Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator

TL;DR

This study investigates how ferromagnetism affects surface states and quantum corrections in magnetically doped topological insulators, revealing complex magnetic behavior and surface state suppression.

Contribution

It provides detailed experimental insights into the magnetic and electronic properties of Mn-doped topological insulator thin films, highlighting surface inhomogeneity and quantum conductance effects.

Findings

Surface ferromagnetism observed between 15 K and 100 K.

Low temperature ferromagnetism below 5 K likely near-surface.

Quantum corrections to conductance align with Dirac gap opening predictions.

Abstract

The breaking of time-reversal symmetry by ferromagnetism is predicted to yield profound changes to the electronic surface states of a topological insulator. Here, we report on a concerted set of structural, magnetic, electrical and spectroscopic measurements of \MBS thin films wherein photoemission and x-ray magnetic circular dichroism studies have recently shown surface ferromagnetism in the temperature range 15 K $\leq T \leq 100$ K, accompanied by a suppressed density of surface states at the Dirac point. Secondary ion mass spectroscopy and scanning tunneling microscopy reveal an inhomogeneous distribution of Mn atoms, with a tendency to segregate towards the sample surface. Magnetometry and anisotropic magnetoresistance measurements are insensitive to the high temperature ferromagnetism seen in surface studies, revealing instead a low temperature ferromagnetic phase at $T \lesssim 5$ K. The absence of both a magneto-optical Kerr effect and anomalous Hall effect suggests that this low temperature ferromagnetism is unlikely to be a homogeneous bulk phase but likely originates in nanoscale near-surface regions of the bulk where magnetic atoms segregate during sample growth. Although the samples are not ideal, with both bulk and surface contributions to electron transport, we measure a magnetoconductance whose behavior is qualitatively consistent with predictions that the opening of a gap in the Dirac spectrum drives quantum corrections to the conductance in topological insulators from the symplectic to the orthogonal class.