Unusual dynamic susceptibility arising from soft ferromagnetic domains in MnBi8Te13 and Sb-doped MnBi2nTe3n+1 (n=2, 3)

TL;DR

This study investigates the magnetic dynamics of ferromagnetic MnBi2nTe3n+1 compounds, revealing universal slow relaxation behaviors, soft ferromagnetic domains, and a rare double-peak AC susceptibility phenomenon linked to domain formations.

Contribution

It provides new insights into the magnetic relaxation and domain structures in ferromagnetic MnBi2nTe3n+1 compounds, highlighting the universality and soft nature of their ferromagnetic domains.

Findings

Universal slow relaxation behavior observed in all compounds

Identification of very soft ferromagnetic domains via magneto-optical imaging

Rare double-peak AC susceptibility linked to soft domain formations

Abstract

MnBi2nTe3n+1(MBT) is the first intrinsic magnetic topological insulator and is promising to hostemergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n >= 4 or with Sb doping. We studied the magnetic dynamics in a few selected ferromag-netic (FM) MBT compounds, including MnBi8Te13and Sb doped MnBi2nTe3n+1(n= 2,3) usingAC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all threecompounds, suggesting its universality among FM MBT. We attribute the origin of the relaxationbehavior to the irreversible domain movements since they only appear below the saturation fieldswhen ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by thelow-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imagedvia our magneto-optical measurements. Finally, we ascribe the rare "double-peak" behavior ob-served in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domainformations.