Tuning strategy for Curie-temperature enhancement in the van der Waals magnet Mn1+xSb2_xTe4

The van-der-Waals antiferromagnetic topological insulator MnBi2Te4 is one of the few materials that realize the sought-after quantum anomalous Hall (QAH) state and quantized surface charge transport. To assess the relevance of its isostructural analog MnSb2Te4 as a potential QAH candidate, the roles of Mn/Sb site mixing and cationic vacancies need to be clarified. Recent findings have shown that non-stoichiometry in Mn1 +/- xSb2 -/+ xTe4 is an efficient tuning knob to achieve a net spin-polarized state and to raise the magnetic ordering temperature well above that of MnBi2Te4. Here, we report the crystal structure, the bulk and the surface magnetism of two new Mn1+xSb2_xTe4 samples: Mn1.08Sb1.92Te4(x approximate to 0.1) with TC = 44 K, and Mn2.01Sb1.19Te4(x approximate to 1.0) with the record TC = 58 K. We quantify the site mixing comprehensively by combining various structural probes on powders and single crystals, and then employ bulk, local (electron spin resonance), and spectroscopic (x-ray magnetic circular dichroism) probes to connect these insights to the magnetism of these materials. We demonstrate that Mn over-stoichiometry up to x = 1.0, in combination with a particular Mn/Sb intermixing pattern and the increasingly three-dimensional character of the magnetic order, push the TC upwards. The tendency towards more robust ferromagnetism mediated by stronger interlayer exchange in Mn1+xSb2_xTe4 upon increasing x is confirmed by bulk magnetometry and by a series of density-functional-theory calculations of model structures with varying intermixing.