Ferromagnetic MnBi4Te7 obtained with low-concentration Sb doping: A promising platform for exploring topological quantum states

Abstract

The tuning of the magnetic phase, chemical potential, and structure is crucial to observe diverse exotic topological quantum states in $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}{({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{m}$ $(m=0--3)$. Here we show a ferromagnetic (FM) phase with a chiral crystal structure in $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{4}{\mathrm{Te}}_{7}$, obtained via tuning the growth conditions and Sb concentration. Unlike previously reported $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{4}{\mathrm{Te}}_{7}$, which exhibits FM transitions only at high Sb doping levels, our samples show FM transitions $({T}_{\mathrm{C}}=13.5\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ at 15%--27% doping levels. Furthermore, our single-crystal x-ray-diffraction structure refinements find Sb doping leads to a chiral structure with the space group of $P3$, contrasted with the centrosymmetric $P\overline{3}m1$ crystal structure of the parent compound ${\mathrm{MnBi}}_{4}{\mathrm{Te}}_{7}$. Through angle-resolved photoemission spectroscopy measurements, we also demonstrated that the nontrivial band topology is preserved in the Sb-doped FM samples. Given that the nontrivial band topology of this system remains robust for low Sb doping levels, our success in making FM $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{4}{\mathrm{Te}}_{7}$ with $x=0.15$, 0.175, 0.2, and 0.27 paves the way for realizing the predicted topological quantum states, such as the axion insulator and Weyl semimetals. Additionally, we also observed magnetic glassy behavior in both antiferromagnetic ${\mathrm{MnBi}}_{4}{\mathrm{Te}}_{7}$ and FM $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{4}{\mathrm{Te}}_{7}$ samples, which we believe originates from cluster spin-glass phases coexisting with long-range antiferromagnetic/FM orders. We have also discussed how the antisite Mn ions impact the interlayer magnetic coupling and how FM interlayer coupling is stabilized in this system.