Ferromagnetism in van der Waals compound MnS b1.8 B i0.2 T e4

The intersection of topology and magnetism represents a new playground to discover novel quantum phenomena and device concepts. In this work, we show that under certain synthetic conditions, a van der Waals single-crystalline compound MnSb1.8Bi0.2Te4 exhibits a net ferromagnetic state with a Curie temperature of 26 K, in contrast to the fully compensated antiferromagnetic order observed previously for other members of the Mn(Sb,Bi)2Te4 family. We employ magneto-transport, bulk magnetization, x-ray and neutron scattering studies to illustrate the structural, magnetic, and electrical properties of MnSb1.8Bi0.2Te4. Our structural analyses reveal considerable Mn-Sb site mixing and suggest a recently proposed mechanism, where Mn occupying the Sb site mediates a ferromagnetic coupling between Mn layers [Murakami, Phys. Rev. B 100, 195103 (2019)10.1103/PhysRevB.100.195103], could be at play. Close comparisons made to an antiferromagnetic compound MnSb2Te4 illustrate the subtle magnetic interactions of the system and the important role played by local chemistry. The appearance of an unusual anomalous Hall effect in MnSb1.8Bi0.2Te4 at low temperatures hints at a magnetic ground state different from other members of this family. Our results are an important step in the synthesis and understanding of magnetism in materials with topological characteristics.