Incipient antiferromagnetism in the Eu-doped topological insulator Bi$_2$Te$_3$

TL;DR

This study investigates Eu-doped Bi$_2$Te$_3$, revealing antiferromagnetic ordering at low temperatures and suggesting coexistence of magnetism with topological surface states, which could enable novel quantum effects.

Contribution

It provides combined experimental and theoretical evidence of antiferromagnetism in Eu-doped Bi$_2$Te$_3$, highlighting the potential for magnetic topological insulators with rare-earth doping.

Findings

Eu$^{2+}$ valence confirmed by x-ray absorption spectra

Antiferromagnetic order observed below 10 K

Density functional theory predicts AFM interactions

Abstract

Rare earth ions typically exhibit larger magnetic moments than transition metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet, in a recent photoemission study of Eu-doped Bi$_2$Te$_3$ films, the spectra remained gapless down to $T = 20\;\text{K}$. Here, we scrutinize whether the conditions for a substantial gap formation in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the $M_{4,5}$ x-ray absorption and magnetic circular dichroism spectra reveals a Eu$^{2+}$ valence and confirms a large magnetic moment, consistent with a $4f^7 \; {^8}S_{7/2}$ ground state. At temperatures below $10\;\text{K}$, bulk magnetometry indicates the onset of antiferromagnetic (AFM) ordering. This is in good agreement with density functional theory, which predicts AFM interactions between the Eu impurities. Our results support the notion that antiferromagnetism can coexist with topological surface states in rare-earth doped Bi$_2$Te$_3$ and call for spectroscopic studies in the kelvin range to look for novel quantum phenomena such as the quantum anomalous Hall effect.