# Do topology and ferromagnetism cooperate at the EuS/Bi$_2$Se$_3$ interface?

## TL;DR

This study investigates the magnetic and electronic properties at EuS/Bi$_2$Se$_3$ interfaces, finding that local magnetic fields are similar regardless of topological nature, and revealing interface-induced modifications in the electronic structure.

## Contribution

It provides the first detailed comparison of magnetic properties at EuS/topological insulator and EuS/metal interfaces, and uncovers interface-induced electronic modifications using SX-ARPES.

## Key findings

- Strong local magnetic fields extend into both Bi$_2$Se$_3$ and titanium interfaces.
- Magnetic fields are mostly independent of the topological properties.
- Evidence of interface-induced modifications in Bi$_2$Se$_3$ wave functions.

## Abstract

We probe the local magnetic properties of interfaces between the insulating ferromagnet EuS and the topological insulator Bi$_2$Se$_3$ using low energy muon spin rotation (LE-$\mu$SR). We compare these to the interface between EuS and the topologically trivial metal, titanium. Below the magnetic transition of EuS, we detect strong local magnetic fields which extend several nm into the adjacent layer and cause a complete depolarization of the muons. However, in both Bi$_2$Se$_3$ and titanium we measure similar local magnetic fields, implying that their origin is mostly independent of the topological properties of the interface electronic states. In addition, we use resonant soft X-ray angle resolved photoemission spectroscopy (SX-ARPES) to probe the electronic band structure at the interface between EuS and Bi$_2$Se$_3$. By tuning the photon energy to the Eu anti-resonance at the Eu $M_5$ pre-edge we are able to detect the Bi$_2$Se$_3$ conduction band, through a protective Al$_2$O$_3$ capping layer and the EuS layer. Moreover, we observe a signature of an interface-induced modification of the buried Bi$_2$Se$_3$ wave functions and/or the presence of interface states.