Crystal and magnetic structures of magnetic topological insulators <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>MnBi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Te</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>MnBi</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mi>Te</mml:mi><mml:mn>7</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

Using single-crystal neutron diffraction, we present a systematic investigation of the crystal structure and magnetism of the van der Waals topological insulators ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ and ${\mathrm{MnBi}}_{4}{\mathrm{Te}}_{7}$, where rich topological quantum states have been recently predicted and observed. Structural refinements reveal that considerable Bi atoms occupied on the Mn sites in both materials, distinct from the previously reported antisite disorder. We show unambiguously that ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ orders antiferromagnetically below 24 K, featured by a magnetic symmetry ${R}_{I}\text{\ensuremath{-}}3c$, while ${\mathrm{MnBi}}_{4}{\mathrm{Te}}_{7}$ is antiferromagnetic below 13 K with a magnetic space group ${P}_{c}\text{\ensuremath{-}}3c1$. They both present antiferromagnetically coupled ferromagnetic layers with spins along the $c$ axis. We put forward a stacking rule for the crystal structure of an infinitely adaptive series ${\mathrm{MnBi}}_{2n}{\mathrm{Te}}_{3n+1}$ ($n\ensuremath{\ge}1$) with a building unit of $[{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}]$. By comparing the magnetic properties between ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ and ${\mathrm{MnBi}}_{4}{\mathrm{Te}}_{7}$, together with recent density-functional theory calculations, we concluded that a two-dimensional magnetism limit might be realized in the derivatives. Our work may promote theoretical studies of topological magnetic states in the series of ${\mathrm{MnBi}}_{2n}{\mathrm{Te}}_{3n+1}$.

Topics & Concepts