Surface magnetic ordering in topological insulators with bulk magnetic dopants

TL;DR

This paper demonstrates that topological insulators doped with magnetic impurities can exhibit surface magnetic order without bulk magnetism, due to topologically protected surface states, explaining recent experimental observations.

Contribution

It reveals a novel regime where surface and bulk magnetic orders differ in topological insulators, using mean field models to explain experimental phenomena.

Findings

Surface magnetic order can exist without bulk magnetism in topological insulators.

Topologically protected surface states influence magnetic ordering.

Experimental spectral gaps in surface states can occur without bulk magnetism.

Abstract

We show that a three dimensional topological insulator doped with magnetic impurities in the bulk can have a regime where the surface is magnetically ordered but the bulk is not. This is in contrast to conventional materials where bulk ordered phases are typically more robust than surface ordered phases. The difference originates from the topologically protected gapless surface states characteristic of topological insulators. We study the problem using a mean field approach in two concrete models that give the same qualitative result, with some interesting differences. Our findings could help explain recent experimental results showing the emergence of a spectral gap in the surface state of Bi2Se3 doped with Mn or Fe atoms, but with no measurable bulk magnetism.