Magnetic Dirac semimetal state of (Mn,Ge)Bi2Te4

The ability to finely tune the properties of magnetic topological insulators (TIs) is crucial for quantum electronics. We studied solid solutions with a general formula Ge_xMn_1-xBi₂Te₄ between two isostructural Z₂ TIs, magnetic MnBi₂Te₄ and nonmagnetic GeBi₂Te₄ with Z₂ invariants of 1;000 and 1;001, respectively. We observed linear x-dependent magnetic properties, composition-independent pairwise exchange interactions, and topological phase transitions (TPTs) between topologically nontrivial phases and the semimetal state. The TPTs are driven purely by the variation of orbital contributions. By tracing the x-dependent Bi 6p contribution to the states near the fundamental gap, the effective spin-orbit coupling variation is extracted. The gapless state observed at x = 0.42 closely resembles a Dirac semimetal above the Néel temperature and shows a magnetic gap below, which is clearly visible in raw photoemission data. The observed behavior demonstrates an ability to precisely control topological and magnetic properties of TIs.