Magnetization relaxation and search for the magnetic gap in bulk-insulating V-doped (Bi, Sb)$_2$Te$_3$

TL;DR

This study investigates V-doped (Bi,Sb)$_2$Te$_3$'s magnetic properties and searches for a magnetic gap, finding no gap at low temperatures and revealing magnetization relaxation effects that impact the QAHE.

Contribution

The paper provides the first direct magnetic characterization of V-doped (Bi,Sb)$_2$Te$_3$ and links magnetization relaxation to the absence of a magnetic gap.

Findings

No magnetic gap detected at the Dirac point down to 1 K.

V-doped (Bi,Sb)$_2$Te$_3$ exhibits high coercivity and magnetization relaxation.

Magnetization decay occurs on a minutes timescale, affecting the QAHE.

Abstract

V-doped (Bi,Sb)$_2$Te$_3$ has a ten times higher magnetic coercivity than its Cr-doped counterpart and therefore is believed to be a superior system for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic band gap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD) we directly probe the magnetism at the V site and in zerofield. Hysteresis curves of the XMCD signal show a strong dependence of the coercivity on the ramping velocity of the magnetic field. The XMCD signal decays on a time scale of minutes which we conclude contributes to the absence of a detectable magnetic gap at the Dirac point.