Crystal and magnetic structures of magnetic topological insulators MnBi2Te4 and MnBi4Te7

Using single-crystal neutron diffraction, we present a systematic investigation of the crystal structure and magnetism of the van der Waals topological insulators MnBi2Te4 and MnBi4Te7, 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 MnBi(2)Te4 orders antiferromagnetically below 24 K, featured by a magnetic symmetry R-I-3c, while MnBi4Te7 is antiferromagnetic below 13 K with a magnetic space group P-c-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 MnBi2nTe3n+1 (n >= 1) with a building unit of [Bi2Te3]. By comparing the magnetic properties between MnBi2Te4 and MnBi4Te7, 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 MnBi2nTe3n+1.