Fluctuation-induced N\'eel and Bloch skyrmions at topological insulator surfaces

TL;DR

This paper theoretically demonstrates that Dirac fermion fluctuations at topological insulator-ferromagnet interfaces induce effective interactions that can stabilize skyrmions even above the magnetic ordering temperature.

Contribution

It introduces a theoretical framework showing how fluctuation-induced interactions lead to skyrmion formation at topological insulator surfaces above magnetic ordering temperatures.

Findings

Effective Dzyaloshinskii-Moriya interaction emerges from Dirac fermion fluctuations.

Skyrmions and skyrmion lattices can form at TI-ferromagnet interfaces.

Skyrmion formation occurs above the magnetic ordering temperature.

Abstract

Ferromagnets in contact with a topological insulator have become appealing candidates for spintronics due to the presence of Dirac surface states with spin-momentum locking. Because of this bilayer Bi$_2$Se$_3$-EuS structures, for instance, show a finite magnetization at the interface at temperatures well exceeding the Curie temperature of bulk EuS. Here we determine theoretically the effective magnetic interactions at a topological insulator-ferromagnet interface {\it above} the magnetic ordering temperature. We show that by integrating out the Dirac fermion fluctuations an effective Dzyaloshinskii-Moriya interaction and magnetic charging interaction emerge. As a result individual magnetic skyrmions and extended skyrmion lattices can form at interfaces of ferromagnets and topological insulators, the first indications of which have been very recently observed experimentally.