet (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ - EuS Bilayers

TL;DR

This study investigates how magnetic proximity effects induce a two-stage gap opening in topological insulator/ferromagnet bilayers, revealing complex magnetic and electronic interactions above the ferromagnet's Curie temperature.

Contribution

It introduces a novel two-stage gap-opening mechanism in topological insulator-ferromagnet bilayers driven by magnetic proximity effects.

Findings

Magnetic anomalies observed between 30K and 60K above EuS Curie temperature.

Resistance increases sharply when Fermi level is near the Dirac point.

Magnetoresistance crossovers occur at Curie temperature related to sheet resistance.

Abstract

To further investigate the interplay between ferromagnetism and topological insulators, thin films of the low-carrier topological insulator (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ were deposited on the insulating ferromagnet EuS (100) in situ. AC susceptibility indicates magnetic anomalies between $T\approx30~\mathrm{K}$ and $T\approx60~\mathrm{K}$, well above the Curie temperature $T_C \approx 15~\mathrm{K}$ of EuS. When the Fermi level is close to the Dirac point and the surface state dominates the electric conduction, sharp increases in resistance with decreasing temperatures were observed concurrently with the magnetic anomalies. Positive-negative magnetoresistance crossovers were observed at the Curie temperature, which seem only to appear when the sheet resistance exceeds the Mott-Ioffe-Regel limit $h/e^2$. A two-stage gap-opening process due to magnetic proximity is proposed.