Magnetic topological materials are a hotbed for exotic quantum phenomena such as the quantum anomalous Hall effect (QAHE), the topological magneto-electric effect, new topological states like axion insulators and magnetic Weyl semimetals. In reply to the high demand for optimized material systems, magnetic topological insulators made a decade-long journey [1] from extrinsically doped Bi2Te3 and (Cr,V)Bi2(Se,Te)3 heterostructures, on which the QAHE was experimentally discovered [2], to the intrinsically magnetic van der Waals material MnBi2Te4 [3]. The QAHE was observed in MnBi2Te4 thin films at notably higher temperatures of 6 K [4] than in [2], pointing at a perspective pathway of materials optimization towards more robust quantum effects. Since the bulk MnBi2Te4 is an A-type antiferromagnet with TN = 25 K, the task of fabricating structurally similar ferri- or ferromagnets with an increasing TC is pertinent.

MnBi2Te4 is the progenitor of a family of van der Waals materials (MnX2Te4)(X2Te3)n, X = Sb or Bi, n = 0–4, which I will introduce in my talk. Their crystal lattices are ordered stacking variants of septuple (MnX2Te4) layers hosting an ordered magnetic sublattice of Mn(II) atoms and of n quintuple (X2Te3) spacers. Varying intralayer and interlayer magnetic exchange couplings foster a rich palette of possible magnetic ground states, including ferri- and ferromagnetic. Besides the stacking order, a more subtle factor – Mn/X site intermixing [5] – influences the long-range magnetic order greatly. This phenomenon is particularly prominent in Mn1±xSb2Te4 where it raises the Curie temperature of a ferrimagnetic-to-paramagnetic transition from 27 to 58 K, while x varies in the range of 0.1–1.0 [6-9]. At higher x values, we document a structure transition to the cubic lattice symmetry in Mn2.7Sb1.3Te4 and Mn2.1Ge0.4Sb0.9Te4 which alters the band topology and magnetic order notably. Mn-enrichment results in very different ground states: whereas Mn2.7Sb1.3Te4 is a ferromagnet with the record high TC = 73 K, that bring magnetic topological materials close to the liquid nitrogen limit, Mn2.1Ge0.4Sb0.9Te4 is an antiferromagnet with TN = 25 K. With an aid of the ab initio calculations we look into the origins of this strong difference.